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Why not self pollination for finding the genotype instead of test cross?

Why not self pollination for finding the genotype instead of test cross?


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Test cross can tell us what's the genotype of a plant is. But we can know that even by self pollinating the plant. For example, If a garden pea plant has the genotype TT, then self pollinating them will give all tall plants. If the plant has the genotype Tt, then self pollinating them will give both tall and dwarf plants in 3:1 ratio. So we can simply say that by self pollination if all the plants are of same type then the parent plant is homozygous. If the plants are in ratio them the parent plant is heterozygous.


Because for test cross you need to phenotype far less progeny to be reasonably sure about parent genotype. Lets say you decide to phenotype N progeny. If the tested parent is heterozygote the random chance of you getting all N plants tall is (1/2)N for test cross and (3/4)N for self pollination.

So for example if you decide to phenotype N=8 progeny plants in case of self pollination (3/4)8=10% of your heterozygous parents will be wrongly assigned as homozygous which is error too high for almost anything serious. In case of test cross (1/2)8=0.4% of your heterozygous parents will be wrongly assigned as homozygous. To achieve the similar level of accuracy with self pollination you would have to phenotype N=19 or N=20 progeny plants.

The difference does not seem like much when testing one parrent but if you need to test many the costs will add up.


Cross Pollination versus Self Pollination comparison chart
Cross PollinationSelf Pollination
Definition Cross pollination is the transfer of pollen grains from the anther of a flower to the stigma of a flower of a different plant of the same species. Self pollination is the transfer of pollen grains from the anther to the stigma of the same flower
Seen in Insects: Apples, grapes, plums, pears, raspberries, blackberries, strawberries, runner beans, pumpkins, daffodils, tulips, lavender Wind: grasses, catkins, dandelions, maple trees, and goat’s beard. Some legumes, e.g. peanuts. Orchids, peas and sunflowers, wheat, barley, oats, rice, tomatoes, potatoes, apricots and peaches.
Transfer Wind, insects, water, animals, etc. Shed pollen directly onto stigma.
Plant differences Brightly colored petals, nector and scent, long stamens and pistils. Smaller flowers.
Results More variety in species. It allows for diversity in the species, as the genetic information of different plants are combined. However, it relies on the existence of pollinators that will travel from plant to plant. More uniform progeny. Allows plant to be less resistant as a whole to disease. However, it does not need to expend energy on attracting pollinators and can spread beyond areas where suitable pollinators can be found.
Number of pollen grains large number small number
Type of reproduction Allogamy Autogamy, Geitonogamy
Occurs in. Either perfect or imperfect flowers Perfect flowers


Mendel&rsquos Experiment with Garden Pea Plant | Genetics

In this article we will discuss about the Mendel’s experiment with garden pea plant.

In 1856 Mendel began his experiments on plant hybridisation with garden peas in the monastery garden. Although similar work had already been done by contemporary botanists, the significant features of all these experiments had been overlooked because the investigators made overall observations of all inherited characters instead of collecting and analysing data in a systematic, mathematical way.

This is how Mendel achieved what his predecessors could not. First of all he concentrated his attention on a single character in his experiments on inheritance. Secondly, he kept accurate pedigree records for each plant. And third, he counted the different kinds of plants resulting from each cross. Fourthly, he analysed his data mathematically.

Mendel’s success is in part also attributed to his choice of material. The garden pea (Pisum sativum) used in his experiments (Fig. 1.1) offers certain advantages: it is an easily growing, naturally self fertilising plant it is well suited for artificial cross pollination therefore hybridisation (crossing of two different varieties) is easily accomplished it shows pairs of contrasting characters which do not blend to produce intermediate types and can be traced through successive generations without confusion.

For example tall and dwarf are a pair of contrasting conditions for the character height similarly round and wrinkled seeds are contrasting forms for the character seed texture. On self pollination each character breeds true. Mendel worked with seven pairs of characters so that he had 14 pure breeding varieties.

Monohybrid Cross:

Mendel crossed varieties of edible peas which showed clear-cut differences in morphological characters (Fig. 1.2) such as colour of flowers (red vs. white), shape of pod (inflated vs. constricted), colour of pod (green vs. yellow), texture of seed (round vs. wrinkled), colour of cotyledons (yellow vs. green), flower position (axial vs. terminal) and height of plant (tall vs. dwarf).

He was dusting the pollen of one variety on the pistil of the other. To prevent self-pollination of the female parent, he removed its stamens before the flowers had opened and shed the pollen. After making the cross he would enclose the flowers in bags to protect them from insects and foreign pollen.

Mendel’s first experiments explain how a single gene segregates in inheritance. When Mendel crossed a true breeding tall plant (female parent) with a true breeding plant of the dwarf variety (male parent), he got tall plants like one parent in the first filial generation designated F1.

He used the term “dominant” for the tall character which dominated in the F1 generation, and “recessive” for the character of dwarfness which remained hidden (latent) in the F1 generation.

Self fertilisation of the F1 hybrids produced the second filial generation F2 consisting of a total of 1064 plants of which 787 were tall and 277 were dwarf. That is tall and dwarf plants appeared in F2 in the proportion of 2.84:1 which is roughly equal to 3:1.

When he performed the reciprocal cross by reversing the sexes of the parents, the same results were obtained showing thereby that it did not matter which plant was used as male or as female parent.

Similarly, Mendel crossed pea plants differing in other characters such as colour of flowers (red flowered versus white flowered), texture of seed (round versus wrinkled), colour of cotyledons (yellow versus green). Such a cross which involves only one character from each parent is called a monohybrid cross.

