Question

In Drosophila melanogaster, the gene for white eyes, \(\mathrm{w}\), is recessive and X-linked. Red eyes result from the wild-type allele at this locus. (a) On chromosomes, symbolize the genotypes of a white-eyed male, a red-eyed female (both possible types), and a white-eyed female. (b) Diagram a cross between a homozygous red-eyed female and white-eyed male. Carry this to the \(\mathrm{F}_{2}\) generation and give phenotype ratios for each sex. (c) Show a cross between an \(\mathrm{F}_{1}\) female and white-eyed male. Give phenotype ratios. (d) Show a cross between an \(\mathrm{F}_{1}\) female and wild-type male. Also give ratios.

Short answer

The genotypes are as follows: white-eyed male (X^wY), homozygous red-eyed female (XX^W^W), heterozygous red-eyed female (X^W^w), and white-eyed female (XX^w^w). For the F2 generation, the phenotype ratios are 1 red-eyed female: 1 red-eyed male: 1 white-eyed female: 1 white-eyed male. For the F1 female x white-eyed male cross, the phenotype ratios are 1 red-eyed female: 1 white-eyed female: 1 red-eyed male: 1 white-eyed male. For the F1 female x wild-type male cross, the phenotype ratios are 2 red-eyed females: 1 red-eyed male: 1 white-eyed male.

Step-by-step solution

White-eyed male

A white-eyed male has only one X chromosome carrying the recessive white eye allele (\(\mathrm{w}\)). So, its genotype would be X^wY.

Red-eyed female (homozygous)

A homozygous red-eyed female would have the wild-type red eye allele (\(\mathrm{W}\)) on both of its X chromosomes. So, its genotype would be XX^W^W.

Red-eyed female (heterozygous)

A heterozygous red-eyed female would have one wild-type allele (\(\mathrm{W}\)) and one recessive white allele (\(\mathrm{w}\)). So, its genotype would be X^W^w.

White-eyed female

A white-eyed female would have the recessive white eye allele (\(\mathrm{w}\)) on both of its X chromosomes. So, its genotype would be XX^w^w. #b) Perform a cross between a homozygous red-eyed female and a white-eyed male. Carry this to the F2 generation and give phenotype ratios for each sex. #

P (Parental) generation cross

A homozygous red-eyed female (XX^W^W) is crossed with a white-eyed male (X^wY).

F1 (First filial) generation

All the offspring will inherit an X^W chromosome from the mother and either an X^w or Y chromosome from the father. So, all females will be heterozygous red-eyed (X^W^w) and all males will be red-eyed (X^WY).

F2 (Second filial) generation

If we cross F1 heterozygous red-eyed females (X^W^w) with F1 red-eyed males (X^WY), we get the following F2 phenotypes: - 1 Red-eyed female (X^W^W): gets X^W from both parents. - 1 Red-eyed male (X^WY): gets X^W from the mother and Y from the father. - 1 White-eyed female (XX^w^w): gets X^w from both parents. - 1 White-eyed male (X^wY): gets X^w from the mother and Y from the father. Phenotype ratios for F2 generation are: 1 red-eyed female: 1 red-eyed male: 1 white-eyed female: 1 white-eyed male. #c) Perform a cross between an F1 female and a white-eyed male. Give phenotype ratios. #

F1 female x white-eyed male cross

A heterozygous red-eyed F1 female (X^W^w) is crossed with a white-eyed male (X^wY).

Offspring

We get the following phenotypes: - 1 Red-eyed female (X^W^w): gets X^W from the mother and X^w from the father. - 1 White-eyed female (XX^w^w): gets X^w from both parents. - 1 Red-eyed male (X^WY): gets X^W from the mother and Y from the father. - 1 White-eyed male (X^wY): gets X^w from the mother and Y from the father. Phenotype ratios for this cross are: 1 red-eyed female: 1 white-eyed female: 1 red-eyed male: 1 white-eyed male. #d) Perform a cross between an F1 female and a wild-type male. Also, give ratios. #

F1 female x wild-type male cross

A heterozygous red-eyed F1 female (X^W^w) is crossed with a wild-type male (X^WY).

Offspring

We get the following phenotypes: - 1 Red-eyed female (X^W^W): gets X^W from both parents. - 1 Red-eyed female (X^W^w): gets X^W from the father and X^w from the mother. - 1 Red-eyed male (X^WY): gets X^W from the mother and Y from the father. - 1 White-eyed male (X^wY): gets X^w from the mother and Y from the father. Phenotype ratios for this cross are: 2 red-eyed females: 1 red-eyed male: 1 white-eyed male.

Key concepts

Drosophila Melanogaster Genetics

Understanding genetics in fruit flies, or Drosophila melanogaster, provides invaluable insights into hereditary patterns and gene function. These tiny organisms have become a staple in genetic research due to their rapid life cycle, ease of care, and well-documented genome.

Drosophila genetics often focuses on phenotypic traits such as eye color, which are determined by specific alleles on chromosomes. In the case of eye color, the allele for white eyes is both recessive and located on the X chromosome, making it an excellent example of an X-linked trait. This means that the absence of a dominant allele on one of the sex chromosomes can result in the white-eyed phenotype, especially pronounced in males due to their XY chromosome composition.

Phenotype Ratios

Phenotype ratios offer a quantitative measure of the occurrence of different phenotypic traits in the offspring, following a particular genetic cross. Understanding these ratios can tell us how traits might be inherited by future generations.

For geneticists, calculating these ratios helps predict the likelihood of traits appearing within a population. In our fruit fly example, a cross involving X-linked alleles can result in distinct phenotype ratios between males and females due to the presence of a single X chromosome in males. This becomes particularly clear when examining the ratios from the F1 and F2 generations in the given crosses.

Recessive Allele

A recessive allele is a variant of a gene that must be inherited from both parents in order to manifest its trait, especially when the corresponding dominant allele masks its expression. In sex-linked traits, such as the white eye color in Drosophila, the recessive nature becomes more apparent due to the sex chromosome differences between males (XY) and females (XX).

In males, a single recessive allele on the X chromosome will result in the expression of the trait, as there is no corresponding allele on the Y chromosome. Females, however, must carry the recessive allele on both of their X chromosomes to exhibit the recessive trait. This is why males are often more likely to display X-linked recessive traits.

Punnett Square

The Punnett square is a graphical representation used by geneticists to predict the outcome of a particular cross or breeding experiment. It's essentially a grid that allows us to map out all possible genotypes of offspring from parental alleles.

By aligning the possible gametes of each parent on either side of the square, we create a visual matrix of possible genotypic combinations and thus can infer phenotype probabilities. In the context of our Drosophila exercise, using a Punnett square helps clarify the outcome of crosses between red-eyed and white-eyed flies, especially when tracking X-linked traits.