Drosophila melanogaster has one pair of sex chromosomes (XX or XY) and three pairs of autosomes, referred to as chromosomes II, III, and IV. A genetics student discovered a male fly with very short (sh) legs. Using this male, the student was able to establish a pure breeding stock of this mutant and found that it was recessive. She then incorporated the mutant into a stock containing the recessive gene black (b, body color located on chromosome II) and the recessive gene pink (p, eye color located on chromosome III). A female from the homozygous black, pink, short stock was then mated to a wild-type male. The F₁ males of this cross were all wild type and were then backcrossed to the homozygous b, p, sh females. The F₂ results appeared as shown in the following table. No other phenotypes were observed. Wild Pink* Black, Black, Pink, Short* Short Females 63 58 55 69 Males 59 65 51 60 *Other trait or traits are wild type. The student repeated the experiment, making the reciprocal cross, F₁ females backcrossed to homozygous b, p, sh males. She observed that 85 percent of the offspring fell into the given classes, but that 15 percent of the offspring were equally divided among b + p, b + +, + sh p, and + sh + phenotypic males and females. How can these results be explained, and what information can be derived from the data?

Key Concepts

Sex-linked Inheritance

In Drosophila melanogaster, sex-linked inheritance refers to traits that are determined by genes located on the sex chromosomes (X or Y). Since males have one X and one Y chromosome (XY), any recessive traits on the X chromosome will be expressed in males, as they lack a second X chromosome to mask the effect. Understanding this concept is crucial for interpreting the inheritance patterns observed in the crosses described in the question.

Recessive and Dominant Alleles

Alleles can be classified as dominant or recessive based on their expression in the phenotype. A recessive allele, such as the short legs (sh) in this case, will only manifest in the phenotype when present in a homozygous state (two copies). In contrast, dominant alleles will express their traits even in a heterozygous state. This concept is essential for analyzing the results of the genetic crosses and understanding the ratios of phenotypes in the offspring.

Backcrossing and F2 Generation Analysis

Backcrossing involves mating a hybrid organism with one of its parents or an individual genetically similar to its parents to determine the genotype of the offspring. The F2 generation results from crossing F1 hybrids, and analyzing their phenotypic ratios can reveal information about the inheritance patterns of traits. In this scenario, the observed ratios and the presence of unexpected phenotypes in the offspring provide insights into gene linkage and potential recombination events.