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. Based on these results, the student was able to assign short to a linkage group (a chromosome). Which one was it? Include your step-by-step reasoning.

Key Concepts

Linkage Groups

Linkage groups refer to sets of genes that are inherited together due to their physical proximity on the same chromosome. Genes located close to each other on a chromosome tend to be inherited together during meiosis, leading to a higher likelihood of co-segregation. Understanding linkage is crucial for interpreting genetic crosses, as it helps in predicting the inheritance patterns of traits and identifying which traits are associated with specific chromosomes.

Recessive Traits

Recessive traits are phenotypic expressions that occur only when an individual has two copies of the recessive allele. In the case of Drosophila, the short legs (sh), black body color (b), and pink eye color (p) are all recessive traits. This means that a fly must inherit the recessive allele from both parents to express the trait, which is essential for understanding the inheritance patterns observed in the F1 and F2 generations of the genetic cross.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this scenario, the observed ratios of wild-type and mutant traits in the F2 generation provide insights into the genetic linkage and segregation of the alleles. Analyzing these ratios helps determine whether the traits are independently assorted or linked, which is key to identifying the chromosome associated with the short leg phenotype.