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 \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of the following crosses: (a) vermilion females \(\times \quad\) brown males (b) brown females \(\times\) vermilion males (c) white females \(\times\) wild males

Short answer

In cross (a), all F1 offspring will have vermilion eye color. This is because they will inherit at least one dominant vermilion allele on the X chromosome, which causes the vermilion eye color. Their brown alleles (Aa) won't be expressed as it is a recessive trait.

Step-by-step solution

Identify the genotypes of the parents

Vermilion females have the genotype X^V X^V (vermilion is X-linked and dominant), while brown males have the genotype X^+ Y and aa (brown is autosomal recessive).

Determine the genotypes of the F1 offspring

All F1 offspring will inherit one X chromosome from their mother and one autosome from their father. So, the F1 offspring will have the following genotypes: Females: X^V X^+, Aa Males: X^V Y, Aa

Determine the phenotypes of the F1 offspring

All F1 offspring will have vermilion eyes due to the presence of at least one dominant vermilion allele on the X chromosome. Their brown alleles (Aa) won't be expressed as it is recessive.

Determine the genotypes of the F2 offspring

To obtain the F2 generation, we need to cross the F1 offspring (X^V X^+, Aa) with themselves. Using a Punnett square for both the X chromosome and the autosomes, we get the following F2 genotypes: Females: (1/4) X^V X^V, AA; (1/2) X^V X^+, Aa; (1/4) X^V X^+, aa; Males: (1/2) X^V Y, AA; (1/2) X^V Y, Aa

Determine the phenotypes of the F2 offspring

All F2 offspring will have the vermilion eye color, since none of them received both the brown and vermilion mutations. #(b) brown females \(\times\) vermilion males#

Identify the genotypes of the parents

Brown females have the genotype X^+ X^+ and aa, while vermilion males have the genotype X^V Y.

Determine the genotypes of the F1 offspring

All F1 offspring will inherit one X chromosome from their mother and one autosome from their father. So, the F1 offspring will have the following genotypes: Females: X^+ X^V, Aa Males: X^+ Y, Aa

Determine the phenotypes of the F1 offspring

All F1 offspring will have vermilion eyes due to the presence of at least one dominant vermilion allele on the X chromosome. Their brown alleles (Aa) won't be expressed as it is recessive. #(c) white females \(\times\) wild males#

Identify the genotypes of the parents

White females have the genotype X^V X^V and aa, while wild males have the genotype X^+ Y and AA.

Determine the genotypes of the F1 offspring

All F1 offspring will inherit one X chromosome from their mother and one autosome from their father. So, the F1 offspring will have the following genotypes: Females: X^V X^+, aa Males: X^V Y, Aa

Determine the phenotypes of the F1 offspring

Females will have white eyes because they inherited both vermilion (X^V) and brown (aa) mutations, causing a lack of pigmentation. Males will have vermilion eyes due to the presence of one dominant vermilion allele on the X chromosome.

Key concepts

X-linked Inheritance

In Drosophila genetics, X-linked inheritance refers to traits that are carried on the X chromosome. Since males have only one X chromosome (XY) and females have two (XX), the inheritance patterns can differ between genders. In the case of the vermilion eye color mutation, this trait is X-linked and dominant. Males exhibiting this trait have a single X chromosome carrying the vermilion allele, while females can have one or two such alleles. This means that if a female Drosophila is heterozygous with one vermilion allele (X^V X^+), she will still express the vermilion phenotype due to the dominance of the trait. On the other hand, a male requires only one vermilion allele on his single X chromosome to express the trait. This pattern is crucial in predicting the outcome of genetic crosses, as it explains why certain traits might appear in offspring depending on the parental genotypes.

Autosomal Recessive Mutation

The brown eye mutation in Drosophila signifies an autosomal recessive trait. Unlike X-linked traits, autosomal recessive mutations are carried on one of the non-sex chromosomes, and both sexes have them in pairs. For the brown eye color to manifest, the Drosophila must be homozygous recessive (aa) since a dominant allele (A) would mask the trait. In genetic crosses involving brown mutations, offspring need to inherit two copies of the recessive allele for the trait to be apparent. This underscores the importance of understanding the inheritance patterns of autosomal traits, as they operate differently from X-linked ones in Drosophila. When performing a cross between a heterozygous individual and a homozygous one, the chance of recessive traits showing up in the offspring is significantly affected by the presence or absence of dominant alleles.

Phenotype Prediction

Predicting the phenotype of offspring involves understanding the genetic composition of the parents and how their alleles will combine. Phenotypes are the observable traits, like eye color in Drosophila, which arise from the interaction of alleles. In a cross involving a vermilion (X-linked dominant) female and a brown (autosomal recessive) male, the vermilion allele typically dominates, causing most offspring to show vermilion eyes. It's the combination of homozygous and heterozygous allele pairs that will suggest the phenotype. For instance, if one parent is vermilion and the other is brown, you will observe offspring expressing the vermilion phenotype thanks to the X-linked dominant trait's presence. This prediction process necessitates attention to the dominance and locus of each allele, thereby offering insights into how genetic traits are expressed in progeny.

Punnett Square Analysis

The Punnett square is an essential tool in genetics for predicting the genotypes of offspring and, consequently, their phenotypes. By laying out possible allele combinations from each parent, you are able to visualize how X-linked or autosomal traits may be inherited. In the example of Drosophila genetic crosses, establishing the possible genotypes of the F1 and F2 generations involves setting up standard Punnett squares for X-linked (X^V, X^+) and autosomal (A, a) alleles. The rows and columns represent the parent's alleles. By filling in the squares, you can identify all potential genotypes. This visual aid makes it easier to see why vermilion can mask brown, or why a phenotype might manifest only if both recessive alleles are present. In essence, the Punnett square provides a clear framework for genetic predictions and helps simplify the complex nature of trait inheritance.