Question

While vermilion is X-linked in Drosophila and causes eye color to be bright red, brown is an autosomal recessive mutation that causes the eye to be brown. Flies carrying both mutations lose all pigmentation and are white-eyed. Predict the \(F_{1}\) and \(F_{2}\) results of the following crosses: (a) vermilion females \(\times\) brown males (b) brown females \(\times\) vermilion males (c) white females \(\times\) wild males

Short answer

Answer: 1. For cross (a), the expected \(F_{2}\) phenotypes are vermilion females, brown females, vermilion males, and white males. 2. For cross (b), the expected \(F_{2}\) phenotypes are white females, vermilion females, wild-type males, and brown males. 3. For cross (c), the expected \(F_{2}\) phenotypes are white females, vermilion females, brown males, and vermilion males.

Step-by-step solution

(a) Cross vermilion females with brown males

To analyze this cross, assign the following symbols: V (vermilion mutation), B (brown mutation), and + (wild-type or normal allele). Since vermilion is X-linked, vermilion females are represented as XX\(_{V}\), and wild-type males are represented as XY. The brown mutation is autosomal recessive, so brown males are represented as bb. The parental cross is XX\(_{V}\) (vermilion females) \(\times\) XY;bb (brown males). The \(F_{1}\) generation will include females with one vermilion allele and one wild-type allele, and males with one brown allele (but no vermilion allele, since it is X-linked): X\(_{V}\)X (vermilion females) and X\(_{V}\)Y;bb (brown males).

(b) Cross brown females with vermilion males

This cross includes females with the brown mutation (bb) and males with the vermilion mutation (X\(_{V}\)Y). The offspring in the \(F_{1}\) generation will inherit one X chromosome from each parent: X\(_{V}\)X;bb (white females due to both mutations) and XY;Bb (wild-type males with one brown allele).

(c) Cross white females with wild males

In this case, the white females have both the vermilion and brown mutations (XX\(_{V}\);bb) and are crossed with wild-type males (XY;BB). The offspring in the \(F_{1}\) generation will inherit one X chromosome and one brown allele: X\(_{V}\)X;Bb (vermilion females with one brown allele) and X\(_{V}\)Y;Bb (brown males with one vermilion allele). Now, let's analyze the \(F_{2}\) generations for each cross: 1. Cross \(F_{1}\) vermilion females (X\(_{V}\)X) with \(F_{1}\) brown males (X\(_{V}\)Y;bb) from scenario (a). The expected \(F_{2}\) phenotypes would be: - Vermilion females (X\(_{V}\)X;BB) - Brown females (XX;Bb) - Vermilion males (X\(_{V}\)Y;BB) - White males (X\(_{V}\)Y;bb) 2. Cross \(F_{1}\) white females (X\(_{V}\)X;bb) with \(F_{1}\) wild-type males (XY;Bb) from scenario (b). The expected \(F_{2}\) phenotypes would be: - White females (X\(_{V}\)X;bb) - Vermilion females (X\(_{V}\)X;Bb) - Wild-type males (XY;Bb) - Brown males (XY;bb) 3. Cross \(F_{1}\) vermilion females (X\(_{V}\)X;Bb) with \(F_{1}\) brown males (X\(_{V}\)Y;Bb) from scenario (c). The expected \(F_{2}\) phenotypes would be: - White females (X\(_{V}\)X;bb) - Vermilion females (X\(_{V}\)X;Bb) - Vermilion females (X\(_{V}\)X;BB) - Brown males (XY;bb) - Vermilion males (X\(_{V}\)Y;Bb) - Vermilion males (X\(_{V}\)Y;BB).

Key concepts

Drosophila

Drosophila, commonly known as fruit flies, are small insects frequently used in genetic studies. They are ideal for research due to their short life cycle and easily observable traits. Scientists have studied them extensively to understand genetic inheritance, as they reproduce quickly and have well-defined, observable traits such as eye color. These traits can be linked to specific genetic mutations. In genetics, studying Drosophila can help us grasp concepts about mutations and inheritance patterns.

One of the reasons Drosophila are such a popular model organism is because they have only four pairs of chromosomes. This simplicity allows for easier analysis of genetic mutations and their effects, making Drosophila genetics a cornerstone of genetic experimentation. Additionally, they exhibit clear, measurable differences in traits such as eye color, which can result from simple genetic mutations or complex interactions between alleles.

• Short life cycle: Just about two weeks from egg to adult.
• Small genome: Four pairs of chromosomes.
• Observable traits: For example, eye color can change with specific mutations.

X-linked traits

X-linked traits are those that are located on the X chromosome. In Drosophila, as in humans, females have two X chromosomes, while males have one X and one Y chromosome. This difference in chromosome composition leads to unique patterns of inheritance for traits linked to the X chromosome.

When discussing X-linked traits in Drosophila, one classic example is the vermilion eye color mutation. This mutation affects eye color and is located on the X chromosome. Since males have only one X chromosome, any gene located there is expressed whether it's dominant or recessive. Females can have one mutated gene on one X chromosome and a normal gene on the other, potentially masking the effect if the normal gene is dominant.

• Females: XX, can be carriers without displaying the trait if they have one normal and one mutated gene.
• Males: XY, will express X-linked traits from the mother since they have no second X chromosome to mask a recessive trait.

Understanding X-linked inheritance helps explain why some traits appear more frequently in one sex than the other. In our study of Drosophila, these principles help predict inheritance patterns involving traits like vermilion eyes.

Autosomal recessive mutation

Autosomal recessive mutations occur in genes located on the non-sex chromosomes. These are called autosomes. In Drosophila, just like in other organisms, autosomal traits follow their own inheritance patterns. For a recessive trait to be expressed in an organism, it often requires two copies of the recessive allele, one from each parent.

In the case of the brown eye mutation in Drosophila, this mutation is autosomal recessive. This means that a fly must inherit two copies of the brown eye allele to actually display the brown eye phenotype. If it has one brown allele and one normal allele, the normal allele will typically dominate, masking the expression of the brown allele.

• Needs two copies: Individuals must have two copies of the recessive allele for the phenotype to be observable.
• Carrier state: Individuals with one normal and one recessive allele do not show the trait but can pass on the recessive allele to offspring.

Understanding autosomal recessive mutations is crucial for predicting how certain traits can be passed down through generations, helping us map out inheritance in flies and other organisms.