Question

Drosophila females homozygous for the third chromosomal genes pink eye (p) and ebony body (e) were crossed with males homozygous for the second chromosomal gene dumpy wings (dp). Because these genes are recessive, all offspring were wild type (normal). \(F_{1}\) females were testcrossed to triply recessive males. If we assume that the two linked genes ( \(p\) and \(e\) ) are 20 mu apart, predict the results of this cross. If the reciprocal cross were made (F1 males-where no crossing over occurs-with triply recessive females), how would the results vary, if at all?

Short answer

Answer: The results of testcrosses between F1 dihybrid females and triply recessive males differ from the reciprocal cross involving F1 males with triply recessive females because of the differences in recombination rates and the number of possible phenotypes. The F1 female testcross has 4 possible phenotypes and a 20% chance of recombination, while the reciprocal cross involving F1 males has only 2 possible phenotypes and no crossing over occurs.

Step-by-step solution

Analyze the P generation cross

We are given the following initial cross: - Female: pink eye (p), ebony body (e), homozygous for these genes - Male: dumpy wings (dp), homozygous for this gene Since pink eye, ebony body, and dumpy wings are all recessive, the offspring of this cross will be wild type with normal eyes, body color, and wings. These offspring make up the F1 generation.

Determine characteristics of the F1 females

The F1 females will be heterozygous for all three genes: - Pink eye: Pp - Ebony body: Ee - Dumpy wings: Dpdp (since the female will have chromosomes with only normal alleles for the dumpy wing gene) The genotypes of the F1 females will be Pp Ee Dpdp.

Perform the F1 female testcross with triply recessive males

Now, the F1 heterozygous females Pp Ee Dpdp are crossed with males that are triply recessive (pp ee dpdp). Due to the 20 mu distance between the linked genes p and e, there will be a 20% chance of recombination between them. Therefore, the expected phenotype percentages of the offspring are as follows: - 40% wild type (normal eye, body, and wings) - P_E_ Dpdp - 20% pink eye, ebony body, dumpy wings (pp ee dpdp) - 20% normal eye, normal body, dumpy wings (P_E_ dpdp) - 20% pink eye, ebony body, normal wings (pp ee Dpdp)

Perform the reciprocal cross and predict the results

In the reciprocal cross, F1 males (Pp Ee DpDp) are crossed with triply recessive females (pp ee dpdp). Since no crossing over occurs in male Drosophila, there will be no recombination for the p and e genes. Therefore, the expected phenotype percentages of the offspring are as follows: - 50% normal eye, normal body, dumpy wings (P_E_ dpdp) - 50% pink eye, ebony body, dumpy wings (pp ee dpdp)

Compare the results of the two crosses

Comparing the results of the F1 female testcross with the reciprocal cross, we can see that the outcomes do vary: - In the F1 female testcross, there are 4 possible phenotypes and a 20% chance of recombination due to crossing over. - In the reciprocal cross with F1 males, there are only 2 possible phenotypes and no crossing over occurs. Thus, the results do vary between the two crosses because of the differences in recombination rates and the number of possible phenotypes.