Two flies were crossed one wildtype another double mutant sepia(se) and vestigial(vg) yielding 40% wild type 40% se vg, 10% se and 10% vg. The genes show-

a.Complete linkage b.Incomplete linkage c.Independent assortment d. epistasis

My attempt at a solution

Since no. of combination of phenotypes appeared =/= 2 ( there are non-parental traits so) it is not complete linkage.

It is neither epistasis because two genes are controlling two diffrenet phenotypes, not one.

It can be independent assortment or incomplete linkage.

Since the question says double mutant sepia(se) and vestigial(vg) (I am getting the idea that) it is a female and the wildtype individual (with no genotype concerned hint) is a male. The genes are likely on X chromosome.

So..... it is incomplete linkage.

But I'm not sure why not independent assortment.

  • $\begingroup$ The word independent in "independent assortment" means that the assortment of the alleles of one gene doesn't depend on the assortment of the alleles of the other gene, which in that case would give you equal percentages for the offspring phenotypes (25% each). Besides that, your assumption regarding the sexes and the chromosome where the genes are found is not correct. $\endgroup$ – user24284 Apr 28 '17 at 3:18
  • $\begingroup$ @GerardoFurtado Could you elaborate why it is not correct? $\endgroup$ – Tyto alba Apr 28 '17 at 6:48
  • $\begingroup$ Why would it be? $\endgroup$ – user24284 Apr 28 '17 at 6:58
  • $\begingroup$ @GerardoFurtado Nothing much about the wild type flies has been mentioned to determine it's genotype so considering it male would be right- was the thought behind my assumption. $\endgroup$ – Tyto alba Apr 28 '17 at 7:13

It doesn't really matter on which chromosomes the where concerned genes are located, Now coming to your question. A> Complete linkage > Complete linkage says that the concerned genes are right next to each other and no crossingover is possible so the expected genotype of the offspring would be 100% parental type. 50% Wild (se+ vg+) and 50% Mutated (se vg)

B> Incomplete linkage > This is the correct answer. According to incomplete linkage, the genes are close but not right next to each other so there is space for crossingover to take place. However, they are still close, close enough to be considered as linked and thus chance of obtaining recombinant offspring would always be less than that of the offspring with parental genotype, parental type always remains over 50% and here the parental type offspring is 80% (40% wild type parental genotype + 40% mutated parental genotype) and 20% recombinant genotype [(10% se & wild type vg+) + (10%vg & wild type se+)]

C> Independent assortment says that there is no linkage and thus no crossingover, this is possible only when the concerned genes are located on different chromosomes but here in the obtained data of offspring it is clear that crossingover did take place and 20% of the offspring is result of the crossingover so this option is not correct

D> Epistasis : In your attempt you mentioned that it is not epistasis because two genes are controlling separate phenotype thus it is not epistasis. your explanation is wrong. Don't get me wrong, the answer is not epistatsis, I am just saying your explanation is wrong, epistasis is the phenomenon where one gene interferes with the expression of another gene, look at the term carefully. I am calling them as genes not alleles. Alleles are different version of the same gene that control the same phenotypic character. In epistasis two different gene that control completely different phenotypic expression are interacting. One gene if expressed prevents the other gene from getting expressed, this not about the dominant alleles where if the dominant allele expresses recessive allele wont express. For example, assume gene A codes for white flower colour and gene B codes for sweet smell. If B gene expresses then A gene wont express. This is epistasis


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