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What is the relationship between independent assortment and cross over?


During meiosis, the independent assortment will be made first and then cross over will be made. I am so confused, what is the difference between this two process? I looked at the diagram on internet.It is said that law of independent assortment is alleles of genes on non homologous chromosomes assort independently during gametes formation, but I don't quite understand that. So, another way for understanding this topic, independent assortment is tetrads can line up four different ways between homologous separate. Is that right. Appreciate your help.


During meiosis, the independent assortment will be made first and then cross over will be made.

No, independent assortment occurs after crossing over. Crossing over occurs in prophase I while independent assortment occurs in metaphase I and anaphase I.

I am so confused, what is the difference between this two process?

During prophase I, a process called synapsis occurs. That is, homologous chromosomes (one from your mother and one from your father) come together and bind together, forming a tetrad. This binding is due to crossing over, which is when the genetic material on the maternal chromosome and the paternal chromosome exchange places. The maternal chromosome gives some of its genes to the paternal chromosome, and the paternal chromosome gives some of its genes to the maternal chromosome. Crossing over increases genetic variety by creating a new chromosome with a new combination of genes.

During metaphase I, the tetrads line up at the equator of the cell. During anaphase one, the tetrads are broken apart and each homologous chromosome that made each tetrad up is pulled toward one end of the cell. Look at the image below, where the blue represents paternal chromosomes and the red represents maternal chromosomes.

Independent assortment describes the phenomenon where the paternal and maternal chromosomes can be lined up randomly at the equator. There are many combinations. For example,

  • Both red chromosomes could face the top pole and both blue chromosomes could face the bottom pole.
  • Both blue chromosomes could face the top pole and both red could face the bottom.
  • One red on the left and one blue on the right could face the top, as shown in the picture.
  • One blue on the left and one red on the right could face the top.

This random alignment at the equator makes it so that during anaphase, random proportions of maternal and paternal chromosomes get assorted into each of the resulting daughter cells. Some cells could get more maternal chromosomes and the other would get more paternal chromosomes, and vice versa. On the other hand, they could get similar amounts. Independent assortment increases genetic variation by allowing daughter cells to each randomly receive a different proportion of paternal and maternal chromosomes.

In conclusion, crossing over and independent assortment (sometimes called random assortment) are different independent processes that both lead to an increase in genetic diversity.


References

  • Reece, Jane A., Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Robert B. Jackson. “Meiosis and Sexual Life Cycles." Campbell Biology. 10th ed. Glenview: Pearson, 2014. 252-266. Print.

  • https://www.quia.com/files/quia/users/dschnepp/Meiosis/Metaphase-I


Linkage vs Crossing Over (Difference between Linkage and Crossing Over)

Linkage and crossing over are two different but inter-related genetic events in Eukaryotic organisms.

Genetic Linkage: The tendency of genes (DNA sequences) to stay together in a chromosome is called genetic linkage. The genes linked together in a chromosome are called the Linkage Group. The linkage group is equivalent to a chromosome. Thus, due to the linkage, the genes present in a particular chromosome will inherit together when the gametes are formed. The genetic linkage is compromised by the Crossing Over.

Crossing Over: The exchange of genetic material between the non-sister chromatids of a homologous chromosome is called crossing over. Crossing over is the tendency of genes to stay apart and inherit separately when the cell produces gametes. Crossing over is a natural genetic recombination process occurs during the pachytene stage of the prophase I of Meiosis.

The process of genetic linkage and crossing over are related to each other. When the linkage between two genes increases, the chance of crossing over between these two are reduced and vice versa. Linkage and crossing over are considered as one of the most important exceptions in Mendel’s low of Independent Assortment. The present post describes the Difference between Genetic Linkage and Crossing over with Comparison Table.


Principles of Inheritance and Variation Class 12 MCQs Questions with Answers

Solving the Principles of Inheritance and Variation Multiple Choice Questions of Class 12 Biology Chapter 5 MCQ can be of extreme help as you will be aware of all the concepts. These MCQ Questions on Principles of Inheritance and Variation Class 12 with answers pave for a quick revision of the Chapter thereby helping you to enhance subject knowledge. Have a glance at the MCQ of Chapter 5 Biology Class 12 and cross-check your answers during preparation.

Select the correct answer

Question 1.
All genes located on the same chromosome:
(a) form different groups depending upon their relative distance
(b) form one linkage group
(c) will not form any linkage groups
(d) form interactive groups that affect the phenotype

Answer: (b) form one linkage group

Question 2.
Conditions of a karyotype 2n + 1 and 2n ± 2 are called:
(a) Aneuploidy
(b) Polyploidy
(c) Allopolyploidy
(d) Monosomy.

