A testcross is performed on an individual to examine three l

A testcross is performed on an individual to examine three linked genes. The most frequent phenotypes of the progeny were Abc and aBC, and the least frequent phenotypes were abc and ABC. What was the genotype of the heterozygous individual that is testcrossed with the correct order of the three genes?

A) Abc aBC

B) BAC/bac

C) bcA/BCa

D) aBc/AbC

E) bAc/BaC

**Please explain the answer so I understand how to solve these kind of problems. Thank you

Solution

A test cross is performed between the offsprings of F1 generation with the homozygous recessive parent in order to determine the genotype of the parents/offsprings. According to the information, in a test cross of three linked genes, the most frequent phenotypes were Abc and aBC. Also, the least frequent phenotypes were abc and ABC.

Since the genes are linked, it is suggested that none of the parents should have genotype ABC or abc. Thus, majority of the parents should have the genotype Abc and aBC i.e. heterozygous at atleast one position. This gives that the choice C is most correct answer since these genotypes match to those of the test-cross progeny.

It is important to note here that this assumption is based on the fact that the information in the question suggests that all the three genes are linked. However, Mendel\'s genetic ratios are based upon the assumption that there is no linkage between genes and they assort independently. This is why it is very likely that the offsprings and the parents would have a very similar genotypes because the genes are linked. Only a very few or none of the offspring will have a differential genotypes than the parents in this case.