Contribution of crossing over and random assortment to


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According to Darwin,Genetic diversity leads to evolution through natural selection. Meiosis contributes towards the genetic diversity through crossing over and random assortment. Random fusion of gametes also leads to genetic diversity.

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Contribution of crossing over and random assortment to

  1. 1. Contribution of Crossing Over and Random Assortment to Genetic Diversity
  2. 2. Genetic Diversity in Human Beings
  3. 3. Reasons behind Genetic Diversity <ul><li>Mutations </li></ul><ul><ul><li>Produces new alleles of genes to increase variation </li></ul></ul><ul><li>Meiosis </li></ul><ul><li>Crossing-over </li></ul><ul><li>Independent assortment </li></ul><ul><li>Random fertilization of the sperm and ovum </li></ul><ul><ul><li>Mixes up existing combinations of the alleles of all the genes to increase the range of genotypes to increase variation </li></ul></ul>
  4. 4. Importance of Genetic Diversity <ul><li>Darwin recognized the importance of genetic diversity in evolution via natural selection. </li></ul><ul><li>A population evolves through the differential reproductive success of its variant members. </li></ul><ul><li>This natural selection results in adaptation, the accumulation of favorable genetic variations. </li></ul>
  5. 5. An Overview of Meiosis
  6. 6. Concept of Random Assortment <ul><li>Let us start with a cell having haploid number 2. </li></ul><ul><li>The alleles of a gene on the homologous chromosomes are different. </li></ul><ul><li>Let us consider two genes A and B whose alleles on homologus chromosomes are Aa </li></ul><ul><li>and Bb. </li></ul>
  7. 7. Concept of Random Assortment <ul><li>Homologous pairs of chromosomes have dominant and recessive alleles of a particular gene. </li></ul><ul><li>During Prophase-I of meiosis-I the homologous pairs associate. </li></ul><ul><li>During metaphase-I these pairs are arranged randomly at the equatorial plate. </li></ul>
  8. 8. Concept of Random Assortment <ul><li>50% of the cells undergoing meiosis will adopt alignment 1 and the rest will take alignment 2. </li></ul><ul><li>The four genotypes AB, Ab, ab, aB are present with equal probability among the four haploid cells. </li></ul><ul><li>Thus alleles of the genes A and B assort independently during meiosis </li></ul>
  9. 9. Concept of Random Assortment <ul><li>The number of possible combinations of maternal and paternal homologues is 2n, where n = the haploid number of chromosomes. </li></ul><ul><li>When haploid number n=2, number of different combinations produced are (2 2 ) 4. </li></ul><ul><li>In human beings haploid number is 23 hence the number of possible combinations of chromosomes are 223 (8,388,608). </li></ul>
  10. 10. Concept of Crossing Over <ul><li>Crossing over consists of a mutual exchange of </li></ul><ul><li>equal quantity (segments) of chromosomal material </li></ul><ul><li>between two non-sister chromatids. </li></ul><ul><li>It involves the following events: </li></ul><ul><ul><li>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). </li></ul></ul><ul><ul><li>The broken segments are of equal lengths. </li></ul></ul><ul><ul><li>The broken segments may rejoin or there may be an exchange of the segments between the non-sister chromatids. </li></ul></ul>
  11. 11. Concept of Crossing Over <ul><li>An enzyme known as ligase helps in the fusion of the chromosomal segment of one with another. </li></ul><ul><li>In this process, the genes located on the segments are exchanged between the two chromatids. </li></ul><ul><li>A little amount of DNA synthesis (about 3% of the total genome) takes place during crossing over which repairs the broken chromosomes. </li></ul><ul><li>Thus, crossing over results in the recombination of genes (characters). </li></ul><ul><li>Crossing over does not take place between sister chromatids. </li></ul><ul><li>The point where the non sister chromatids exchange is known as chaisma. </li></ul>
  12. 12. Concept of Crossing Over <ul><li>Frequency of recombination is not the same for all gene combinations. </li></ul><ul><li>It depends on the genetic distance. </li></ul>Image reference -
  13. 13. Concept of Crossing Over Image reference - http://
  14. 14. Significance of Crossing Over <ul><li>The gametes produced through meiosis receive a new combination of characters (genes). </li></ul><ul><li>Therefore individuals with new combination of characters are produced in each generation. </li></ul><ul><li>This forms the genetic basis for variations and plays important role in evolution. </li></ul>
  15. 15. Random Fusion of Gametes During Fertilisation <ul><li>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. </li></ul><ul><li>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. </li></ul><ul><li>Each fertilisation event produces a unique genotype. </li></ul><ul><li>Random fertilisation further increases the variety of genotypes which will be present in the population </li></ul>
  16. 16. Summary of the Class
  17. 17. Thank you