Question

Recombination via crossing over is being studied in a diploid species for four genes known to be on the same chromosome. Prior study demonstrates that the genes are arranged as follows with only the two chromosomal genotypes shown being initially present in the population.

The two chromosomes genotypes are     A-------B--------c---------D

                       

                                                            a--------b-------C---------D

Which of the following is the best answer to the question: Which chromosomal genotypes could be observed in the progeny of matings in a population following reciprocal crossing over:

A) These 6 chromosomal genotypes could be observed:

            ABCD, abcD, AbCD, aBcD, ABcD, abCD

B) These 8 chromosomal genotypes could be observed:

            ABCD, abcD, AbCD, aBcD, ABcD, abCD, ABcD and abCD

C) Only these four chromosomal genotypes could be observed: (ABcD and abCD) and (AbCD and aBcD)

D) Only two chromosomal genotypes could be observed and these are: ABcD and abCD because all reciprocal crossover products die.

E) These two chromosomal genotypes could be observed following reciprocal crossover: AABcD, abbcDD

F) None of the above

G) All of the above

Answer 1

Reciprocal crossing over includes exchange of parts between the chromosome which results in variatuion. The given two chromosomal genotypes are ABcD and abCD. When matings in individuals of population is done the following observation can be deduced:

(i) When cross over takes place between locus A and B, then the chromosomal genotypes (AbCD and aBcD) is observed. The cross over is given below:

(ii) When the cross over takes place between B and c locus, then the chromosomal genotypes formed are (ABCD and abcD). The cross cover is given below:

So the reciprocal crossing over will give six chromosomal genotypes which are ABCD, abcD, AbCD, aBcD, ABcD,abCD.