In Drosophila, white eyes is an X-linked recessive trait. A...

In Drosophila, white eyes is an X-linked recessive trait. A whiteeyed female is crossed with a male with normal red eyes. An $\mathrm{F}_{1}$ female from this cross is mated with her father, and an $\mathrm{F}_{1}$ male is mated with his mother. What will be the phenotypic ratios for eye color in the two sexes of the offspring of the last two crosses?

Key Concepts

Sex-Linked Inheritance

Sex-linked inheritance refers to the transmission of genes located on the sex chromosomes, typically the X or Y chromosome. In species like Drosophila, where females have two X chromosomes and males have one, the pattern of inheritance of X-linked traits is distinct. Males, having only one X chromosome, will express recessive traits coded on the X even if there is no corresponding allele on the Y chromosome, while females require two copies of the recessive allele to express these traits.

X-Linked Recessive Traits

X-linked recessive traits manifest predominantly in males because they possess only a single X chromosome, meaning any recessive allele present will be unmasked phenotypically. Females, on the other hand, need two copies of the recessive allele to show the trait. This concept is critical in understanding traits such as white eyes in Drosophila, where the mutant allele only exhibits its phenotype under hemizygous or homozygous conditions.

Genetic Crosses

Genetic crosses are experimental methods used to predict the outcome of mating between organisms with known genotypes. Techniques such as Punnett squares aid in visualizing possible allele combinations and their associated phenotypes. In problems involving multiple crosses, as well as backcrosses with specific parental genotypes, understanding these methods is essential for determining the likely genotypic and phenotypic ratios of the progeny.

Phenotypic Ratio Calculation

Phenotypic ratio calculation involves determining the proportion of offspring that display a particular trait based on the inheritance patterns of dominant and recessive alleles. In X-linked crosses, separate analyses by sex are necessary because the presence or absence of a second X chromosome alters the expression of recessive traits. This concept is crucial for predicting the distribution of traits, such as eye color in Drosophila, among male and female offspring.

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