In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.


A wild-type female fruit fly with the genotype y⁺w⁺f/ywf⁺ is crossed to a male fruit fly that has yellow body, white eye, and forked bristles. Predict the frequency of each progeny phenotype class produced by this mating.

Verified step by step guidance

1. Step 1: Understand the genetic setup. The female is heterozygous for all three loci (y⁺w⁺f/ywf⁺), while the male is hemizygous for the mutant alleles (ywf). Since these genes are X-linked, males inherit their X chromosome from their mother and their Y chromosome from their father, while females inherit one X chromosome from each parent.
2. Step 2: Determine the parental and recombinant gametes. The female can produce four types of gametes due to recombination: parental types (y⁺w⁺f and ywf⁺) and recombinant types (y⁺wf⁺ and ywf). The male can only produce one type of gamete (ywf) because he is hemizygous for the X chromosome.
3. Step 3: Calculate the recombination frequencies. Use the map distances to estimate the recombination frequencies between the loci. The distance between y and w is 1.5 m.u., corresponding to a 1.5% recombination frequency. The distance between w and f is 56.7 - 1.5 = 55.2 m.u., corresponding to a 55.2% recombination frequency. Assume that recombination events between y and f are independent.
4. Step 4: Predict the frequency of each gamete type. Use the recombination frequencies to calculate the proportion of parental and recombinant gametes. For example, the parental gametes (y⁺w⁺f and ywf⁺) will occur with a frequency of (1 - recombination frequency), while the recombinant gametes (y⁺wf⁺ and ywf) will occur with a frequency equal to the recombination frequency.
5. Step 5: Determine the progeny phenotypes and their frequencies. Combine the gametes from the female with the single type of gamete from the male (ywf). For each combination, determine the genotype and corresponding phenotype of the progeny. Multiply the frequency of the female gamete by the male gamete frequency to calculate the frequency of each progeny phenotype.

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. This is important in understanding the inheritance patterns of traits, as linked genes do not assort independently. In the context of the question, the proximity of the body color, eye color, and bristle form genes on the Drosophila X chromosome affects the expected phenotypic ratios in the offspring.

Map Units and Recombination Frequency

Map units (m.u.) are a measure of genetic distance between loci on a chromosome, where 1 m.u. corresponds to a 1% chance of recombination occurring between two genes during meiosis. The distances provided in the question indicate how likely it is for recombination to separate the alleles of the linked genes. Understanding these distances is crucial for predicting the phenotypic ratios of the progeny from the cross.

Dominance and Allelic Interactions

Dominance describes the relationship between alleles of a gene, where the dominant allele masks the expression of the recessive allele in a heterozygous individual. In this scenario, the wild-type alleles (y⁺, w⁺, f⁺) are dominant over their respective mutant alleles (y, w, f). This concept is essential for determining the phenotypes of the progeny based on the genotypes of the parents in the cross.