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Contribution of Crossing Contribution of Crossing Over and Random Over and Random

Assortment to Genetic Assortment to Genetic DiversityDiversity

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Genetic Diversity in Human Genetic Diversity in Human BeingsBeings

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Reasons behind Genetic Reasons behind Genetic DiversityDiversity

Mutations– Produces new alleles of genes to increase

variationMeiosis• Crossing-over• Independent assortment• Random fertilization of the sperm and

ovum– Mixes up existing combinations of the alleles of

all the genes to increase the range of genotypes to increase variation

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Importance of Genetic Importance of Genetic DiversityDiversity

• Darwin recognized the importance of genetic diversity in evolution via natural selection.

• A population evolves through the differential reproductive success of its variant members.

• This natural selection results in adaptation, the accumulation of favorable genetic variations.

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An Overview of MeiosisAn Overview of Meiosis

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Concept of Random AssortmentConcept of Random Assortment

• Let us start with a cell having haploid number 2.

• The alleles of a gene on the homologous chromosomes are different.

• Let us consider two genes A and B whose alleles on homologus chromosomes are Aa

and Bb.

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Concept of Random AssortmentConcept of Random Assortment• Homologous pairs of

chromosomes have dominant and recessive alleles of a particular gene.

• During Prophase-I of meiosis-I the homologous pairs associate.

• During metaphase-I these pairs are arranged randomly at the equatorial plate.

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Concept of Random AssortmentConcept of Random Assortment• 50% of the cells

undergoing meiosis will adopt alignment 1 and the rest will take alignment 2.

• The four genotypes AB, Ab, ab, aB are present with equal probability among the four haploid cells.

• Thus alleles of the genes A and B assort independently during meiosis

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Concept of Random AssortmentConcept of Random Assortment

• The number of possible combinations of maternal and paternal homologues is 2n, where n = the haploid number of chromosomes.

• When haploid number n=2, number of different combinations produced are (22) 4.

• In human beings haploid number is 23 hence the number of possible combinations of chromosomes are 223 (8,388,608).

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Concept of Crossing OverConcept of Crossing Over

Crossing over consists of a mutual exchange of equal quantity (segments) of chromosomal materialbetween two non-sister chromatids. It involves the following events:

The non-sister chromatids, taking part in crossing over, first break at the corresponding points due to the activity of a nuclear enzyme called endonuclease (Stern and Hotta, 1969).

The broken segments are of equal lengths. The broken segments may rejoin or there may be an

exchange of the segments between the non-sister chromatids.

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Concept of Crossing OverConcept of Crossing Over An enzyme known as ligase helps in the fusion of the

chromosomal segment of one with another. In this process, the genes located on the segments are

exchanged between the two chromatids. A little amount of DNA synthesis (about 3% of the total

genome) takes place during crossing over which repairs the broken chromosomes.

Thus, crossing over results in the recombination of genes (characters).

Crossing over does not take place between sister chromatids.

The point where the non sister chromatids exchange is known as chaisma.

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Concept of Crossing OverConcept of Crossing Over Frequency of recombination is not the same for

all gene combinations. It depends on the genetic distance.

Image reference - http://www.microbiologyprocedure.com/genetics/crossing-over/mechanism-of-meiotic-crossing-over.htm

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Concept of Crossing OverConcept of Crossing Over

Image reference - http://www.accessexcellence.org/RC/VL/GG/comeiosis.php

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Significance of Crossing Significance of Crossing OverOver

The gametes produced through meiosis receive a new combination of characters (genes).

Therefore individuals with new combination of characters are produced in each generation.

This forms the genetic basis for variations and plays important role in evolution.

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Random Fusion of Gametes During Random Fusion of Gametes During FertilisationFertilisation

• Each parent is genetically different, and will produce huge numbers of gametes, which themselves are genetically different in terms of the combinations of alleles they carry.

• Which gamete fuses with which is totally random thus generating a huge number of possible combinations of the alleles of the male and female in the resultant offspring.

• Each fertilisation event produces a unique genotype.

• Random fertilisation further increases the variety of genotypes which will be present in the population

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Summary of the ClassSummary of the Class

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