Question

An attached-X female fly, \(\overline{X X} Y\) (see the "Insights and Solutions" box), expresses the recessive X-linked white-eye mutation. It is crossed to a male fly that expresses the X-linked recessive miniature-wing mutation. Determine the outcome of this cross in terms of sex, eye color, and wing size of the offspring.

Short answer

#Short Answer# The offspring resulting from the cross between a female fly with attached-X genotype and white-eye mutation (\(\overline{X X} Y\)) and a male fly with the X-linked recessive miniature-wing mutation (XmY) will display the following phenotypes: 1. Female with white eyes and miniature wings. 2. Female with red eyes and miniature wings. 3. Male with white eyes and normal wings. 4. Male with white eyes and miniature wings.

Step-by-step solution

Identify the genotypes of the parent flies

The female fly has an attached-X genotype with a recessive white-eye mutation and a normal wing size, which is represented as \(\overline{X X} Y\). The male fly has a normal eye color but carries the X-linked recessive miniature-wing mutation, represented as \(Xm Y\).

Set up a Punnett square for the cross

To determine the possible genotypes of the offspring, we will use a Punnett square. The Punnett square will have the female fly's genotypes on one side (rows) and the male fly's genotypes on the other side (columns). ``` Xm Y --------- \X | | | --------- \X | | | --------- Y | | | --------- ```

Fill in the Punnett square with the results of the cross

Now, we will fill in the Punnett square with the possible genotypes resulting from the cross between the female and male flies: ``` Xm Y --------- \X | Xm\X | \Y | --------- \X | XmX | XY | --------- Y | XmY | YY | --------- ```

Analyze the results and determine the phenotypes of the offspring

Based on the filled Punnett square, the possible genotypes of the offspring are as follows: 1. Xm\X (female with attached-X and miniature-wing mutation) 2. \Y (non-viable offspring due to missing X chromosome) 3. XmX (female with normal eye color and miniature-wing mutation) 4. XY (male with normal eye color and wing size) 5. XmY (male with white-eye mutation and miniature-wing mutation) 6. YY (non-viable offspring due to missing X chromosome) Now, we can predict the phenotypes of the surviving offspring: 1. Female with white eyes and miniature wings. 2. Female with red eyes and miniature wings. 3. Male with white eyes and normal wings. 4. Male with white eyes and miniature wings. Thus, the offspring of this cross will have a combination of different eye colors and wing sizes, with both males and females being produced.

Key concepts

X-linked recessive inheritance

Understanding X-linked recessive inheritance is crucial when analyzing genetic crosses, especially in organisms such as Drosophila melanogaster, commonly known as fruit flies. In X-linked recessive inheritance, the gene causing the trait or disorder is located on the X chromosome and is not dominant. This means females (XX), who have two X chromosomes, must have two copies of the recessive allele to express the trait, while males (XY), with only one X chromosome, express the trait when they inherit one copy of the recessive allele.

X-linked recessive traits often exhibit unique patterns in pedigree charts due to the difference in the number of X chromosomes between males and females. For instance, males cannot pass the trait to their sons because they contribute a Y chromosome to male offspring. However, they can pass the allele to all their daughters, who will be carriers if the other X chromosome is normal. Daughters who are carriers do not usually express the trait, but they have a 50% chance of passing the recessive allele to their own offspring. This pattern often results in more males than females expressing the trait in a population.

Genotype determination

The genotype of an organism refers to its specific genetic makeup—in other words, the combination of alleles it has for a certain gene. Determining genotypes is a fundamental step in genetics, serving as the starting point for predicting phenotype outcomes. The use of a Punnett square, a simple grid-based tool, greatly aids in visualizing the possible genotypes resulting from a cross.

For the Punnett square to serve its purpose, we begin by denoting the genotypes of the parental organisms on the rows and columns of the grid. In the case of sex-linked traits, such as the white-eye mutation in flies, it is important to differentiate between the X and Y chromosomes and recognize that males and females contribute different types of gametes. As the genes are located on the X chromosome, and males have only one X, their genotype directly determines the phenotype. Females, however, might have different combinations of the recessive and dominant alleles on their two X chromosomes, which can lead to various genotypic combinations. Once the Punnett square is populated with all possible combinations of parental alleles, we can easily interpret the matrix to list the potential genotypes of the offspring.

Phenotypic outcomes

After determining the potential genotypes using a Punnett square, the next step is to predict the phenotypic outcomes, meaning the physical expression of the genotypes. Phenotypes are influenced by the genotypes but can be also affected by factors such as environment and genetic interactions. In the context of X-linked traits, phenotypes become gender-specific due to the sex chromosomes involved.

In the given exercise, the genotypic combinations obtained from the Punnett square guide us to understand the likely appearance of the offspring in terms of eye color and wing size. For example, since the white-eye mutation is recessive, it will only be expressed phenotypically in males carrying the allele on their single X chromosome or in females carrying the allele on both X chromosomes. Similarly, the miniature-wing mutation's expression is contingent on its presence in the genotype of the fly. As such, males with the mutation on their X chromosome will exhibit miniature wings. Ultimately, analyzing the outcomes of each possible genotype allows us to grasp the inheritance pattern and predict phenotypes accurately for both sexes.