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

(a) vermilion females × brown males, (b) brown females × vermilion males, and (c) white females × wild-type males. For crosses (a) and (b), the F2 phenotypic ratios will be: - 3/4 female offspring with vermilion eyes and 1/4 female offspring with wild-type eyes. - 1/2 male offspring with vermilion eyes and 1/2 male offspring with wild-type eyes. For cross (c), the F2 phenotypic ratios will be: - 1/2 female offspring with brown eyes and 1/2 female offspring with white eyes. - 1/4 male offspring with white eyes, 1/4 male offspring with vermilion eyes, and 1/2 male offspring with brown eyes.

Step-by-step solution

Determine the genotypes of the parents

Since vermilion is an X-linked mutation and the female is vermilion, her genotype is \(X^{v}X\) (v for vermilion). The male is brown, an autosomal recessive mutation, so his genotype must be \(bb\).

Predict the F1 offspring genotype and phenotype

The Punnett square should consider the expected genotypes for the offspring: Female: \(X^{v}X \times X^{+}Y = X^{v}X^{+}\) (heterozygous, vermilion eyes) Male: \(X^{v}X \times X^{+}Y = X^{v}Y\) (vermilion eyes) All female offspring will have vermilion eyes, while all male offspring will also have vermilion eyes.

Cross the F1 generation

To predict the F2 generation, we will cross the F1 offspring (heterozygous females × vermilion males (\(X^{v}X^{+} \times X^{v}Y\))): Female: \((X^{v}X^{+}) \times (X^{v}Y) = X^{v}X^{v}, X^{v}X^{+}, X^{v}X^{+}, X^{+}X^{+}\) (vermilion, vermilion, vermilion, wild-type) Male: \((X^{v}X^{+}) \times (X^{v}Y) = X^{v}Y, X^{+}Y\) (vermilion, wild-type)

Calculate the phenotypic ratio in F2

In the F2 generation, the phenotypic ratio will be: \(\frac{3}{4}\) female offspring with vermilion eyes and \(\frac{1}{4}\) female offspring with wild-type eyes, \(\frac{1}{2}\) male offspring with vermilion eyes and \(\frac{1}{2}\) male offspring with wild-type eyes. #Cross (b): Brown females × Vermilion males# We won't go through all the steps in full detail, but the process will be similar to cross (a). In this case, we will have: Parents: \(bbX^{+}X \times X^{v}Y\) F1 generation females: \(bbX^{v}X\) (brown eyes) F1 generation males: \(bbX^{+}Y\) (brown eyes) The F2 generation will result in the same phenotypic ratio as in cross (a). #Cross (c): White females × Wild-type males# Again, we will follow a similar process to determine the genotypes of the parents, predict the F1 and F2 offspring genotype and phenotype, and calculate the phenotypic ratio: Parents: \(bbX^{v}X \times X^{+}Y\) F1 generation females: \(bbX^{v}X^{+}\) (brown eyes) F1 generation males: \(bbX^{v}Y\) (white eyes) The F2 generation will result in a different phenotypic ratio: \(\frac{1}{2}\) female offspring with brown eyes and \(\frac{1}{2}\) female offspring with white eyes, \(\frac{1}{4}\) male offspring with white eyes, \(\frac{1}{4}\) male offspring with vermilion eyes, and \(\frac{1}{2}\) male offspring with brown eyes.

Key concepts

Understanding the Punnett Square

A Punnett square is a straightforward visual representation used in genetics to predict the possible genotypes of offspring from a particular cross. Imagine it as a sort of a genetic grid chart. How does it work? Well, you put the alleles of one parent along the top and the alleles of another parent along the side. Then, you fill in the squares by combining these alleles to see all potential combinations for their offspring.

For many students, the Punnett square can seem confusing at first. A helpful tip is to carefully distinguish between the capital letters (which often represent dominant alleles) and lowercase letters (which typically signify recessive alleles). Also, remember that for X-linked traits, males have only one X chromosome, so they express whatever allele is present on that X, which influences the inheritance pattern you see in these crosses.

X-linked Inheritance Explained

X-linked inheritance refers to genes that are located on the X chromosome. What stands out in X-linked inheritance is how traits are passed down differently in males and females. Males have only one X chromosome (XY), while females have two (XX). This means that if a male inherits a recessive allele for an X-linked trait, he will express that trait, since there's no 'backup' allele like in females. For females to express an X-linked recessive trait, they must inherit two copies of the recessive allele.

Understanding this is vital for grasping why diseases like hemophilia and colorblindness are more common in males. In the exercise, vermilion eyes are an X-linked trait in Drosophila flies, leading to distinctive inheritance patterns that are quite different from autosomal inheritance.

Autosomal Recessive Mutation

What do we mean by an 'autosomal recessive mutation'? It's a change in the DNA sequence on one of the autosomes (non-sex chromosomes) that can lead to a certain trait only if a person has two copies of that mutated gene. For the trait to manifest, an individual must receive one copy of the gene from each parent. In genetic jargon, we call these individuals homozygous recessive.

When working with autosomal recessive mutations, it's essential to remember that carriers, or heterozygotes, don’t show the trait but can pass the mutation on to their offspring. The exercise references brown eyes in Drosophila as an autosomal recessive trait, meaning that flies need two copies of the 'b' allele to have this eye color.

Figuring Out the Phenotypic Ratio

The phenotypic ratio is essentially the ratio of different physical traits (phenotypes) that show up in the offspring from a genetic cross. It's important to realize that this ratio can tell us a lot about the underlying genetics at play. In simpler terms, these numbers give us hints about the dominance and recessiveness of alleles, as well as whether traits are autosomal or X-linked.

In the exercise, we get different phenotypic ratios depending on the type of cross. Understanding how to calculate these ratios comes from carefully analyzing the crosses and the probability of each phenotype. Practice makes perfect here, so if at first these ratios seem puzzling, performing more Punnett square exercises and working through more genetic cross problems will help you interpret these ratios like a pro.