In each case Mendel found one parental character dominating in the F1 hybrid, and after self fertilisation in F2 generation both parental characters appeared in the proportion of three-fourths to one-fourth. He performed each experiment on several thousand plants and counted all the plants in F2 progeny which gave an average ratio of 3:1.

From his experiments Mendel concluded that each parent contributes one factor for a character to the F1 hybrid. In this way the F1 hybrid has two factors for each character.

When the F1 hybrid forms gametes the two factors separate from each other. There is no mixing up of factors thus emphasizing the purity of gametes. The phenomenon of separation became Mendel’s First Principle and was later termed as the Law of Segregation.

This is explained diagrammatically as follows:

Terms Used in Mendel’s Crosses:

Dominant versus Recessive:

When two pure breeding varieties are crossed, the parental character that expresses itself unchanged in the F1 generation hybrids is dominant: the one that does not appear in F1 but appears in F2 is called recessive. In the above cross three-fourths of the F2 progeny show the dominant character and one-fourth the recessive character.

Factors which control contrasting expressions of a character are said to be alleles or allelomorphs of each other. In the above cross the character in consideration is height, and factors T and t which control tallness and dwarfness are alleles of each other.

Homozygous and Heterozygous:

These terms were coined by Bateson and Saunders in 1902. Mendel had concluded that each character is controlled by a pair of factors. When both factors are identical such as TT and tt, the individual is said to be homozygous for that character. When the factors are different (for example Tt), the term heterozygous is used. Mendel used capital letter of the alphabet to denote dominant factors, and small letters for recessive alleles.

Mendel’s factors were later replaced by the term ‘gene’ by a Danish botanist Johannsen in 1909. The word genotype refers to the genetic constitution of an individual, whereas phenotype refers to the external appearance or manifestation of a character.

Mendel selfed members of the F2 progeny and found that out of the dominant types, one-third bred true for the dominant character, whereas two-thirds segregated into dominants and recessives in the ratio of 3: 1. All the recessive plants of F2 generation when selfed bred true for the recessive character.

Mendel found similar results in monohybrid crosses with all the seven pairs of contrasting characters in Pisum sativum. After eight years of detailed investigations on thousands of pea plants, Mendel published his results in a paper entitled “Experiments in Plant Hybridisation” in the Proceedings of the Brunn Natural History Society in 1866.

However, his work received no attention for 34 years until three scientists, De-Vries in Holland, Correns in Germany and Tschermak in Austria working independently published their findings in 1900 and confirmed Mendel’s results.

Not satisfied with his work, Mendel himself subjected his results to a test. In the cross between tall and dwarf pea plants, the F1 hybrids were all phenotypically tall but their genotypes were not only TT but also Tt. Consider a heterozygous hybrid plant Tt. When it forms gametes, the factors T and t segregate in the gametes in a 1: 1 ratio.

This means that 50% of the gametes of an F1 heterozygous hybrid carry the factor T and 50% the factor t. Mendel crossed such a hybrid plant (Tt) with a plant of the true breeding, dwarf variety (tt). All the gametes of the homozygous dwarf plant carried the recessive factor t.

Every gamete of the recessive parent has 50% chance of combining with a gamete carrying T and 50% chance to combine with a t gamete from the heterozygous parent.

This should result in 50% of progeny showing the tall phenotype and genetic constitution Tt, whereas 50% of the progeny should be phenotypically dwarf with genotype tt as explained diagrammatically below:

Indeed Mendel’s results of this cross agreed with the theoretical expectations thus providing additional experimental proof of the correctness of his interpretations. Such a cross where an individual is crossed to a double recessive parent to test and verify the individual’s genotype is called a testcross or backcross.

In order to determine genotypes of the F2 progeny, Mendel allowed the F2 plants to self- fertilize and produce a third filial or F3 generation. He found that the homozygous F2 tall plants could produce only tall plants on self-fertilisation. This indicated their genotype to be TT.

Similarly the F2 dwarf homozygotes yielded only dwarf plants on selling their genotype was tt. The F2 heterozygotes on self fertilizing behaved identical to the F1 hybrids and gave rise to tall and dwarf phenotypes in the ratio 3:1. This proved that their genotype was identical to that of F1 hybrids i.e. Tt.

It is noteworthy that the genotypes of the parents are written as TT and tt instead of single T and t. This is in accordance with Mendel’s hypothesis that each parent has two factors for a character. There is also a cytological explanation. The somatic chromosomes of all plants and animals exist in homologous pairs, one member of each pair coming from the paternal parent, other from maternal parent.

A gene is a section of the chromosomal DNA which has information necessary for determination of a specific genetic trait. Suppose a hypothetical gene A occupies a particular site or locus on a given chromosome. The homologous chromosome contains at the identical locus an alternative gene a which controls the same trait as gene A, but in such a way as to produce a different phenotype for the same trait.

The alternative genes at the same locus A and a are also called alleles. It is an astonishing fact that though Mendel knew nothing about genes, he could predict the existence of factors, which later turned out to be genes. During the reduction division of meiosis (Metaphase I), chromosomes of a pair separate and go to the opposite poles. Consequently genes or alleles segregate from each other and pass into different gametes.

Demonstration of Genetic Segregation:

Mendel’s F1 hybrids (Tt) were all tall plants indistinguishable phenotypically. Sometimes homozygous and heterozygous plants show phenotypic differences. There is a seedling character for green pigment in soybeans. The homozygous (GG) soybean plant is dark green, the heterozygous (Gg) plant light green.

The homozygous recessive (gg) produces a golden lethal seedling which dies in early stages due to lack of green pigment. If the heterozygous plants are grown to maturity and self-pollinated, their progeny will again segregate as dark green, light green and lethal golden in the ratio of 1: 2: 1.

