Question

In the fruit fly, Drosophila melanogaster, the gene for white eyes is sex- linked recessive, \(w\). Its wild-type allele, \(w^{+}\), gives dull red eyes. The gene for black body, \(b\), is an autosomal recessive and contrasts its wild- type allele, \(\mathrm{b}^{+}\), which gives a gray body. For a cross between females who are heterozygous for both genes, and white-eyed, black-bodied males, determine the phenotypes of the progeny, and in what proportions they occur.

Short answer

The phenotypes of the progeny and their proportions are as follows: \(50\%\) will have red eyes and gray body (WwBb), \(25\%\) will have red eyes and black body (Wwbb), \(12.5\%\) will have white eyes and gray body (wwBb), and \(12.5\%\) will have white eyes and black body (wwbb).

Step-by-step solution

Identify the Parents' Genotypes

The genotypes of the parents are given in the problem. The females are heterozygous for both genes (WwBb), and the males are white-eyed, black-bodied (wwbb). Please note, for the males, the first allele is lowercase because the white eyes is a sex-linked recessive trait.

Determine Possible Gametes for Each Parent

Each parent can produce different types of gametes (sex cells) due to independent assortment. For the heterozygous female, the possible gametes are WB, Wb, wB, wb. For the white-eyed, black-bodied male, the only possible gamete is wb.

Create a Punnett Square

To determine the genotypes of the progeny, a Punnett square can be used to calculate the frequencies of each possible combination of alleles that result from the cross. The female's possible gametes are across the top, and the male's possible gametes down the side.

Determine the Progeny Genotypes

After filling in the Punnett square, the genotypes of the progeny can be determined. Each box in the Punnett Square represents a potential offspring genotype: WWBB, WWBb, WWbb, WwBB, WwBb, Wwbb, wwBB, wwBb, wwbb.

Determine the Progeny Phenotypes

Finally, the physical characteristics (phenotypes) associated with each genotype should be determined. Here are simplified assumptions: 1. A genotype with at least one 'W' will have red eyes because 'W' is dominant over 'w'. 2. A genotype with 'ww' will have white eyes because it shows the recessive characteristic. 3. A genotype with at least one 'B' will have a gray body. 4. A genotype with 'bb' will have a black body.

Determine Proportions

Count the number of each phenotype to determine their proportions. Remember that these proportions are theoretical, based on probability; the actual proportions in a real cross can vary due to chance. Doing these steps carefully, one can figure out what proportion of the progeny will display each phenotype.

Key concepts

Drosophila melanogaster

Drosophila melanogaster, commonly known as the fruit fly, is a small insect that has become a cornerstone of genetic research. It is particularly favored in laboratories due to its short life cycle and simplicity of breeding. Researchers appreciate how its genetics exhibit clear inheritance patterns, which can be observed over many generations in a short period. This makes fruit flies excellent subjects for studying principles of genetic crosses.

Moreover, Drosophila melanogaster shares a surprising amount of genetic similarity with humans, making it relevant for understanding fundamental biological processes. Its well-mapped genome and easily recognizable mutations, like eye color, allow scientists and students alike to explore genetic concepts such as dominance, recessiveness, and the effects of sex-linked and autosomal genetic traits.

Punnett square

The Punnett square is a simple yet powerful tool used to predict the potential outcomes of a genetic cross. By laying out possible gametes (or sex cells) from each parent, this grid allows you to visualize how different combinations of alleles can form in the offspring.

In the context of the original problem, the Punnett square helps demonstrate the genetic possibilities when crossing Drosophila with sex-linked and autosomal recessive traits. You'd list the female's gametes across the top and the male's along the side. By filling in the grid, you calculate the likelihood of each genotype—thus determining the potential phenotypes of the progeny.

• Dominant traits are typically represented by uppercase letters.
• Recessive traits use lowercase letters.
• Each square represents a possible genotype that could result from the parental cross.

This structured approach simplifies complex inheritance patterns for better understanding.

Sex-linked traits

Sex-linked traits, often associated with the X chromosome, display unique inheritance patterns because their expression can differ between genders. In Drosophila melanogaster, the gene responsible for eye color is located on the X chromosome. White eyes ( ws ) appear due to a recessive allele, while dull red eyes ( w^{+} ) result from the dominant allele.

In males, with only one X chromosome, just one recessive white-eye allele is enough for the trait to be expressed, since they lack a second X chromosome to potentially carry the dominant allele. In females, both X chromosomes must carry the recessive allele for the trait to be visible. This results in a higher frequency of sex-linked recessive traits appearing in males than in females.

• Males: Represented by XwY (white eyes, recessive) or Xw⁺Y (red eyes, dominant).
• Females: Require two Xw alleles for a white-eyed phenotype.

Understanding these patterns is essential in predicting the outcomes of sex-linked genetic crosses.

Autosomal recessive traits

Autosomal recessive traits are inherited through non-sex chromosomes (autosomes) and require two copies of the recessive allele for expression. In the provided exercise, the gene for a black body is autosomal recessive, denoted by 'b'.

An individual must inherit 'bb' to exhibit the black body trait. If a Drosophila fly carries one or two of the dominant 'B' allele, it will display the gray body phenotype.

• Dominant allele (B): Results in a gray-bodied phenotype.
• Recessive allele (b): Results in a black-bodied phenotype, visible only when the genotype is 'bb'.

Autosomal traits, unlike sex-linked traits, have equal probability to be passed on to both male and female progeny. Understanding these principles helps unravel the inheritance patterns in broader genetic studies.