Question

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

Short answer

Short Answer: When an attached-X female fly with white eyes (X^w X^w Y) is crossed with a male fly with miniature wings (X^m Y), the offspring will have the following genotypes and phenotypes: Females: - 50% with Wild-type eyes and Miniature wings (genotype X^w X^m). Males: - 50% with White eyes and Wild-type wings (genotype X^w Y). - 50% with Wild-type eyes and Miniature wings (genotype X^m Y).

Step-by-step solution

Identify the genotypes of the parent flies

The problem states that we have an attached-X female fly with genotype XXY, showing the X-linked recessive white-eye phenotype. This means the genotype of this female fly is (X^w X^w Y), with X^w representing the allele for white eyes. We also have a male fly expressing the X-linked recessive miniature wing phenotype, which means its genotype is (X^m Y), where X^m represents the allele for miniature wings.

Determine the possible gametes

Now, let's consider the possible gametes each parent fly can produce. The female fly (X^w X^w Y) can produce two types of gametes: X^w and Y. The male fly (X^m Y) can also produce two types of gametes: X^m and Y.

Use a Punnett square to predict offspring genotypes

To predict the genotypes of the offspring, we will use a Punnett square. In this case, we have a 2x2 Punnett square with female gametes on the top and male gametes on the side: X^w Y ____________ X^m | | X^w X^m | X^m Y | ____________ Y | | X^w Y | Y Y | ____________

Analyze the genotypes and determine the phenotypes of the offspring

We can now analyze the genotypes and phenotypes of the offspring. The four possible genotypes are: 1. X^w X^m: Female with one dominant eye-color allele and one recessive miniature wing allele. (Wild-type eyes, Miniature wings) 2. X^w Y: Male with a recessive eye color allele (White eyes, Wild-type wings) 3. X^m Y: Male with a recessive wing-size allele (Wild-type eyes, Miniature wings) 4. Y Y: This genotype is not viable in flies – no offspring. Overall, the offspring will have the following genotypes and phenotypes: Females: - 50% with Wild-type eyes and Miniature wings (genotype X^w X^m). Males: - 50% with White eyes and Wild-type wings (genotype X^w Y). - 50% with Wild-type eyes and Miniature wings (genotype X^m Y).

Key concepts

X-linked Genes

X-linked genes are genes located on the X chromosome, one of the two sex chromosomes in many organisms, including fruit flies and humans. These genes are inherited differently in males and females, due to their presence on the X chromosome only. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).

This means that a male has only one copy of each X-linked gene, whereas a female has two. As a result, recessive X-linked traits appear more frequently in males. This is because they do not have a second X chromosome to potentially "mask" the recessive allele if it is present. In the context of the fruit fly exercise, examples of X-linked traits include white eyes and miniature wings, where both are recessive phenotypes.

Understanding the nature of X-linked inheritance helps explain the differences in phenotypes between male and female offspring when dealing with X-linked recessive traits.

Punnett Square

A Punnett square is a simple yet powerful diagram used in genetics to predict the genotypes of offspring from a particular cross or breeding experiment. It provides a visual representation of all possible combinations of alleles from the male and female parents.

In the fruit fly exercise, a 2x2 Punnett square was used because each parent produces two types of gametes. The female fly (X^w X^w Y) can produce X^w and Y gametes, while the male fly (X^m Y) can produce X^m and Y gametes.

The Punnett square organizes these possible gametes along the top and side, and allows for the determination of potential genotypes for their offspring. This visual method helps clarify which traits may appear in future generations based on the genetic information from the parents.

Phenotypic Outcomes

Phenotypic outcomes refer to the observable traits of an organism resulting from the interaction of its genotype with the environment. Phenotype includes attributes such as eye color, wing size, and also more complex traits.

In the given exercise, various phenotypic outcomes were predicted for the offspring of the cross between the fruit flies. These include:

• Females with wild-type eyes and miniature wings from the X^w X^m genotype. Though carrying recessive alleles for miniature wings, they exhibit wild-type wings due to the presence of dominant alleles from the male parent.
• Males with white eyes from the X^w Y genotype, highlighting their recessive nature due to the lack of a corresponding "normal" allele on the non-existent second X chromosome.
• Males with wild-type eyes and miniature wings from the X^m Y genotype exhibit dominant traits, having only one X chromosome from the father with the recessive allele for wild-type eyes and miniature wings as a phenotype.

Understanding phenotypic outcomes allows scientists to predict how traits are passed on and express themselves in future generations.

Genotype Analysis

Genotype analysis involves examining the genetic makeup of an organism to understand its heritable traits better. In this context, it looks at the specific alleles present on the chromosomes that determine an organism's phenotype.

In the exercise with the fruit flies, genotype analysis helps determine the potential characteristics of offspring from a given set of parent flies. For example, a female fly with an X^w X^m genotype results in wild-type eyes despite having a recessive allele for white eyes. This is because she has one dominant allele from the father that masks the recessive trait.

Male offspring have simpler genotype analysis, as they possess only one X chromosome. Thus, any recessive X-linked trait they inherit will immediately appear as their phenotype, such as white eyes in the case of genotype X^w Y.

Genotype analysis in studies like these is crucial for understanding how genetic variations influence traits and for predicting outcomes in genetics, breeding, and medicine.