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.

Could the F₂ female offspring be used to construct the map? Why or why not?

Verified step by step guidance

1. Understand the problem: The question involves a genetic cross in Drosophila with three X-linked genes (yellow, white, and cut). The goal is to determine whether F₂ female offspring can be used to construct a genetic map and explain why or why not.
2. Recall the concept of X-linked inheritance: In Drosophila, males have one X chromosome (XY) and females have two X chromosomes (XX). This means that males inherit their X chromosome from their mother and their Y chromosome from their father, while females inherit one X chromosome from each parent.
3. Analyze the F₁ generation: The F₁ females are wild type for all three traits, indicating they are heterozygous for the X-linked genes. The F₁ males express the yellow-body and white-eye traits, which means they inherited the recessive alleles for these traits from their mother.
4. Consider the F₂ generation: The F₂ male offspring are tallied in the data provided. Since males inherit their X chromosome from their mother, the phenotypes of the F₂ males directly reflect the combinations of alleles present on the X chromosome of the F₁ females. This makes it possible to construct a genetic map using the F₂ male data.
5. Explain why F₂ females cannot be used: F₂ females inherit one X chromosome from their mother and one from their father. This means their phenotypes are influenced by the combination of alleles on both X chromosomes, making it difficult to determine the specific recombination events that occurred in the F₁ females. Therefore, F₂ females cannot be used to construct the genetic map.

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 will manifest in males if they inherit the recessive allele, while females may be carriers without expressing the trait if they have one dominant allele.

Phenotypic Ratios

Phenotypic ratios are the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this case, the observed ratios of male offspring phenotypes provide insights into the linkage and recombination frequencies of the genes involved. Analyzing these ratios helps in constructing a genetic map by indicating how closely linked the genes are based on the frequency of recombination.

Genetic Mapping

Genetic mapping is the process of determining the relative positions of genes on a chromosome based on the frequency of recombination events during meiosis. By analyzing the offspring's phenotypes and their ratios, researchers can infer the distances between genes. In this scenario, the F₂ female offspring cannot be used for mapping because their phenotypes do not provide the necessary information about the male-specific inheritance patterns of the X-linked traits.