Question

 1. Identify four sources of genetic variation found in sexually reproducing organisms. Select one of these sources and (i) describe what it is and (ii) how it generates genetic variation. 

 2. In a series of mapping experiments, the recombination frequencies for five different linked genes of Bison bison were determined. Y-W had a recombination frequency of 4%, W-M of 47% and M-Y of 51%. The recombination frequencies (in percents) are shown below for Y, W, M, V and R. Given all the information below, where do the V and R genes fit on this chromosome map? (6 points)

 Y-V: 44

 Y-R: 70

 W-V: 39

 W-R: 66

 V-M: 8

 V-R: 27

 M-R: 19

Answer 1

Ans 1:

Four sources of genetic variability in a sexually reproducing organism are:

1. During first meiosis the independent assortment of homologous chromosomes.
2. During second meiosis the independent assortment of nonidentical sister chromatids.
3. During first meiosis prophase I the Crossing over incident between homologous chromosomes.
4. A random fertilization process of an ovum by a sperm.

Description of one source:

The exchange of genetic materials between the homologous chromosome which results in the recombinant chromosome during meiosis cell division in germ cells of sexually reproducing organism is termed as crossing over or crossover event.

It occurs in the pachytene stage of of Prophase-1 of meiosis by pairing of two homologous chromosomes a process called synapsis. Synapsis helps to matching up of homologous pairs before their segregation and crossing over. Synapsis starts before formation of synaptonemal complex and continues up to end of prophase 1.During crossing over the matching region of of the homologous chromosome are inter change by chromosome break and rejoining method.

Reason for generating genetic variation:

During crossing over process the chromosomal segments are exchanged which results in change in genetic material between the chromosomes as the chromatids joined by the centromere are no more identical to each other. Thus during the independent assortment during the second meiotic phase daughter cells receive daughter chromosomes with recombined alleles. These daughter chromosomes are not exactly identical to the mother counterpart. As a result of this genetic recombination the offspring receive a different subset of gene than their parents.

Ans 2:

Total allele no 5; Y,W,V,M,R

Y-W =4

Y-M=51

Y-V=44

Y-R=70

W-M=47

W-V=39

W-R=66

V-M=8

V-R=27

M-R=19

Then, According to the rule less recombination frequency closely located gene pair and high recombination frequency rarely located gene pair, the longest distance between all of them is Y-R =70 , they located at two ends of these series. Based on the data other can be arranges in the following sequence

Y---⁴----W-----------------------³⁹----------------------V--------⁸------M----------------¹⁹-------------R