In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F₁, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F₂ generation, and 1000 offspring were counted, with the results shown in the following table. Phenotype Offspring sc s v 314 + + + 280 + s v 150 sc + + 156 sc + v 46 + s + 30 sc s + 10 + + v 14 No determination of sex was made in the data. Are there more or fewer double crossovers than expected?

Key Concepts

X-linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. In Drosophila, males have one X chromosome and one Y chromosome, while females have two X chromosomes. This means that recessive traits on the X chromosome will manifest in males if present, while females may be carriers without expressing the trait if they have one dominant allele.

Genetic Mapping and Recombination

Genetic mapping involves determining the relative positions of genes on a chromosome based on the frequency of recombination events during meiosis. Recombination can lead to new combinations of alleles, and the frequency of double crossovers can be calculated to assess genetic linkage. Understanding these concepts is crucial for interpreting the results of genetic crosses and predicting offspring phenotypes.

Phenotypic Ratios and Chi-Square Analysis

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring of a genetic cross. In this context, analyzing the observed ratios against expected ratios can help determine if the results deviate from what is expected due to random assortment or linkage. Chi-square analysis is a statistical method used to evaluate these deviations and assess the significance of the results.