Differences between homozygous and heterozygous genotypes can sometimes be observed in the gametes. In rice, sorghum and maize, effect of the gene for waxy endosperm is visible in the pollen grains. Maize kernels which have waxy endosperm produce starch and stain blue with iodine non-waxy endosperm does not produce starch and stains red with iodine. In maize gene for waxy endosperm is located on chromosome 9.

A homozygous plant with genetic constitution WxWx produces starch in endosperm and stains blue with iodine. In the heterozygous plant (Wx wx) the dominant gene causes starch production and the kernels stain blue with iodine. But kernels on homozygous recessive plants (wx wx) have no starch and stain red with iodine. If anthers of these plants are treated with iodine, the pollen grains stain in a similar way.

In homozygous plants all the pollen grains stain blue. In heterozygous plants 50% of pollen grains stain blue (i.e. those containing Wx), whereas 50% stain red (i.e. pollen grains having wx).

In the homozygous recessive plant, all the pollen grains stain red. If breeding tests are done by self-pollinating the heterozygous F1 plants, the progeny consists of blue staining kernels (WxWx and Wxwx plants) and red staining kernels (wxwx plants) in the ratio 3:1.

The Dihybrid Cross:

Mendel made crosses between pea plants differing in two characters such as texture of seed and colour of cotyledons. Such a cross in which inheritance of two characters is considered is called a dihybrid cross.

First of all Mendel crossed a pea plant that was breeding true for round seeds with a plant that bred true for wrinkled seeds. The F1 indicated that roundness was dominant over wrinkled texture of seed coat. Similarly, by another cross he could determine that yellow colour of cotyledons was dominant over green.

He now used as male parent a plant which bred true for both round and yellow characters and crossed it with a female parent that bred true for wrinkled green. As expected from the results of his single crosses, the F1 was round yellow. When he selfed the F1 hybrids, the F2 progeny showed all the parental characters in different combinations with each other.

Thus plants with round yellow seeds, round green seeds, wrinkled yellow seeds and wrinkled green seeds all appeared in the ratio 9:3:3:1. Reciprocal cross in which the female parent was round yellow and male parent wrinkled green gave the same results.

Mendel applied the principle of a monohybrid cross and argued that in the dihybrid cross the true breeding round yellow parent must be homozygous RRYY, and the wrinkled green parent rryy. Since each character is determined be two factors, in a dihybrid cross there must be four factors present in each parent.

Likewise the F1 hybrid must be RrYy. But the question remained as to how did the four different combinations of parental phenotypes appear in the progeny? Mendel argued that the pair of factors for roundness must be behaving independently of the pair of factors for yellow colour of seeds. In other words, one factor for a character must be passing independently of a factor for another character.

Thus in the F1 hybrids, R and r pass into different gametes.

Now the probability of an R gamete formed is one-half, and of r gamete also one-half. Similar probabilities exist for Y and y gametes. It follows that the probability that R and Y should go to the same gamete is one-fourth, as also of R and y, r and Y, and r and y. Therefore, gametes containing factors RY, Ry, rY and ry should form in equal proportions.

The F1 hybrid producing the four types of gametes mentioned above was selfed. The results expected in the F2 progeny can be predicted by making a checkerboard or a Punnett Square. Gametes produced by one parent are plotted on top of the checkerboard, and gametes of the other parent on the side.

The sixteen squares of the checkerboard are filled up by making various possible combinations of male and female gametes during fertilisation. The phenotypes read out from the checkerboard indicate a 9: 3: 3: 1 ratio exactly as observed by Mendel.

As in the case of the monohybrid cross, Mendel verified his results by performing the test cross. He crossed the F1 hybrid heterozygous for both characters with a double recessive parent (rryy) which should produce only one type of gamete ry. The uniformity in the gametes of the recessive parent determines the differences in the types of gametes produced by the heterozygous parent.

Now the hybrid RrYy produces gametes carrying RY, Ry, rY and ry with equal frequency. It follows that during fertilisation if all these four types of gametes unite with ry gamete of the recessive parent, the resulting progeny should show all the four combinations of characters also in equal proportions. Indeed, Mendel observed the testcross progeny to consist of Round Yellow, Round Green, Wrinkled Yellow and Wrinkled Green plants in the ratio 1:1:1:1.

From the results of his dihybrid crosses, Mendel realised the following facts. At the time of gamete formation the segregation of alleles R and r into separate gametes occurs independently of the segregation of alleles Y and y.


Functions of Punnett Squares

In large-scale experiments, such as those conducted by Mendel, Punnett squares can accurately predict the ratios of various observable traits as well as their underlying genetic composition. For instance, when a true-breeding tall pea plant is cross fertilized with pollen from a true-breeding short pea plant, the Punnett square can predict that all the offspring will be tall, and all of them will be heterozygous with both the allele for shortness and tallness. It can further predict that if these heterozygous plants are allowed to self-fertilize, approximately seventy-five percent of the second generation plants will be tall, and the remaining twenty-five percent will be short. Among the tall plants, one-third will remain true-breeding while the remaining two-thirds will be heterozygous. This tool is therefore used by plant and animal breeders to choose appropriate specimens in order to obtain offspring carrying a desired trait.

They are also used in genetic counseling to help couples make the decision about having children. For example, in cases where both parents are carriers for an autosomal recessive disease such as cystic fibrosis, there is a twenty-five percent chance of their child suffering from the illness and a fifty-percent chance that their offspring will be carriers. However, if one parent has the disease and the other is neither a carrier nor suffering from the illness, the couple can be reassured that their child will not develop cystic fibrosis since she will carry only one copy of the abnormal gene.


