True-breeding Drosophila strains exist for the mutant phenotypes curly wings and

Question

True-breeding Drosophila strains exist for the mutant phenotypes curly wings and orange eyes. The curly wing trait is dominant to wildtype wings and the orange eye trait is recessive to wildtype eyes A dihybrid cross was setup for these two traits. In the P1 generation, a curly wing female was mated with an orange-eyed male. In the F1 generation all the flies had curly wings and wildtype eyes These F1 flies were interbred and the following F2 phenotypes were obtained. What are the genotypes of the male and the female F1 flies? 6. Phenotypes of F2 Progeny Phenotvpes Wildtype Curly Orange Orange Curly Male 63 190 65 Females 125 377 0 180

Explanation / Answer

Since the strains are true breeding, the parents with curly wings must have the phenotype CC (curly is dominant) and the ones with wild type eyes phenotype EE (wild type eye is dominant). Since we do not have the genotype for eye-colour of P1 female and wing of P1 male, let us denote them by Xe and Xc, respectively. The F1 cross will be as follows -

P1 -> Curley x orange => CCXeXe x XcXcee

Gametes -> CXe x  Xce

F1 -> CXcXee

Since all the F1 individuals have wild type eyes (dominant trait), the value of Xe in the F1 genotype is E. Thus, the genotype of the F1 individuals is CXcEe. There could be two values of Xc, C or c. To find this, let us go to the F2 results -

By adding the male and female individuals of each group, we have

Wildtype : 188

Curley: 567

Orange: 65

Curley: 180

This looks very similar to a di-hybrid cross. Dividing each value with the minimum value (=65) we get:

567/65 : 188/65 :  180/65 : 65/65 = 8.72 : 2.89 : 2.77 : 1, which is nearly equal to 9:3:3:1. The one with 'one' part in the ratio being the individuals with orange eye colour (and the recessive wild type wings). Thus, the genotype for this all-recessive individuals is ccee. If we look into the F1 gametes, there is only one possibility from which the 'c' allele could have come, which is the unknown Xc allele. Thus, the value of Xc is 'c'. putting this value in the F1 genotype (above in bold and underline), we get CcEe, which is the desired answer.

The solution is a little complicated. If anymore clarifications required, please comment.

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