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In: Biology

You are a researcher who is interested in three genes in drosophila: The a gene, the...

You are a researcher who is interested in three genes in drosophila: The a gene, the b gene and the c gene. You think the genes may be linked, so you decide to do a cross using a true-breeding a+/b+/c+ fly and an a/b/c fly. The F1 generation is all a+/b+/c+. However, when the F1 generation is bred to a true-breeding a/b/c fly, it yields the following offspring phenotypes: 596 a+/b+/c+ ; 589 a/b/c ; 183 a+/b/c ; 176 a/b+/c+ ; 0 a/b+/c ; 0 a+/b/c+ ; 0 a+/b+/c ; 0 a/b/c+. Which What is a possible interpretation of your results?

Group of answer choices

The b and c genes demonstrate 100% (complete) linkage.

The b and c genes are recessive lethal alleles.

The a gene is a dominant lethal allele.

The a and b genes demonstrate 100% (complete) linkage.

Solutions

Expert Solution

The correct answer is option a.

Test cross is the cross made with homozygous recessive parent and is a common method to test the linkage between genes.

Cross was made between true-breeding a+/b+/c+ fly and an a/b/c fly. The F1 generation is all a+/b+/c+

The test cross was then performed between F1 and homozygous recessive parent:

a+/b+/c+ x   a/b/c

F2:

a+/b+/c+ = 596 (Parent type)

a/b/c = 589 (Parent type)

a+/b/c = 183 (recombinant type)

a/b+/c+ = 176 (recombinant type)

a/b+/c = 0

a+/b/c+ = 0

a+/b+/c = 0

a/b/c+. = 0

When the genes are linked, probability of crossing over is drastically reduced. and if genes are completely linked, no crossing over takes place and only parent type offsprings are obtained.

In the present condition, Parent type genetype is contributing major share (i.e., a+/b+/c+ = 596 and a/b/c = 589). However recombinant types are also present but in low frequency (i.e., a+/b/c = 183 and a/b+/c+ = 176).

There are three genes a, b and c and hence the possibility of crossing over is between gene a and b and gene b and c.

Image below show the recombinant types after crossing over.

This image shows that the recombinant types obtained in the present test cross is due to crossing over betweeb gene a and gene b. Recombinants of crossing over between gene b and gene c was not obtained in the test cross progenies hence, gene b and c demonstrate 100 % (complete linkage).

Option b is incorrect. Genes are showing linkage between b and c and are not lethal.

Option C is incorrect. if a will be dominant lethal, all the progenies with a will not survive and this is not the case here. hence this option is incorrect.

Option d is incorrect. If a and b genes are completely linked then the recombinants obtained was not possible as they are product of crossing over between gene a and b.

gladiator answered 1 year ago
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