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.

No determination of sex was made in the data. Calculate the coefficient of coincidence. Does it represent positive or negative interference?

Verified step by step guidance

1. Step 1: Identify the parental and recombinant phenotypes from the table. The parental phenotypes are the most frequent classes, which are 'sc s v' (314 offspring) and '+++ ' (280 offspring). The double crossover classes are the least frequent, which are 'sc + +' (10 offspring) and '++ v' (14 offspring).
2. Step 2: Calculate the recombination frequencies between the gene pairs. Use the single crossover classes to find the recombination frequency between sc and s, and between s and v. For example, the recombinants between sc and s are the phenotypes where only one of these genes is recombinant (e.g., '+ s v' and 'sc ++'). Calculate the recombination frequency as the sum of these single crossover offspring divided by the total offspring (1000).
3. Step 3: Calculate the expected number of double crossovers (DCO) assuming independence. Multiply the recombination frequencies of the two intervals to get the expected DCO frequency, then multiply by the total number of offspring to get the expected DCO count.
4. Step 4: Calculate the coefficient of coincidence (c) using the formula: \(c = \frac{\text{observed DCO}}{\text{expected DCO}}\). The observed DCO is the sum of the two double crossover classes (10 + 14).
5. Step 5: Determine the interference (I) using the formula: \(I = 1 - c\). If I is positive, it indicates positive interference (fewer double crossovers than expected). If I is negative, it indicates negative interference (more double crossovers than expected).

Key Concepts

Genetic Linkage and Recombination

Genetic linkage refers to genes located close together on the same chromosome that tend to be inherited together. Recombination occurs during meiosis when crossing over between homologous chromosomes can separate linked genes, producing new allele combinations. The frequency of recombination between genes is used to map their relative positions on chromosomes.

Coefficient of Coincidence and Interference

The coefficient of coincidence measures the observed double crossover frequency divided by the expected frequency, indicating how often two crossover events occur together. Interference describes how one crossover event affects the likelihood of another nearby; positive interference reduces double crossovers, while negative interference increases them.

X-linked Inheritance in Drosophila

In Drosophila, X-linked traits are inherited differently in males and females because males have one X chromosome and females have two. Recessive X-linked traits appear in males if they inherit the mutant allele, while females must inherit two copies. This pattern helps interpret crosses involving X-linked genes and their phenotypic ratios.