Another cross in Drosophila involved the recessive, X-linked genes yellow (y), white (w), and cut (ct). A yellow-bodied, white-eyed female with normal wings was crossed to a male whose eyes and body were normal but whose wings were cut. The F₁ females were wild type for all three traits, while the F₁ males expressed the yellow-body and white-eye traits. The cross was carried to an F₂ progeny, and only male offspring were tallied. On the basis of the data shown here, a genetic map was constructed.

Were any double-crossover offspring expected?

Verified step by step guidance

1. Step 1: Understand the genetic cross. The problem involves three X-linked genes in Drosophila: yellow (y), white (w), and cut (ct). These genes are recessive and located on the X chromosome. The parental cross involves a yellow-bodied, white-eyed female with normal wings (y w +) and a male with normal body and eyes but cut wings (+ + ct).
2. Step 2: Analyze the F₁ generation. The F₁ females are wild type for all traits (+ + +), indicating that the female inherited the dominant alleles from the male parent. The F₁ males express yellow body and white eyes (y w +), which indicates that they inherited the recessive alleles from the female parent.
3. Step 3: Examine the F₂ progeny. The F₂ generation includes male offspring with various phenotypes. The phenotypes and their frequencies are provided in the table. These frequencies will help determine the genetic map and identify crossover events.
4. Step 4: Identify parental, single-crossover, and double-crossover classes. The most frequent phenotypes represent the parental classes (+ + ct and y w +). The less frequent phenotypes represent single-crossover events, and the least frequent phenotypes (or zero-frequency phenotypes) represent double-crossover events. Double crossovers occur when two recombination events happen between the three genes.
5. Step 5: Determine if double-crossover offspring were expected. Double crossovers are expected based on the genetic distance between the genes. If the genes are far enough apart, double crossovers should occur. However, the absence of double-crossover phenotypes (y + + and + w ct) in the data suggests that the genetic distance between the genes may be small or that the sample size was insufficient to detect them.

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 linked to the X chromosome are more likely to be expressed in males, as they do not have a second X chromosome to mask the recessive allele.

Genetic Mapping

Genetic mapping is the process of determining the relative positions of genes on a chromosome based on the frequency of recombination between them. By analyzing offspring phenotypes from genetic crosses, researchers can estimate distances between genes, which helps in constructing a genetic map. The closer two genes are on a chromosome, the less likely they are to be separated by recombination during meiosis.

Double Crossover

A double crossover occurs when two separate crossover events happen between two linked genes during meiosis. This can lead to the exchange of genetic material between homologous chromosomes, potentially resulting in offspring with recombinant phenotypes. The frequency of double crossovers can be calculated using the recombination frequencies of the genes involved, and their occurrence can affect the expected ratios of phenotypes in the offspring.