Question

In Drosophila , the \(\mathrm{X}\) -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 wild-type eyes. In the absence of vermilion alleles, \(s u-v\) has no effect on eye color. Determine the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) 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 wild-type alleles at the \(s u-v\) locus.

Short answer

Answer: The phenotypic ratios for the F1 and F2 generations are 100% wild-type eyes for each generation.

Step-by-step solution

Determining the genotypes of the parents

The female has wild-type alleles at the vermilion (V) locus and is homozygous for the suppressor of vermilion (su-v/su-v) mutation. She can be represented as V X_su-v X_su-v. The male has a vermilion (v) mutation at the vermilion locus and wild-type alleles at the suppressor of vermilion (Su-v/Su-v) locus. He can be represented as v Y Su-v/Su-v.

Determining the genotypes of the F1 generation

The female can produce two types of gametes: V X_su-v and X_su-v. The male can produce two types of gametes: v Y and Su-v Y. We can set up a Punnett square to find the genotypes of the F1 generation. V X_su-v | X_su-v ---------|------- v Y | Vv Y Su-v/Su-v (wild-type eyes) | VvX_su-v Su-v/Su-v (wild-type eyes) Su-v Y | V Su-v Y Su-v/Su-v (wild-type eyes) | VX_su-v Su-v/Su-v (wild-type eyes) All individuals in the F1 generation have wild-type eyes.

Determining the genotypes and phenotypes of the F2 generation

To find the genotypes of the F2 generation, we need to perform a cross within the F1 generation. For simplicity, we will use the abbreviated symbols: VvYS = Vv Y Su-v/Su-v VvXs = Vv X_su-v Su-v/Su-v VSYS = V Su-v Y Su-v/Su-v VXsS = VX_su-v Su-v/Su-v Setting up a Punnett square for a dihybrid cross: VvYS | VSYS VvXs | Vv Y Su-v/Su-v (wild-type eyes) | V Su-v Y Su-v/Su-v (wild-type eyes) | Vv X_su-v Su-v/Su-v (wild-type eyes) | VX_su-v Su-v/Su-v (wild-type eyes) VvYS | VSYS VXsS | Vv Y Su-v/Su-v (wild-type eyes) | V Su-v Y Su-v/Su-v (wild-type eyes) | Vv X_su-v Su-v/Su-v (wild-type eyes) | VX_su-v Su-v/Su-v (wild-type eyes) In the F2 generation, all individuals also have wild-type eyes.

Determining the phenotypic ratios

For both the F1 and F2 generations, all individuals have wild-type eyes. Therefore, the phenotypic ratios for both generations are: F1: 100% wild-type eyes F2: 100% wild-type eyes

Key concepts

X-linked Recessive Inheritance

X-linked recessive inheritance is a pattern of trait expression that is tied to genes located on the X chromosome. Since females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY), this inheritance pattern showcases different implications for each sex.

For a male to display a trait governed by an X-linked recessive gene, he only needs to inherit one copy of the mutated gene—one from the X chromosome he inherits from his mother. Due to having just one X chromosome, the recessive trait will almost always be expressed in males when they carry it. In contrast, females must inherit two copies of the recessive allele, one on each of their X chromosomes, to express the X-linked recessive trait, due to the presence of a second potentially normal allele 'shielding' the recessive one.

For example, in the textbook exercise, the vermilion eye color in Drosophila is caused by an X-linked recessive mutation (v). A male with a vermilion allele will always have vermilion eyes since he possesses only one X chromosome. However, a female with only one vermilion allele (heterozygous) will have the wild-type eye color since the dominant allele on the other X chromosome can mask the recessive phenotype.

Autosomal Recessive Mutation

Another key genetic concept is the autosomal recessive mutation. This type of mutation occurs on one of the 22 pairs of autosomes—the non-sex chromosomes—and to express the recessive trait, an individual must possess two copies of the recessive allele, one from each parent.

These mutations are not sex-linked and thus affect both males and females with equal probability, given that both sexes have two copies of each autosome. Recessive traits only manifest phenotypically when an organism is homozygous for the recessive allele. If an organism is heterozygous, carrying one normal and one mutated allele, the normal allele typically masks the presence of the recessive one, resulting in a wild-type phenotype.

As demonstrated in the Drosophila exercise, the suppressor of vermilion (su-v) is an autosomal recessive mutation. Flies need two copies of this suppressor mutation to revert the vermilion phenotype to wild-type. This concept is crucial for understanding how certain phenotypes can be rescued or altered by interactions between different genes.

Punnett Square

A Punnett square is a powerful tool used in genetics to predict the genotypes and probabilities of offspring from parental crosses. It allows students to visually map out all potential combinations of parental alleles and their chance of inheritance by the offspring.

The square is comprised of a grid, where each column and row represents the potential gametes from each parent. By filling out the grid with possible allele combinations, the Punnett square helps simplify complex genetic calculations. It methodically illustrates how alleles are separated during gamete formation and how they recombine at fertilization.

In the Drosophila example given, the Punnett square helps to visualize the outcome of both the F1 and F2 generations, showing the inheritance patterns for the X-linked vermilion eye color and autosomal suppressor mutation. By examining the patterns in the Punnett square, one can predict that all F1 and F2 offspring will have wild-type eyes.

Phenotypic Ratios

The phenotypic ratio is a genetic term used to describe the relative number of offspring manifesting different traits following a cross. When analyzing genetic crosses, we're often interested in the observable traits - the phenotypes - that result from the complex interactions of alleles.

These ratios provide an insight into the dominant and recessive nature of alleles, as well as how they interact when paired. In a simple scenario, a classic Mendelian monohybrid cross between two heterozygous organisms typically yields a phenotypic ratio of 3:1, with three offspring showing the dominant phenotype for every one exhibiting the recessive trait.

However, as is apparent from the Drosophila exercise solution, the presence of genotypic interactions can lead to phenotypic ratios that deviate from these Mendelian expectations. In this case, the F1 and F2 generations both show a 100% prevalence of the wild-type phenotype for eye color, indicating a non-Mendelian phenotypic ratio that is influenced by the suppressor mutation and its interaction with the X-linked gene affecting eye color.