AP Biology Campbell Active Reading Guide Chapter 14 – Mendel and the Gene Idea

1. Explain the concept of blending, and then describe how Mendel’s "particulate" gene hypothesis was different.

Blending – The genetic material of the two parents mix in a manner analogous to the way blue and yellow paints mix to make green. Over many generations, a freely mating population will give rise to a uniform population of individuals. Mendel’s Hypothesis – Parents pass on discrete heritable units, genes, that retain their separate identities in offspring.

2. Explain how using pea plants allowed Mendel to control mating.

The reproductive parts of pea plants are enclosed in a flower, petals hindering cross-pollination, with plants usually self-fertilizing. Mendel removed the immature stamens of a plant before they produced pollen and then dusted pollen from another plant onto the altered flowers. Mendel could thus always be sure of the parentage of new seeds.

3. What is the difference between a character and a trait?

A character is a heritable factor that varies among individuals, eg flower color. Traits are each variant of a character, eg purple or white colored flowers.

4. Define P generation, F₁ generation, and F₂ generation.

P generation – true breeding parents in a genetic cross F₁ generation – offspring of P generation (first filial generation) F₂ generation – offspring of F₁ generation (second filial generation)

5. Explain how Mendel’s simple cross of purple and white flowers did the following:
a) refuted blending
b) determined dominant and recessive characteristics
c) demonstrated the merit of experiments that covered multiple generations

a) If blending was true, the F₁ hybrid from a cross between purple and white flowered pea plants would have had pale purple flowers, but all F₁ offspring had purple flowered plants and the white flower trait appeared in the F₂ generation. b) Mendel reasoned that the heritable factor for white flowers did not disappear in the F₁ plants, but was somehow hidden, or masked, when the purple-flower factor was present. In Mendel’s terminology, purple flower color is a dominant trait, and white flower color is a recessive trait. c) If Mendel had stopped the experiment at F₁ instead of continuing through to F₂, he would not have discovered the law of segregation and the law of independent assortment.

7. In sexually reproducing organisms, why are there exactly two chromosomes in each homologous pair?

One chromosome is paternal, one is maternal.

8. Mendel’s model consists of four concepts. Describe each concept in the appropriate space below. Indicate which of the concepts can be observed during meiosis by placing an asterisk by the concept.

First concept – Alternative versions of genes cause variation in inherited characters Second concept – For each character, every organism inherits two copies of a gene, one from each parent Third Concept – If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance the other, the recessive allele, has no noticeable effect on the organism’s appearance. Fourth concept (law of segregation) * – the two alleles for each heritable character separate during gamete formation and end up in different gametes.

9. a) What is the F₂ phenotypic and genotypic ratio for a PP x pp cross?
b) Which generation is completely heterozygous?
c) Which generation has both heterozygous and homozygous offspring?

a) phenotype = 3:1 genotype = 1:2:1 b) F₁ generation c) F₂ generation

10. In pea plants, T is the allele for tall plants, while t is the allele for dwarf plants. If you have a tall
plant, demonstrate with a testcross how it could be determined if the plant is homozygous tall or
heterozygous tall.

Breed plant in question with short homozygous, and if F₁ is all tall, then the mystery plant was homozygous, otherwise if 1/2 are tall and 1/2 are short, it was heterozygous.

11. Explain the difference between a monohybrid cross and a dihybrid cross.

Monohybrid cross – cross involving study of only one character Dihybrid cross – cross involving two characters, eg flower color and seed shape

14. Explain Mendel’s Law of Independent Assortment.

Each pair of alleles segregates independently of each other pair of alleles during gamete formation.

16. In probability, what is an independent event?

The outcome of any particular trial is unaffected by what has happened in previous trials.

17. State the multiplication rule and give an original example.

The probability that two or more independent events will occur together in a specific combination is found by multiplying probabilities of each of two events. For example, the probability that two coin tosses will result in heads is 1/2 * 1/2 = 1/4.

18. State the addition rule and give an original example.

The addition rule states that the probability that any two or more mutually exclusive events will occur is calculated by adding their individual probabilities.

19. What is the probability that a couple will have a girl, a boy, a girl, and a boy in this specific
order?

20. Explain how incomplete dominance is different from complete dominance, and give an example of
incomplete dominance.

In incomplete dominance, neither allele is completely dominant in the phenotype, and the F₁ generation has a phenotype that is somewhere between the those of the two parental varieties. In complete dominance, the heterozygote and homozygote for the dominant allele are indistinguishable. An example of incomplete dominance is a red parent flower and white parent flower cross to form pink flowered-offspring.

21. Compare and contrast codominance with incomplete dominance.

In codominance two alleles are dominant and affect the phenotype in two different, but equal ways, eg human blood type AB.

22. Dominant alleles are not necessarily more common than recessive alleles in the gene pool. Explain why this is true.

Natural selection determines how common an allele is in the gene pool. For example, having six fingers (polydactyly) is dominant to five fingers, but the presence of six fingers is not common in the human gene pool.

23. Explain what is meant when a gene is said to have multiple alleles. Blood groups are an excellent
human example of this.

Most genes exist in more than two allelic forms, for example, ABO blood groups.

26. What is pleiotropy? Explain why this is important in diseases like cystic fibrosis and sickle-cell disease.

Property of a gene that causes it to have multiple phenotypic effects. For example, sickle cell disease and cystic fibrosis have multiple symptoms all due to a single defective gene.

The phenotypic effects of a gene at one locus alters the effects of a gene at another locus.

28. Explain why the dihybrid cross detailed in Figure 14.12 in your text has four yellow Labrador retrievers instead of the three that would have been predicted by Mendel’s work.

