Question

Explain how crossing over and independent assortment contributed to genetic variation. Your explanation should be based on ONE (1) living organism (you may choose any species that undergoes sexual reproduction as an example, except human).

                                                                                                                       

The details of explanation should be concise and informative. The length of the explanation should be less than 500 words.                                                                                                

Answer 1

Answer

Genetic variation is the root of any evolutionary process. New characters are developed in an offspring of a sexually reproducing organism by genetic variation. Genetic variation is mainly produced during meiosis by two processes namely Crossing over and Independent assortment. For explaining how these processes contribute to genetic variation, fruit fly (Drossophila ) having 2n = 8 is taken as an example.

Meiosis produces four daughter cells from the parent cell and each daughter cell have half the number of chromosomes. These daughter cells are different from one another and also differ from their parent cell. These genetic variation is produced by Crossing over and independent assortment.

During meiosis, homologous chromosomes pair each other by synapsis and each pair is called bivalent. In fruit fly, four homologous pairs are formed. Then crossing over takes place in the pachytene stage of prophase I. Crossing over is the exchange of chromosomal segments / genes between the non - sister chromatids of homologous chromosomes. After the crossing over, recombination of the exchanged parts takes place and so the sister chromatids become differ from one another. Thus new combination of alleles are produced in each chromatids. During anaphase II, the two chromatids of each chromosome separate and four daughter cells have different combination of alleles.

Second process contribute to genetic variation is independent assortment. It means alleles for one trait segregate independently from alleles of another trait in metaphase I. During metaphase I, chromosome pairs are randomly aligned at the equator of the cell. In the anaphase I, these chromosomes of the homologous pairs separate independently of the other.

In fruit fly, there are 8 chromosomes and so forms four homologous pairs during meiosis. The independent alignment and separation of these chromosomes during metaphase I and anaphase I produces different combinations of chromosomes. The number of combinations produced by the random distribution is 2n , where 'n' is the number of homologous pairs. In fruit fly there is 4 homologous pairs and possible combinations are 24 = 16. The number of combinations increases as the number of homologous pairs increases. By the random distribution of chromosomes at anaphase I, alleles on different chromosomes sorted into different combinations.

The crossing over makes new combinations of alleles in the same homologous pair and independent assortment makes new combinations of alleles on different chromosomes. Both these together produces genetic variation.