Question

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?

Short answer

The phenotypic ratio for the offspring of an F1 female mated with her father is all females with red eyes and a 1:1 ratio of red-eyed to white-eyed males. The offspring of an F1 male mated with his mother are all white-eyed, both males and females.

Step-by-step solution

Identify Genetic Information

Establish the genotype for white eyes in Drosophila. White eyes is an X-linked recessive trait, so we represent it with 'w' for the mutant allele and 'W' for the normal allele. The white-eyed female is 'ww' and the male with red eyes is 'WY', since males have only one X chromosome.

Determine F1 Generation

The cross of a white-eyed female (XwXw) with a red-eyed male (XWY) results in all females having genotype XwXW (red eyes, since the trait is recessive) and all males having genotype XwY (white eyes). This is because females receive one X chromosome from each parent, while males receive the X chromosome from their mother and the Y chromosome from their father.

Cross F1 Female with Father

An F1 female (XwXW) is mated with her father (XWY), so female offspring could be XwXW or XWXW (red eyes, since they have at least one normal allele) and males could be XwY (white eyes) or XWY (red eyes).

Cross F1 Male with Mother

An F1 male (XwY) is mated with his mother (XwXw), so all female offspring will be XwXw (white eyes, since they receive one 'w' allele from each parent) and males will be XwY (white eyes, since they receive the 'w' allele from their mother and the Y from their father).

Phenotypic Ratios in Offspring of Last Two Crosses

From the third step's cross, the phenotypic ratio for females is all red eyes and for males is 1 red eyes: 1 white eyes. From the fourth step's cross, both females and males show all white eyes.

Key concepts

Phenotypic Ratios

Understanding phenotypic ratios is key to predicting the outcome of genetic crosses. Phenotypic ratio refers to the relative number of offspring manifesting a particular trait or combination of traits. It is the visible expression of an organism's genes, which can be calculated after certain types of genetic crosses. In the exercise regarding X-linked inheritance in Drosophila, the phenotypic ratios provide insights into the distribution of eye color among the offspring.

For instance, when an F1 female with genotype XwXW (where W is the dominant allele for red eyes) is mated with an XWY male, the offspring’s eye color phenotypic ratio is all red-eyed females to half red-eyed and half white-eyed males. Conversely, an F1 male XwY mating with a homozygous recessive white-eyed female XwXw produces offspring with a phenotypic ratio showing all white-eyed males and females.

Analyzing phenotypic ratios allows researchers and students alike to predict the likelihood of certain traits appearing in the next generation, which is crucial in understanding genetics and evolution.

Recessive Traits

Recessive traits are expressed phenotypically only when an organism has two copies of the recessive allele. In relation to the exercise, the white eye color in Drosophila is a recessive trait, marked by the 'w' allele. A recessive trait can be masked by the presence of a dominant allele, denoted here by 'W' for the red eye color.

For a trait to be X-linked means that the genes associated with it are located on the X chromosome. In the case of Drosophila, since males (XY) have only one X chromosome, the presence of a single 'w' allele will suffice to express the white eye trait. Females (XX), on the other hand, must possess two 'w' alleles to display the trait.

This explains why the male offspring of a white-eyed female (XwXw) and a red-eyed male (XWY) will always exhibit the white-eyed phenotypic trait, while female offspring will be red-eyed but carry one copy of the 'w' allele, becoming carriers of the recessive trait.

Genotype Determination

Genotype determination involves deducing the combination of alleles that an organism has for a particular gene. It is a fundamental step in understanding inheritance patterns. In the exercise provided, the genotypes of Drosophila are established through notation where 'W' and 'w' represent alleles for eye color.

The initial determination of genotypes in the exercise shows that genotypic information is critical to anticipate the outcomes of a genetic cross. An X-linked trait like white eyes simplifies this process for males since their single X chromosome reveals their genotype directly. However, females can be homozygous or heterozygous for the traits in question, requiring careful analysis of parental genotypes to determine their genetic makeup.

Determining genotypes of further generations requires understanding of the parents' genotypes and the possible gametes they can produce. This preparation enables more accurate predictions of offspring phenotypes.

Cross-Breeding Experiments

Cross-breeding experiments are utilized to study inheritance patterns of traits. They involve mating two individuals with different genotypes and phenotypes to observe the traits of the offspring. This classic genetic tool helps establish the mode of inheritance of specific traits, such as X-linked, autosomal dominant, or autosomal recessive.

The exercise presents two cross-breeding experiments. The first cross involves an F1 female that has red eyes but carries the allele for white eyes (a heterozygote), and her father, who has red eyes. The second cross involves an F1 male that has white eyes (a hemizygote due to having only one X chromosome), and his white-eyed mother. Predictions of offspring phenotypes depend on combining the understanding of X-linked inheritance with expected independent assortment and fertilization events.

Such experiments are critical for genetic research and breeding programs, helping us unveil complex inheritance patterns and the influence of specific genes.