The E/e gene is epistatic to the B/b gene.

29. Why is height a good example of polygenic inheritance.

Two or more genes have an additive effect on a single character in the phenotype, such as height.

30. Quantitative variation usually indicates _________.

31. Using the terms norm of reaction and multifactorial, explain the potential influence of the
environment on phenotypic expression.

Norms of reaction are broadest for polygenic characters. Both genetic and environment factos contribute to the quantitative nature of those characters.

33. In the pedigree you completed above, explain why you know the genotype of one female in the third
generation, but are unsure of the other.

The presence of a free earlobe could indicate either an FF or Ff genotype, as F is the dominant allele, resulting in free earlobes. The female with the recessive trait can only have one genotype. The female with the dominant trait could be homozygous or heterozygous.

34. Describe what you think is medically important to know about the behavior of recessive alleles.

Thousands of genetic disorders are known to be inherited as simple recessive traits. These disorders range in severity from relatively mild, such as albinism (lack of pigmentation, which results in susceptibility to skin cancers and vision problems) to life-threatening, such as cystic fibrosis.

35. You are expected to have a general knowledge of the pattern of inheritance and the common symptoms of a number of genetic disorders. Provide this information for the disorders listed below.
a) cystic fibrosis
b) sickle-cell disease
c) achondroplasia
d) Huntington’s disease

a) A human genetic disorder caused by a recessive allele for a chloride channel protein characterized by an excessive secretion of mucus and consequent vulnerability to infection fatal if untreated. b) A recessively inherited human blood disorder in which a single nucleotide change in the β-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals. c) A form of dwarfism that occurs in one of every 25,000 people. Heterozygous individuals have the dwarf phenotype. Like the presence of extra fingers or toes, achondroplasia is a trait for which the recessive allele is much more prevalent than the corresponding dominant allele. d) A human genetic disease caused by a dominant allele characterized by uncontrollable body movements and degeneration of the nervous system usually fatal 10 to 20 years after the onset of symptoms.

36. Amniocentesis and chorionic villus sampling are the two most widely used methods for testing a fetus
for genetic disorders. Use the unlabeled diagram below to explain the three main steps in
amniocentesis and the two main steps of CVS.

Amniocentesis: 1. A sample of amniotic fluid can be taken starting at the fourteenth to sixteenth week of pregnancy. 2. Biochemical and genetic tests can be performed immediately on the amniotic fluid or later on the cultured cell. 3. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. CVS: 1. A sample of chorionic villus tissue can be taken as early as the eighth to tenth week of pregnancy. 2. Karyotyping and biochemical and genetic tests can be performed on the fetal cells immediately, providing results within a day or so.

37. What are the strengths and weaknesses of each fetal test?

Strength of amniocentesis: In addition to fetal cells, amniotic fluid is also collected. Amniotic fluid can be used to detect additional enzymatic or developmental problems not detectable from the karyotype. Weakness of amniocentesis: Cells must be cultured for several weeks before karyotyping, and the test cannot be performed until the fourteenth to sixteenth week. Strength of CVS: These cells proliferate rapidly enough to allow karyotyping to be carried out immediately, and CVS can be performed as early as the eighth to tenth week. Weakness of CVS: No amniotic fluid is collected with this technique.

38. What are the symptoms of phenylketonuria (PKU)? How is newborn screening used to identify
children with this disorder?

The symptoms of phenylketonuria include an inability to metabolize the amino acid phenylalanine, causing severe mental intellectual disability. Some genetic disorders, including phenylketonuria, can be detected at birth by simple biochemical tests that are now routinely performed in most hospitals in the United States.


Limitations of True Breeding

True breeding comes with a number of limitations, most of which stem from a lack of genetic variation. Many true breeding specimens are susceptible to disease and can suffer from a number of crippling illnesses as they grow older including bone and blood disorders. In addition, many traits may appear to breed true, such the temperament for being good guard dogs, but any characteristic that is multi-genic or influenced by the environment can show variation.


Inheritance of Two Genes

Mendel also researched with inheritance of two genes, but crossing over pea plant with two contrasting traits, such as plant with seeds with round and green color and plant with seeds with yellow and wrinkled shape and found that the seeds resulted from this crossing over was yellow colored and round shaped. He thus, concluded that yellow color is dominant over green and round shape is dominant over wrinkled shape.

Now let us consider several genotype symbols –

Y = dominant yellow seed color

y = recessive green seed color

The genotype of parents is written as follows – RRYY and rryy.

Following figure shows the cross between these two parents produced the following result:

Result of the Dihybrid Cross

Round Yellow: Round Green: Wrinkled Yellow: Wrinkled Green

In the above figure, the gametes RY and ry unite on fertilization and produce RrYy hybrid at F1. When Mendel self – hybridized F 1 plants, he found 3/4 th of F 2 plants had yellow seeds while 1/4 th had green. Thus, the yellow and green color segregated in a ratio 3:1. In the similar manner, round and wrinkled seeds also segregated in the ratio 3:1.


Assorted References

An egg cell in an ovule of a flower may be fertilized by a sperm cell derived from a pollen grain produced by that same flower or by another flower on the same plant, in either of which two cases fertilization is…

A flower is self-pollinated (a “selfer”) if pollen is transferred to it from any flower of the same plant and cross-pollinated (an “outcrosser” or “outbreeder”) if the pollen comes from a flower on a different plant. About half of the more important cultivated plants are naturally cross-pollinated, and…

Occurrence in

…do not open, thus enforcing self-pollination (cleistogamous). In the chasmogamous flowers, the sepals are most commonly partly fused, and the five petals alternate in position with the sepals. There are commonly 10 stamens, but there may be fewer or more. The stamens may remain free or they may be fused…

…chief kinds of pollination: (1) self-pollination, the pollination of a stigma by pollen from the same flower or another flower on the same plant and (2) cross-pollination, the transfer of pollen from the anther of a flower of one plant to the stigma of the flower of another plant of…

…placed onto the stigma, effecting self-pollination. This is a remarkable instance in which seed formation is ensured by self-pollination if necessary, but cross-pollination is first attempted. This is an important adaptation in a genus of plants growing in arid areas where their pollinators might not be present or abundant.

