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. Phenotype Male Offspring y + ct 9 + w + 6 y w ct 90 + + + 95 + + ct 424 y w + 376 y + + 0 + w ct 0 Were any double-crossover offspring expected?

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 have only one copy of the X chromosome. Understanding this concept is crucial for analyzing the inheritance patterns of traits in the given cross.

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 genetic linkage and the inheritance of traits. By analyzing these ratios, one can infer the presence of linked genes and predict the expected outcomes of various genetic crosses, which is essential for answering the question about double-crossovers.

Genetic Mapping and Double Crossovers

Genetic mapping involves determining the relative positions of genes on a chromosome based on recombination frequencies. Double crossovers occur when two separate crossover events happen between two genes, potentially altering the expected phenotypic ratios. By analyzing the offspring data and calculating recombination frequencies, one can assess whether double crossovers were expected in the given genetic cross, which is key to understanding the genetic relationships among the traits studied.