Drosophila females homozygous for the third chromosomal genes pink and ebony (the same genes from Problem 16) were crossed with males homozygous for the second chromosomal gene dumpy. Because these genes are recessive, all offspring were wild type (normal). F₁ females were testcrossed to triply recessive males. If we assume that the two linked genes, pink and ebony, 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?

Verified step by step guidance

1. Identify the genotypes of the parental flies: The females are homozygous recessive for pink and ebony on chromosome 3 (pp ee) and wild type for dumpy on chromosome 2 (DD), while the males are homozygous recessive for dumpy (dd) and wild type for pink and ebony (P E).
2. Determine the F1 genotype: Since pink and ebony are recessive and on the same chromosome, and dumpy is on a different chromosome, the F1 females will be heterozygous for pink and ebony (Pp Ee) and heterozygous for dumpy (Dd). Because the parents are homozygous recessive for different genes, all F1 offspring will be phenotypically wild type.
3. Understand the testcross: The F1 females (Pp Ee Dd) are testcrossed to males homozygous recessive for all three genes (p e d). Since pink and ebony are linked with a recombination frequency of 20 mu (20%), calculate the expected frequency of parental and recombinant gametes from the F1 females.
4. Calculate gamete types from F1 females: For the linked pink and ebony genes, 80% of gametes will be parental types (P E and p e) and 20% recombinant types (P e and p E). For the unlinked dumpy gene, assume independent assortment, so half the gametes will carry D and half d.
5. Predict offspring phenotypes: Combine the gametes from F1 females with the recessive testcross male gametes (p e d) to determine the expected phenotypic ratios. For the reciprocal cross with F1 males (where no crossing over occurs), only parental types will be produced, so recombinant phenotypes will be absent.

Key Concepts

Genetic Linkage and Recombination Frequency

Genetic linkage occurs when two genes are located close together on the same chromosome and tend to be inherited together. The recombination frequency, measured in map units (mu), indicates the likelihood of crossing over between these genes; 20 mu means a 20% chance of recombination. This concept helps predict offspring genotypes based on parental gene arrangements.

Testcross and Its Purpose

A testcross involves crossing an individual with a dominant phenotype but unknown genotype to a homozygous recessive individual. This reveals the genotype of the tested individual by analyzing offspring phenotypes, especially useful for detecting recombination events between linked genes.

Sex Differences in Recombination in Drosophila

In Drosophila, crossing over occurs only in females, not in males. Therefore, recombination frequencies and offspring genotype ratios differ depending on whether the female or male parent carries the heterozygous chromosomes. This affects predictions of genetic outcomes in reciprocal crosses.