Self-pollination occurs in a significant number of orchids. Several degrees of this phenomenon may be found in a single genus, from species in which accidental self-pollination results in fertilization to those in which the flowers never open, yet are capable of producing fertile seed. In…


MCAT Practice Questions: Biology

The MCAT will present you with 10 passages on biology and biochemistry topics, and ask 4-7 questions about each passage. The questions will address the four skills listed, although not every passage will require you to use each skill. You will also be presented with 15 discrete questions that are not associated with passages. These will also be designed to test both your science knowledge and application of that knowledge based on these four skills. You can find more details on what you need to know about the overall structure of the MCAT here .

The biology/biochem section of the MCAT is scored on a curved scale of 118-132, with the median score of all test takers set at 125. There is no specific number of right or wrong questions that corresponds to a given scaled score instead, each test administration is curved according to its level of difficulty and the performance of the test-takers on that day. The score for this section of the test is combined with the other three sections to give an overall score ranging from 472 to 528 .

MCAT Practice Questions: Biology

A patient presents to the emergency room with an asthma attack. The patient has been hyperventilating for the past hour and has a blood pH of 7.52. The patient is given treatment and does not appear to respond, but a subsequent blood pH reading is 7.41. Why might this normal blood pH NOT be a reassuring sign?
A. The patient’s kidneys may have compensated for the alkalemia.
B. The normal blood pH reading is likely inaccurate.
C. The patient may be descending into respiratory failure.
D. The patient’s blood should ideally become acidemic for some time to compensate for the alkalemia.

The correct answer is: C
When a patient with an asthma attack does not respond to treatment and has been hyperventilating for over an hour, he or she may become fatigued and may not be able to maintain hyperventilation. In this case, the patient begins to decrease his or her breathing rate and is not receiving adequate oxygen. By extension, carbon dioxide is trapped in the blood, and the pH begins to drop. Despite the fact that this pH is normal at the moment, this patient is crashing and may start demonstrating acidemia in the near future. While the kidneys should compensate for alkalemia, this is a slow process and would not normalize the blood pH within an hour further, adequate compensation by the kidneys would actually be a reassuring sign, eliminating choice (A). There is no evidence to believe the measurement was inaccurate, eliminating choice (B). Finally, after treatment, the patient should return to a normal blood pH with adequate ventilation and would not be expected to overcompensate by becoming acidemic, eliminating choice (D).

Suppose that in a mammalian species, the allele for black hair (B) is dominant to the allele for brown hair (b), and the allele for curly hair (C) is dominant to the allele for straight hair (c). When an organism of unknown genotype is crossed against one with straight, brown hair, the phenotypic ratio is as follows:
25% curly black hair
25% straight black hair
25% curly brown hair
25% straight brown hair
What is the genotype of the unknown parent?
A. BbCC
B. bbCc
C. Bbcc
D. BbCc

The correct answer is: D

In this dihybrid problem, a doubly recessive individual is crossed with an individual of unknown genotype this is known as a test cross. The straight- and brown-haired organism has the genotype bbcc and can thus only produce gametes carrying bc. Looking at the F1 offspring, there is a 1:1:1:1 phenotypic ratio. The fact that both the dominant and recessive traits are present in the offspring means that the unknown parental genotype must contain both dominant and recessive alleles for each trait. The unknown parental genotype must therefore be BbCc. If you want to double-check the answer, you can work out the Punnett square for the cross BbCc × bbcc:

MCAT Strategy Tips

Although the designers of the MCAT will provide physiological facts and numbers, that’s not what you’re expected to know before taking the exam. Your MCAT practice will require being able to see the same concepts that you learned in your undergraduate pre-med classes in a very defined and isolated environment—applied in a foreign scope to integrated sciences.

The interplay of scientific disciplines (i.e., how your knowledge of physics or chemistry informs your understanding of how an organ works) is paramount to your success both on the new MCAT and as a future physician. The human body is a network of interdependent physical, chemical, and biological processes, and the MCAT measures your ability to make those connections.

Knowing formulas and reactions is necessary for success on the MCAT, but will in no way be sufficient. Success will only come with practice, so here are a couple of tips:

Tip 1: When studying a “physical” science, think through all of its biological applications.

For example, let’s use reduction and oxidation. We know that LEO the Lion says GER (the Loss of an Electron is Oxidation and the Gain of an Election is Reduction). Therefore, when a metal is losing or gaining electrons, it’s either getting oxidized or reduced, respectively.

Don’t stop there. The ReDox that occurs in the Electron transport chain, with NADH losing an electron to the ETC and getting—you guessed it—oxidized, is the exact same concept. NAD+ is the product of the oxidation of NADH, just as Ag+ is the oxidized product of Ag. It’s the exact same science. So, don’t get thrown off by the fact that you learned it in two different places.

Tip 2: There are a finite number of scientific facts.

There is some truth in the claim that most of biology is rooted in chemistry and physics. Take proteins for example. We think of them as biological molecules because they serve such a prominent role in the body. In reality, they are nothing more than a very specific structural arrangement of carbon, nitrogen, oxygen, hydrogen, and sometimes sulfur.

