Question

In Drosophila, two mutations, Stubble (Sb) and curled (cu), are linked on chromosome III. Stubble is a dominant gene that is lethal in a homozygous state, and curled is a recessive gene. If a female of the genotype \\[ \frac{S b}{+}+ \\] is to be mated to detect recombinants among her offspring, what male genotype would you choose as a mate?

Short answer

Answer: The ideal male genotype for the mating is \(+cu/cu\).

Step-by-step solution

General information about the given genotype

We are given a female Drosophila with genotype \(\frac{Sb}{+}+\), which means she has one dominant Stubble allele, one normal (wild-type) allele and a homozygous wild-type allele for the curled gene. Since the Stubble gene is dominant and lethal in a homozygous state, the female must have a normal (wild-type) allele for survival. Because the curled gene is recessive, it means its phenotype is not visible unless in a homozygous state.

Choosing the male genotype for the mating

To detect recombinants among the offspring, the male must carry the recessive curled allele. This is because if the offspring inherit the curled allele, they will show the curled phenotype which can be an indication of recombination between the Sb and cu genes. The ideal male genotype for the mating would be homozygous for the curled gene (\(cu/cu\)), and carrying a normal (wild-type) allele for the Stubble gene since it's lethal in the homozygous state. Thus, the male genotype should be \(+cu/cu\).

Offspring genotypes and recombinant detection

By mating the female with the genotype \(\frac{Sb}{+}+\) and the male genotype \(+cu/cu\), we should expect offspring with the following genotypes: \(\frac{Sb}{+}+\), \(\frac{Sb}{+}cu\), \(+cu\), and \(+cu/cu\). The offspring that inherit the curled allele from the male parent will show the curled phenotype, indicating a recombination event between the Stubble and curled genes. The offspring genotypes with curled alleles (\(\frac{Sb}{+}cu\) and \(+cu/cu\)) will allow us to detect recombinants among the offspring.

Key concepts

Drosophila Genetics

The study of Drosophila, commonly known as the fruit fly, has been a cornerstone of genetic research for over a century. The reasons for this are manifold: their short life cycle, large number of offspring, and relatively simple genome make them ideal subjects for studying inheritance patterns.

Drosophila genetics often involves tracking mutations through generations to understand how genes are inherited. A female fruit fly, for example, can have various genotypes that will affect not only her phenotype but also the potential inheritance of traits in her offspring.

In the exercise provided, we are considering a female with a genotype indicative of a Stubble (Sb) and normal (+) allele combination. Understanding her genotype is crucial because it will affect the phenotype of her offspring and the detection of recombinants—a key concept in genetics which refers to offspring with combinations of alleles that differ from those in either parent due to crossing over during meiosis.

Linked Genes

Genes that are located close to each other on the same chromosome, such as the Stubble and curled genes in the Drosophila exercise, are said to be linked. This physical proximity means they tend to be inherited together during meiosis, as they are less likely to be separated by recombination.

In cases of linked genes, Mendel's principle of independent assortment does not fully apply because the genes do not assort independently from one another. The likelihood of recombination between these genes is a function of their distance from each other on the chromosome—the closer they are, the less likely it is for them to be separated.

To detect recombination events, genetic crosses are performed, such as the mating of the female fruit fly with a specific male genotype. The male is chosen to highlight the possible separation of linked genes, and the resulting offspring are then analyzed to see if recombination has occurred. It is these genetic shuffles during recombination that can lead to genetic diversity within a population.

Homozygous Lethality

Homozygous lethality occurs when an organism inherits two copies of a mutant allele that is lethal when present in the homozygous state. In the context of the Drosophila problem, the female carries the dominant Stubble (Sb) allele which is lethal when an individual has two copies of it. This means that any offspring inheriting Sb alleles from both parents would not survive.

To ensure survival of the offspring, a male must provide a non-lethal allele. That is why, in the exercise, the ideal male genotype chosen for mating is one with a normal allele for the Stubble gene and homozygous for the recessive curled gene. Such a mating strategy avoids passing on the homozygous condition for the Stubble gene, thus evading the lethal outcome.

Understanding homozygous lethality is important in genetics because it affects the outcomes of genetic crosses and has implications for the survival and genetic health of a population. In breeding experiments, it is crucial to select genotypes that prevent the expression of lethal phenotypes, which would otherwise limit the success of the experiment and the ability to study genetic recombination.