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. Calculate the coefficient of coincidence. Does it represent positive or negative interference?

Key Concepts

X-linked Inheritance

X-linked inheritance refers to the pattern of genetic transmission of traits 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 X-linked recessive traits will manifest in males if they inherit the mutant allele, while females require two copies of the mutant allele to express the trait. Understanding this concept is crucial for interpreting the inheritance patterns observed in the offspring of the cross described.

Phenotypic Ratios and Genetic Mapping

Phenotypic ratios are the relative frequencies of different phenotypes in the offspring of a genetic cross. In this case, the observed ratios can help determine the linkage and distance between the genes for the traits studied. Genetic mapping uses these ratios to infer the arrangement of genes on chromosomes and can indicate whether genes are linked or assort independently. This concept is essential for analyzing the F₂ generation results and calculating the coefficient of coincidence.

Coefficient of Coincidence and Interference

The coefficient of coincidence is a measure used in genetics to quantify the degree of interference in crossover events during meiosis. It is calculated by comparing the observed double crossover frequency to the expected frequency based on the individual crossover rates. A coefficient greater than 1 indicates negative interference (more double crossovers than expected), while a value less than 1 indicates positive interference (fewer double crossovers than expected). Understanding this concept is vital for interpreting the results of the genetic cross and determining the nature of interference.