The way in which they are bonded to each other is through a bonding orbital—just like the ones you learned about in general chemistry. They fold into specific shapes because of attractions and repulsions of the amino acids in their sequence. Those are the same attractions and repulsions seen in chemistry. The proteins themselves are coded from RNA, which is coded from DNA. RNA and DNA are just chemical molecules with the same properties you learned about in chemistry.


Laws of Inheritance

Mendel generalized the results of his pea-plant experiments into four postulates, some of which are sometimes called &ldquolaws,&rdquo that describe the basis of dominant and recessive inheritance in diploid organisms. As you will learn, more complex extensions of Mendelism exist that do not exhibit the same F2 phenotypic ratios (3:1). Nevertheless, these laws summarize the basics of classical genetics.

Pairs of Unit Factors, or Genes

Mendel proposed first that paired unit factors of heredity were transmitted faithfully from generation to generation by the dissociation and reassociation of paired factors during gametogenesis and fertilization, respectively. After he crossed peas with contrasting traits and found that the recessive trait resurfaced in the F2 generation, Mendel deduced that hereditary factors must be inherited as discrete units. This finding contradicted the belief at that time that parental traits were blended in the offspring.

Alleles Can Be Dominant or Recessive

Figure 7. The child in the photo expresses albinism, a recessive trait.

Mendel&rsquos law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively. The recessive allele will remain &ldquolatent&rdquo but will be transmitted to offspring by the same manner in which the dominant allele is transmitted. The recessive trait will only be expressed by offspring that have two copies of this allele (Figure 7), and these offspring will breed true when self-crossed.

Since Mendel&rsquos experiments with pea plants, other researchers have found that the law of dominance does not always hold true. Instead, several different patterns of inheritance have been found to exist, which we will explore this later in the module.

Equal Segregation of Alleles

Observing that true-breeding pea plants with contrasting traits gave rise to F1 generations that all expressed the dominant trait and F2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of segregation. This law states that paired unit factors (genes) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor. For the F2 generation of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous dominant, heterozygous, or homozygous recessive. Because heterozygotes could arise from two different pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes and homozygous dominant individuals are phenotypically identical, the law supports Mendel&rsquos observed 3:1 phenotypic ratio. The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes. The physical basis of Mendel&rsquos law of segregation is the first division of meiosis, in which the homologous chromosomes with their different versions of each gene are segregated into daughter nuclei. The role of the meiotic segregation of chromosomes in sexual reproduction was not understood by the scientific community during Mendel&rsquos lifetime.

Independent Assortment

Mendel&rsquos law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur. The independent assortment of genes can be illustrated by the dihybrid cross, a cross between two true-breeding parents that express different traits for two characteristics. Consider the characteristics of seed color and seed texture for two pea plants, one that has green, wrinkled seeds (yyrr) and another that has yellow, round seeds (YYRR). Because each parent is homozygous, the law of segregation indicates that the gametes for the green/wrinkled plant all are yr, and the gametes for the yellow/round plant are all YR. Therefore, the F1 generation of offspring all areYyRr (Figure 8).

Figure 8. This dihybrid cross of pea plants involves the genes for seed color and texture.

In pea plants, violet flowers (P) are dominant to white flowers (p) and yellow peas (Y) are dominant to green peas (y). What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? How many squares do you need to do a Punnett square analysis of this cross?

[practice-area rows=&rdquo2&Prime][/practice-area]
[reveal-answer q=&rdquo733053&Prime]Show Answer[/reveal-answer]
[hidden-answer a=&rdquo733053&Prime]The possible genotypes are PpYY, PpYy, ppYY, and ppYy. The former two genotypes would result in plants with violet flowers and yellow peas, while the latter two genotypes would result in plants with white flowers with yellow peas, for a 1:1 ratio of each phenotype. You only need a 2 × 2 Punnett square (four squares total) to do this analysis because two of the alleles are homozygous.

For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele. Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed, as follows: YR, Yr, yR, and yr. Arranging these gametes along the top and left of a 4 × 4 Punnett square (Figure 8) gives us 16 equally likely genotypic combinations. From these genotypes, we infer a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green (Figure 8). These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size.

The law of independent assortment also indicates that a cross between yellow, wrinkled (YYrr) and green, round (yyRR) parents would yield the same F1 and F2 offspring as in the YYRR x yyrr cross.

The physical basis for the law of independent assortment also lies in meiosis I, in which the different homologous pairs line up in random orientations. Each gamete can contain any combination of paternal and maternal chromosomes (and therefore the genes on them) because the orientation of tetrads on the metaphase plane is random.

In pea plants, violet flowers (P) are dominant to white flowers (p) and yellow peas (Y) are dominant to green peas (y). What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? How many squares do you need to do a Punnett square analysis of this cross? The possible genotypes are PpYY, PpYy, ppYY, and ppYy. The former two genotypes would result in plants with violet flowers and yellow peas, while the latter two genotypes would result in plants with white flowers with yellow peas, for a 1:1 ratio of each phenotype. You only need a 2 × 2 Punnett square (four squares total) to do this analysis because two of the alleles are homozygous.

Linked Genes Violate the Law of Independent Assortment

Although all of Mendel&rsquos pea characteristics behaved according to the law of independent assortment, we now know that some allele combinations are not inherited independently of each other. Genes that are located on separate non-homologous chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes, organized linearly on chromosomes like beads on a string. The segregation of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair. However, because of the process of recombination, or &ldquocrossover,&rdquo it is possible for two genes on the same chromosome to behave independently, or as if they are not linked. To understand this, let&rsquos consider the biological basis of gene linkage and recombination.