Question 3.
Distance between the genes and advantage of recombination shows:
(a) a direct relationship
(b) an inverse relationship
(c) a parallel relationship
(d) no relationship.

Answer: (a) a direct relationship

Question 4.
If a genetic disease is transferred from a phenotypically normal but carrier female to only some of the male progeny, the disease is:
(a) Autosomal dominant
(b) Autosomal recessive
(c) Sex-linked dominant
(d) Sex-linked recessive.

Answer: (d) Sex-linked recessive.

Question 5.
In sickle cell anaemia glutamic acid is replaced by valine. Which one of the following triplets codes for valine?
(a) GGG
(b) AAG
(c) GAA
(d) GUG.

Question 6.
Person having genotype l A l B would show the blood group as AB. This is because of:
(a) Pleiotropy
(b) Co-dominance
(c) Segregation
(d) Incomplete dominance.

Question 7.
ZZ / ZW type of sex determination is seen in :
(a) Platypus
(b) Snails
(c) Cockroach
(d) Peacock.

Question 8.
A cross between two tall plants resulted in offspring having a few dwarf plants. What would be the genotypes of both the parents?
(a) TT and Tt
(b) Tt and Tt
(c) TT and TT
(d) Tt and tt.

Question 9.
In a dihybrid cross, if you get 9 : 3 : 3 : 1 ratio it denotes that:
(a) The alleles of two genes are interacting with each other
(b) It is a multigenic inheritance
(c) It is a case of multiple alleles m
(d) The alleles of two genes are segregating independently.

Answer: (d) The alleles of two genes are segregating independently.

Question 10.
Which of the following will not result in variations among siblings?
(a) Independent assortment of genes
(b) Crossing over
(c) Linkage
(d) Mutation.

Question 11.
Mendel’s law of independent assortment holds good for genes situated on the :
(a) non-homologous chromosomes
(b) homologous chromosomes
(c) extra nuclear genetic element
(d) same chromosome.

Answer: (a) non-homologous chromosomes

Question 12.
Occasionally, a single gene may express more than one effect. The phenomenon is called :
(a) multiple allelism
(b) mosaicism
(c) pleiotropy
(d) polygeny.

Question 13.
In a certain taxon of insects some have 17 chromosomes and the others have 18 chromosomes. The 17 and 18 chromosome-bearing organisms are:
(a) males and females, respectively
(b) females and males, respectively
(c) all males
(d) all females.

Answer: (a) males and females, respectively

Question 14.
The inheritance pattern of a gene over generations among humans is studied by the pedigree analysis. Character studied in the pedigree analysis is equivalent to :
(a) quantitative trait
(b) Mendelian trait
(c) polygenic trait
(d) maternal trait.

Question 15.
It is said that Mendel proposed that the factor controlling any character is discrete and independent. This proposition was based on the :
(a) results of F3 generation of a cross.
(b) observations that the offspring of a cross made between the plants having two contrasting characters shows only one character without any blending.
(c) self-pollination of F1 offsprings
(d) cross-pollination of parental generations.

Answer: (b) observations that the offspring of a cross made between the plants having two contrasting characters shows only one character without any blending.

Question 16.
Two genes ‘A’ and ‘B’ are linked. In a dihybrid cross involving these two genes, the F1 heterozygote is crossed with homozygous recessive parental type (aa bb). What would be the ratio of offspring in the next generation ?
(a) 1 : 1 : 1 : 1
(b) 9 : 3 : 3 : 1
(c) 3 : 1
(d) 1 : 1.

Question 17.
In the F2 generation of a Mendelian dihybrid cross the number of phenotypes and genotypes are :
(a) Phenotypes -4 genotypes -16
(b) Phenotypes -9 genotypes -4
(c) Phenotypes -4 genotypes -8
(d) Phenotypes -4 genotypes -9.

Answer: (d) Phenotypes -4 genotypes -9.

Question 18.
Mother and father of a person with ‘O’ blood group have ‘A’ and ‘B’ blood groups, respectively. What would be the genotype of both mother and father ?
(a) Mother is homozygous for ‘A’ blood group and father is heterozygous for ‘ B’.
(b) Mother is heterozygous for ‘A’ blood group and father is homozygous for ‘B’.
(c) Both mother and father are heterozygous for ‘A’ and ‘B’ blood group, respectively.
(d) Both mother and father are homozygous for ‘A’ and ‘B’ blood group, respectively.

Answer: (c) Both mother and father are heterozygous for ‘A’ and ‘B’ blood group, respectively.

Assertion and Reason Type Questions

These questions consist of two statements each, printed as Assertion and Reason.
While answering these questions you are required to choose any one of the following four responses.
(a) If both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are true but Reason is not a correct explanation of the Assertion.
(c) If Assertion is true but the Reason is false.
(d) If both Assertion and Reason are false.
Question 19.
Assertion: In humans the gamete contributed by male determines whether the child produced will be male or female.
Reason: Sex in humans is polygenic trait depending upon a cumulative effect of some genes on X-chromosome and some on Y-chromosome.

