Question

While vermilion is X-linked in Drosophila and causes the 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 \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of the following crosses: (a) vermilion females \(\times\) brown males (b) brown females \(\times\) vermilion males (c) white females \(\times\) wild-type males

Short answer

Answer: For the F1 generation, the results are as follows: (a) Vermilion females × brown males: Vermilion-eyed females and white-eyed males. (b) Brown females × vermilion males: Vermilion-eyed females and white-eyed males. (c) White females × wild-type males: Vermilion-eyed females and vermilion-eyed or brown-eyed males. For the F2 generation, the results are as follows: (a) Self-cross of F1 from (a): 1/2 vermilion females, 1/2 white females, 1/2 vermilion males, and 1/2 white males. (b) Self-cross of F1 from (b): 1/2 vermilion females, 1/2 white females, 1/2 vermilion males, and 1/2 white males. (c) Self-cross of F1 from (c): 1/2 vermilion females, 1/2 white females, 1/4 vermilion males, 1/4 brown males, and 1/2 white males.

Step-by-step solution

State parental genotypes

Vermilion females carry the vermilion gene on their X chromosome and can be represented as V-/v, where "V-" represents the wild-type allele (on the other X chromosome) and "v" represents the vermilion gene. Brown males can be represented as b/b and carry their Y chromosome.

Determine F1 genotypes

Perform a cross and use a Punnett square to determine the possible genotypes of the offspring. Vermilion gene Punnett: X-V- X-v Xv X-Y Brown gene Punnett: B- b b b From the Punnett squares, we have F1 genotypes: - Females: V-B-/v-b (Vermilion and heterozygous brown) - Males: v-b/Y (hemizygous vermilion and homozygous brown)

Predict F1 phenotypes

- Females: Vermilion eyes (due to the presence of the vermilion gene) - Males: White eyes (due to the presence of both vermilion and brown genes)

Determine F2 from a self-cross of F1

To determine the F2 phenotypes, you can perform a self-cross of the F1 genotypes obtained in step 2 using Punnett squares again. (V-B-/v-b) × (v-b/Y) The genotypes and phenotypes of F2 offspring would be: - 1/2 vermilion female: VB-/vb - 1/2 white female: vb/vb - 1/2 vermilion male: VY/vY - 1/2 white male: vb/Y (b) brown females × vermilion males Follow the same steps as above (state parental genotypes, determine F1 genotypes, predict F1 phenotypes, and determine F2 from a self-cross of F1). The genotypes and phenotypes of F1 offspring would be: - All females: vermilion and heterozygous brown (V-B-/v-b) - All males: white eyes (v-b/Y) The genotypes and phenotypes of F2 would be exactly the same as in (a) since the F1 generation is identical. (c) white females × wild-type males White females are homozygous recessive for both the vermilion and the brown gene, so their genotype would be (vb/vb, b/b). Wild-type males don't have the vermilion gene, so their genotype would be (V-/Y, B-/B-). The genotypes and phenotypes of F1 offspring would be: - All females: vermilion and heterozygous brown (V-B-/v-b) - All males: hemizygous vermilion and heterozygous brown (v-b/Y, B-/b) The genotypes and phenotypes of F2 would be different from (a) and (b): - 1/2 vermilion female: VB-/vb - 1/2 white female: vb/vb - 1/4 vermilion male: VY/vY - 1/4 brown male: VY/vb - 1/4 white male: vb/Y - 1/4 white male: vb/Y

Key concepts

X-linked Inheritance

X-linked inheritance refers to the pattern of genetic transmission linked to the X chromosome, one of the two sex chromosomes in humans and many other organisms. In the context of Drosophila, or fruit flies, X-linked genes have unique patterns of inheritance because males (XY) have only one X chromosome, while females (XX) have two. This means that males are hemizygous for X-linked traits, carrying only one allele (gene variant), while females can be homozygous or heterozygous, carrying two identical or two different alleles, respectively.

In the exercise involving the vermilion eye color, which is an X-linked trait, this pattern of inheritance plays a crucial role. The vermilion gene (v) is recessive to the wild-type allele (V-), so a female must have two copies of v to express the vermilion phenotype, whereas a male will express it with just one copy because he lacks a second allele to mask it. Conversely, wild-type females can be heterozygous (V-/v) and still have normal eye color, while all males with a normal allele (V-) will also have normal eye color.

Autosomal Recessive Mutation

An autosomal recessive mutation is a change in a gene located on one of the autosomes (non-sex chromosomes) where two copies of the mutant allele are needed for the individual to express the associated trait or phenotype. In Drosophila, the brown eye color is controlled by an autosomal recessive mutation (b).

For a fly to exhibit brown eyes, both alleles must be the mutant form (b/b), as the wild-type allele (B-) is dominant. When crossing individuals, one needs to account for the possibility that offspring can inherit one or two copies of the recessive allele. For example, crossing a homozygous recessive individual (b/b) with a heterozygous carrier (B-/b) will result in a 50% chance of producing an offspring with brown eyes and a 50% chance of an offspring being a carrier like one of the parents, without expressing the brown eye phenotype.

Punnett Square

The Punnett square is a powerful visual tool used in genetics to predict the outcome of crosses between parents for a particular trait or traits. The tool is a simple grid that allows us to align the gametes (sperm and egg cells) from each parent to visualize all possible genetic combinations in their offspring. For example, when considering the X-linked vermilion eye color in Drosophila, one can set up a Punnett square where the male's possible gametes (X and Y chromosomes) are listed on one side, and the female's possible gametes (both X chromosomes, with or without the mutant gene) are listed on another.

Utilizing a Punnett square, when crossing vermilion females with brown males (or vice versa), we can easily see the genotypes and phenotypes expected in the F1 and F2 generations. This tool helps us to understand complex genetic interactions, such as those involving X-linked traits and autosomal recessive mutations, by providing a clear and systematic way to determine potential genetic outcomes.

F1 and F2 Generations

In genetics, the terms F1 and F2 refer to the first and second generations of offspring from a set of parents. The F1 generation is comprised of the immediate progeny from a cross between two parental genotypes. Meanwhile, the F2 generation comes from crossing individuals from the F1 generation, either with each other or with one of the original parents.

In the context of our Drosophila crosses, the F1 generation from a vermilion female and brown male cross will exhibit the phenotype according to the dominant alleles present, while the F2 generation will display a variety of phenotypes based on the combination of alleles from the F1 self-cross. In the step-by-step solution, it’s shown how to use the Punnett squares for both F1 and F2 generations to predict eye color, considering the inheritance patterns of both the X-linked vermilion and the autosomal recessive brown mutations. Since the resulting phenotypes are influenced by the interplay of these genetic principles, carefully analyzing the F1 generation is crucial in predicting the genetic makeup of the F2 generation.