Question

In Drosophila , the X-linked recessive mutation vermilion (v) causes bright red eyes, in contrast to the brick-red eyes of wild type. A separate autosomal recessive mutation, suppressor of vermilion \((s u-v),\) causes flies homozygous or hemizygous for \(v\) to have wildtype eyes. In the absence of vermilion alleles, su-v has no effect on eye color. Determine the \(F_{1}\) and \(F_{2}\) phenotypic ratios from a cross between a female with wild-type alleles at the vermilion locus, but who is homozygous for \(s u\) -v, with a vermilion male who has wildtype alleles at the su-v locus.

Short answer

Short Answer: The F1 generation phenotypic ratio is 1:1 (ancestor wild-type : wild-type). The F2 generation phenotypic ratio is 2:1:2:1 (wild-type female, wild-type male, vermilion female, and vermilion male).

Step-by-step solution

Determine the genotypes of the parents

We are given the following information about the parents: - Female with wild-type alleles at the vermilion locus (wild-type for X-linked allele) and homozygous for su-v (autosomal recessive allele) - Vermilion male (homozygous recessive at the X-linked allele) with wildtype alleles at the su-v locus The genotypes of the parents can be represented as: - Female: X^+ X^+ su-v su-v - Male: X^vY, Su-v+ Su-v+

Determine the gametes produced by each parent

Each parent will produce two different gametes with respect to these alleles: - Female: X^+su-v and X^+su-v - Male: X^vSu-v+ and YSu-v+

Perform the cross of gametes to determine the F1 generation

We perform the cross between female and male gametes: - X^+su-v x X^vSu-v+ -> X^+ X^v Su-v+su-v (Female) - X^+su-v x YSu-v+ -> X^+ Y Su-v+su-v (Male)

Determine the F1 phenotypes and ratios

All F1 individuals will have wild-type eyes. Since the females carry one copy of the suppressor gene, they appear wild-type despite carrying the v allele. The males also appear wild-type due to having only one copy of the Su-v+ allele. Phenotypic ratio for F1 generation is 1:1 (ancestor wild type : wild type).

Perform a cross of F1 individuals to find the F2 generation

Now, we cross the F1 female and F1 male: Female gametes: X^+su-v, X^+su-v, X^vsu-v, X^vsu-v Male gametes: X^+Su-v+su-v, X^vSu-v+su-v, YSu-v+su-v Possible offspring: - X^+ X^+ Su-v+su-v - X^+ X^v Su-v+su-v - X^+ Y Su-v+su-v - X^+ X^v Su-v+su-v - X^v X^v Su-v+su-v - X^v Y Su-v+su-v

Determine the F2 phenotypes and ratios

We can now determine the phenotypes of the F2 generation: - X^+ X^+ Su-v+su-v (Wild-type Female) - X^+ X^v Su-v+su-v (Wild-type Female) - X^+ Y Su-v+su-v (Wild-type Male) - X^+ X^v Su-v+su-v (Wild-type Female) - X^v X^v Su-v+su-v (Vermilion Female) - X^v Y Su-v+su-v (Vermilion Male) Phenotypic ratio for F2 generation is 2:1:2:1 (wild-type female, wild-type male, vermilion female, and vermilion male).

Key concepts

X-linked recessive mutation

When we talk about X-linked recessive mutations, we're referring to a change in a gene that's located on the X chromosome and requires two copies to manifest in females, but only one in males, due to their having just one X chromosome. These mutations are mostly observed in males because they are hemizygous for the X chromosome (meaning they only have one X chromosome, paired with a Y chromosome).

For example, in the exercise, the vermilion eye color in Drosophila, which is controlled by an X-linked gene, demonstrates this pattern. Females need two copies of the recessive vermilion allele to exhibit the trait, whereas males need only one. This underlying mechanism is pivotal in understanding how traits are inherited in genetic crosses, especially those involving sex-linked genes.

Autosomal recessive mutation

Contrary to X-linked recessive mutations, autosomal recessive mutations are located on non-sex chromosomes (autosomes) and require two copies of the allele for the trait to be expressed, irrespective of the individual's sex. In Drosophila, and humans alike, autosomes are present in pairs, so both males and females have two chances to inherit the mutated gene.

In our exercise, the suppressor of vermilion is an autosomal gene. It's recessive, which means for Drosophila to show the suppression effect and have wildtype eyes, it must have two recessive alleles of the suppressor gene. When at least one dominant allele is present at this locus, the suppression does not manifest.

F1 and F2 phenotypic ratios

After establishing the basics of X-linked and autosomal recessive mutations, let's dive into the F1 and F2 phenotypic ratios. These ratios are a cornerstone of genetics, helping us understand the inheritance patterns across generations. F1 refers to the first filial generation, or the immediate offspring from a cross, while F2 is the second filial generation, produced by crossing two F1 individuals.

In the given exercise, the F1 progeny all have wildtype eyes due to the suppressor's presence, leading to a 1:1 phenotypic ratio; all offspring look the same regardless of their gender. However, things get more exciting in the F2 generation where different combinations of X-linked and autosomal genes can be observed, resulting in a more complex phenotypic ratio of 2:1:2:1 for wild-type females, wild-type males, vermilion females, and vermilion males, respectively. This ratio sheds light on the interactions between different types of mutations and their effects on offspring phenotype.

Genetic crosses

Delving into genetic crosses, they are essentially the breeding experiments that scientists conduct to study inheritance patterns. These crosses allow us to predict the genetic makeup and phenotype of offspring from parents with known genotypes. By performing a genetic cross, scientists can track traits across generations and predict how they might manifest in future progeny.

Understanding genetic crosses requires a grasp of how genes are inherited and expressed. In the textbook exercise, by following the principles of Mendelian genetics and considering both X-linked and autosomal genes, we are able to predict the eye color of Drosophila in the F1 and F2 generations. This prediction hinges on knowing the genotype of the parents, the laws of inheritance for X-linked and autosomal genes, and the interaction between different alleles. The practice of constructing Punnett squares, or tabular grids, is often used to visualize the possible outcomes of a genetic cross.