Figure 9. The process of crossover, or recombination, occurs when two homologous chromosomes align during meiosis and exchange a segment of genetic material. Here, the alleles for gene C were exchanged. The result is two recombinant and two non-recombinant chromosomes.

Homologous chromosomes possess the same genes in the same linear order. The alleles may differ on homologous chromosome pairs, but the genes to which they correspond do not. In preparation for the first division of meiosis, homologous chromosomes replicate and synapse. Like genes on the homologs align with each other. At this stage, segments of homologous chromosomes exchange linear segments of genetic material (Figure 9). This process is called recombination, or crossover, and it is a common genetic process. Because the genes are aligned during recombination, the gene order is not altered. Instead, the result of recombination is that maternal and paternal alleles are combined onto the same chromosome. Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles.

When two genes are located in close proximity on the same chromosome, they are considered linked, and their alleles tend to be transmitted through meiosis together. To exemplify this, imagine a dihybrid cross involving flower color and plant height in which the genes are next to each other on the chromosome. If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will go together into a gamete and the short and yellow alleles will go into other gametes. These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes. But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles. If you create the Punnett square with these gametes, you will see that the classical Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not apply.

As the distance between two genes increases, the probability of one or more crossovers between them increases, and the genes behave more like they are on separate chromosomes. Geneticists have used the proportion of recombinant gametes (the ones not like the parents) as a measure of how far apart genes are on a chromosome. Using this information, they have constructed elaborate maps of genes on chromosomes for well-studied organisms, including humans.

Testing the Hypothesis of Independent Assortment

To better appreciate the amount of labor and ingenuity that went into Mendel&rsquos experiments, proceed through one of Mendel&rsquos dihybrid crosses.

Question: What will be the offspring of a dihybrid cross?

Background: Consider that pea plants mature in one growing season, and you have access to a large garden in which you can cultivate thousands of pea plants. There are several true-breeding plants with the following pairs of traits: tall plants with inflated pods, and dwarf plants with constricted pods. Before the plants have matured, you remove the pollen-producing organs from the tall/inflated plants in your crosses to prevent self-fertilization. Upon plant maturation, the plants are manually crossed by transferring pollen from the dwarf/constricted plants to the stigmata of the tall/inflated plants.

Hypothesis: Both trait pairs will sort independently according to Mendelian laws. When the true-breeding parents are crossed, all of the F1 offspring are tall and have inflated pods, which indicates that the tall and inflated traits are dominant over the dwarf and constricted traits, respectively. A self-cross of the F1 heterozygotes results in 2,000 F2 progeny.

Test the hypothesis: Because each trait pair sorts independently, the ratios of tall:dwarf and inflated:constricted are each expected to be 3:1. The tall/dwarf trait pair is called T/t, and the inflated/constricted trait pair is designated I/i. Each member of the F1 generation therefore has a genotype of TtIi. Construct a grid analogous to Figure 10, in which you cross two TtIi individuals. Each individual can donate four combinations of two traits: TI, Ti, tI, or ti, meaning that there are 16 possibilities of offspring genotypes. Because the T and I alleles are dominant, any individual having one or two of those alleles will express the tall or inflated phenotypes, respectively, regardless if they also have a t or i allele. Only individuals that are tt or ii will express the dwarf and constricted alleles, respectively. As shown in Figure 10, you predict that you will observe the following offspring proportions: tall/inflated : tall/constricted : dwarf/inflated : dwarf/constricted in a 9:3:3:1 ratio. Notice from the grid that when considering the tall/dwarf and inflated/constricted trait pairs in isolation, they are each inherited in 3:1 ratios.

Table 3 shows all possible combinations of offspring resulting from a dihybrid cross of pea plants that are heterozygous for the tall/dwarf and inflated/constricted alleles.

Table 3. Dihybrid Cross
TtIi
TI Ti tI ti
TtIi TI TTII TTIi TtII TtIi
Ti TTIi TTii TtIi Ttii
tI TtII TtIi ttII ttIi
ti TtIi Ttii ttIi ttii

Test the hypothesis: You cross the dwarf and tall plants and then self-cross the offspring. For best results, this is repeated with hundreds or even thousands of pea plants. What special precautions should be taken in the crosses and in growing the plants?

Analyze your data: You observe the following plant phenotypes in the F2 generation: 2706 tall/inflated, 930 tall/constricted, 888 dwarf/inflated, and 300 dwarf/constricted. Reduce these findings to a ratio and determine if they are consistent with Mendelian laws.

Form a conclusion: Were the results close to the expected 9:3:3:1 phenotypic ratio? Do the results support the prediction? What might be observed if far fewer plants were used, given that alleles segregate randomly into gametes? Try to imagine growing that many pea plants, and consider the potential for experimental error. For instance, what would happen if it was extremely windy one day?

Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes independently of one another. That is, alleles are generally not more likely to segregate into a gamete with a particular allele of another gene. A dihybrid cross demonstrates independent assortment when the genes in question are on different chromosomes or distant from each other on the same chromosome. For crosses involving more than two genes, use the forked line or probability methods to predict offspring genotypes and phenotypes rather than a Punnett square.

Although chromosomes sort independently into gametes during meiosis, Mendel&rsquos law of independent assortment refers to genes, not chromosomes, and a single chromosome may carry more than 1,000 genes. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel&rsquos law of independent assortment. However, recombination serves to exchange genetic material on homologous chromosomes such that maternal and paternal alleles may be recombined on the same chromosome. This is why alleles on a given chromosome are not always inherited together. Recombination is a random event occurring anywhere on a chromosome. Therefore, genes that are far apart on the same chromosome are likely to still assort independently because of recombination events that occurred in the intervening chromosomal space.



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