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.
Are there more or fewer double crossovers than expected?

Verified step by step guidance

1. Step 1: Identify the parental and recombinant phenotypes. The parental phenotypes are the most frequent classes, which are 'sc s v' (314 offspring) and '+++ ' (280 offspring). The recombinant phenotypes include single and double crossovers with lower numbers.
2. Step 2: Determine the gene order on the X chromosome. Use the single crossover classes to infer the order of the three genes (sc, s, v). Compare the phenotypes that differ by one gene to establish the linear order.
3. Step 3: Calculate the recombination frequencies between each pair of genes. Use the formula for recombination frequency: \(\text{RF} = \frac{\text{Number of recombinant offspring}}{\text{Total offspring}}\). Calculate RF for sc-s and s-v intervals using the appropriate single crossover classes.
4. Step 4: Calculate the expected frequency of double crossovers. Multiply the recombination frequencies of the two intervals: \(\text{Expected double crossover frequency} = \text{RF}_{sc-s} \times \text{RF}_{s-v}\). Then multiply by the total number of offspring (1000) to get the expected number of double crossovers.
5. Step 5: Compare the observed number of double crossovers (sum of the two double crossover classes: 'sc + v' and '+ s +') to the expected number calculated in Step 4. Determine if there are more or fewer double crossovers than expected, which can indicate interference.

Key Concepts

X-linked Inheritance

X-linked inheritance refers to genes located on the X chromosome, which show distinct inheritance patterns, especially in species like Drosophila. Males (XY) express recessive X-linked traits if they inherit a mutant allele, while females (XX) must inherit two copies. This concept is crucial for understanding the phenotypic ratios in crosses involving X-linked traits.

Genetic Recombination and Crossing Over

Genetic recombination occurs during meiosis when homologous chromosomes exchange segments, producing new allele combinations. Crossing over frequency between genes reflects their physical distance on a chromosome. Double crossovers involve two recombination events and are less frequent, affecting the observed offspring phenotypes.

Double Crossover Frequency and Interference

Double crossover frequency is predicted by multiplying the probabilities of single crossovers between gene pairs. Interference occurs when one crossover reduces the likelihood of another nearby, leading to fewer double crossovers than expected. Analyzing observed versus expected double crossovers helps understand chromosome behavior during meiosis.