Answer: (c) If Assertion is true but the Reason is false.

Question 20.
Assertion: Progeny of pure line is heterozygous.
Reason: Pure lines are not the progeny of homozygous organisms.

Answer: (d) If both Assertion and Reason are false.

Question 21.
Assertion: Extranuclear chromosomes are present in mitochondria and plastids.
Reason: They are prochromosomes or organelle chromosomes.

Answer: (a) If both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Question 22.
Assertion: Mendel formulated the laws of heredity.
Reason: Mendel did not perform the study of one character at a time.

Answer: (c) If Assertion is true but the Reason is false.

Question 23.
Assertion: Over-exposure to sun tans the skin in human beings.
Reason: Melanin appears in the epidermal cells which saves the skin from UV rays of sun.

Answer: (a) If both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Question 24.
Assertion: Y chromosomes are called androsome.
Reason: X and Y chromosomes differ in form and size.

Answer: (b) If both Assertion and Reason are true but Reason is not a correct explanation of the Assertion.

Question 25.
Assertion: DNA is a long, double stranded and linear in eukaryotic chromosome.
Reason: The eukaryotic chromosomes are composed of DNA, proteins, RNA, metal ions and enzymes.

Answer: (b) If both Assertion and Reason are true but Reason is not a correct explanation of the Assertion.

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What prevents independent assortment of genes?

Homologous pairs of chromosomes line up at the metaphase plate during metaphase I of meiosis. When genes are close together on a chromosome, the alleles on the same chromosome tend to be inherited as a unit more frequently than not. Such genes do not display independent assortment and are said to be linked.

Secondly, why do genes not assort independently? Genes on separate chromosomes assort independently because of the random orientation of homologous chromosome pairs during meiosis. Homologous chromosomes are paired chromosomes that carry the same genes, but may have different alleles of those genes.

Additionally, what is independent assortment of genes?

The Principle of Independent Assortment describes how different genes independently separate from one another when reproductive cells develop. During meiosis, the pairs of homologous chromosome are divided in half to form haploid cells, and this separation, or assortment, of homologous chromosomes is random.

Does independent assortment occur in meiosis 1 or 2?

During meiosis, 1 diploid cell undergoes 2 cycles of cell division but only 1 round of DNA replication. The result is 4 haploid daughter cells known as gametes. Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis.


Rules for Multihybrid Fertilization

Predicting the genotypes and phenotypes of offspring from given crosses is the best way to test your knowledge of Mendelian genetics. Given a multihybrid cross that obeys independent assortment and follows a dominant and recessive pattern, several generalized rules exist you can use these rules to check your results as you work through genetics calculations (Table). To apply these rules, first you must determine n, the number of heterozygous gene pairs (the number of genes segregating two alleles each). For example, a cross between AaBb and AaBb heterozygotes has an n of 2. In contrast, a cross between AABb and AABb has an n of 1 because A is not heterozygous.

General Rules for Multihybrid Crosses
General Rule Number of Heterozygous Gene Pairs
Number of different F1 gametes 2 n
Number of different F2 genotypes 3 n
Given dominant and recessive inheritance, the number of different F2 phenotypes 2 n


The Single Trait Cross (Monohybrid Cross)

Monohybrid cross (one trait cross) observing the pod shape of peas.

Monohybrid cross (on trait cross) observing the pod color of peas.


Scientific Method Connection

Testing the Hypothesis of Independent AssortmentTo better appreciate the amount of labor and ingenuity that went into Mendel’s experiments, proceed through one of Mendel’s 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, 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, 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.

This figure 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.

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?


How would you explain how independent assortment, crossing over, and random fertilization contribute to genetic variation?

Each factor contributes to a different combination of alleles in a haploid gamete.

Explanation:

During Meiosis I, there are two ways each homologous pairs of chromosomes can line up ( I I' ) & ( I' I ) - we can calculate the possible number of random combinations of chromosomes in each gamete (ie. sperm/egg) using the equation:

number of possible combinations = #2^n#
where n is the number of chromosomes in the system.
Humans have 23 chromosomes so this gives rise to 8,388,608 genetically unique gametes through independent assortment alone

Random fertilization refers to the fact that if two individuals mate, and each is capable of producing over 8million potential gametes, the random chance of any one sperm and egg coming together is a product of these two probabilities - some 70 trillion different combinations of chromosomes in a potential offspring.
think about that for a second

Crossing over occurs during tetrad formation of Metaphase I of meiosis when portions of the homologous pairs get exchanged, it results in the sister chromatids involved being recombined and genetically distinct from the other sister chromatid - this can happen more than once and can happen at random loci (locations) on the chromosome.

Because crossing over can give rise to an additional unique combination of alleles, each occurrence would effectively double the number of genetically unique gametes - and because crossovers occur randomly this means that number earlier (70 trillion) doesn't even begin to describe the potential variation in offspring that can exist given the three concepts listed.


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What is the relationship between independent assortment and cross over? - Biology

Mendelian Genetics:
Laws of Dominance, Segregation & Independent Assortment

Phenotype ( external appearance) is influenced by genotype (hereditary makeup)
or, individual characters are influenced by particular genes
or, individual genes are expressed in such a way as to influence characters (traits)
IMPORTANT: A gene "for" a phenotypic trait is almost always an oversimplification
BTW: Genes are made of DNA located in chromosomes ,
at a particular physical location (a locus : plural, loci)
Genes are often [but not always] expressed as proteins
Molecular phenotype: a gene "for" an enzyme

Alternative forms of genes are called alleles
Most genes exist in multiple allelic variants
Any diploid individual possesses two alleles for each gene .
An individual with two identical alleles is a homozygote and is described as homozygous
an individual with two dissimilar alleles is a heterozygote and is described as heterozygous .

Ex.: S ome people can taste the chemical phenylthiocarbimide ( PTC )
Suppose character "PTC sensitivity" influenced by a gene with two alleles,
one for " taster" and one for " non-taster"

Ex. : Pea seeds have alternative phenotypes green / yellow , or round / wrinkled

Mendel's Law of Dominance

Some alleles *mask" the phenotypic expression of other alleles in in heterozygous combination
Call the former dominant , the latter recessive (IG1 Research Briefing 15.1, pp. 292-293)
That is, heterozygo te phenotype is identical to that of one of the homozygotes
Call allele in that homozygote " dominant ", call other " recessive "

Dominant alleles symbolized with capital letters ( A )
Recessive alleles with lower-case letters ( a )
Genotype described by giving both alleles: AA or Aa or aa
Phenotype can be described by the letter of the expressed allele: "A" or "a"

Ex. : the "taster" allele (T) is dominant to the "non-taster" allele (t) :
Individuals h omozygous TT or heterozygous Tt express the "T" phenotype ("taster"):
only the homozygous tt individual express the "t" phenotype (" non-taster ")
Or, TT homozygotes and Tt heterozygotes show the tasterphenotype, tt homozygotes are non-tasters

Ex .: the " yellow " allele ( Y ) masks the " green " allele ( y )
the " round " allele ( R ) masks the " wrinkled " allele ( r )
Yy and Rr peas are yellow and round , respectively
yy and rr peas are green and wrinkled , respectively
[Alternatively, yellow peas are GG or Gg, round peas are WW or Ww
and green & wrinkled peas are gg ww ]

Do not confuse inheritance of a genotype and expression of a phenotype
Dominance is a relationship between alleles, not between phenotypes
Yellow does not dominate Green

Mendel's Law of Segregation

Mendel showed experimentally:
Alleles separate (segregate) during the formation of gametes (eggs & sperm) in meiosis
half carry one allele , half carry the other
[Mendel did not know about chromosomes , meiosis / mitosis , or DNA ]

R andom union of gametes produces zygotes that develop into new individuals.
Zygotic genotypes occur in characteristic ratios , according to parental genotypes
Ex .: a monohybrid cross between two heterozygotes ( Aa x Aa )
produces expected genotypic ratio of 1 : 2 : 1 among AA, Aa, & aa genotypes.

The genotypic ratios produce characteristic phenotypic ratios ,
according to dominance relationship of alleles involved.
Ex .: if A dominant to a, cross between heterozygotes produces
expected phenotypic ratio of 3 : 1 between " A" and "a" phenotypes.

Mendel's Law of Independent Assortment
Alleles at separate loci inherited independently
This produces characteristic genotypic and phenotypic ratios.
Ex .: a dihybrid cross between two "double heterozygotes" ( AaBb x AaBb ) produces
genotypic ratios of 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
for genotypes AABB AABb AAbb AaBB AaBb Aabb aaBB aaBb aabb
and therefore phenotypic ratios of 9 " AB " : 3 " Ab " : 3 " aB " : 1 " ab "

Homework : What genotypic & phenotypic ratios result for a cross AAbb x aaBB ? AABB x aabb ?
C alculate the genotypic & phenotypic ratios for a trihybrid cross ( AaBbDd x AaBbDd )

This law may not hold if loci are physically adjacent (" linked ") on same chromosome
Linkage alters characteristic ratios: Mendel did not observe linkage
Practice problems in Mendelian Genetics


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