Question

In Drosophila, the two mutations Stubble bristles (Sb) and curled wings ( \(c u\) ) are linked on chromosome III. Sb is a dominant gene that is lethal in a homozygous state, and \(c u\) 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 her mate?

Short answer

Answer: The male genotype to best detect recombinants among the offspring when mated with a female genotype of \(Sb/+\) is \(Sb\ sb\ \ cu\ cu\).

Step-by-step solution

Understanding the information given

We are given a female genotype that has Stubble bristles (Sb) and the wild type (+) paired on her chromosome III. The Stubble bristles gene is dominant, but lethal when homozygous. The curly wings gene (\(cu\)) is recessive. The female's genotype is given as: \\[ \frac{Sb}{+} \\]

Identifying the possible male genotypes

Since the Sb gene is dominant and lethal in a homozygous state, as well as the \(cu\) gene being recessive, there are 3 possible genotypes for a male that could be the mate: 1. Homozygous recessive for both traits: \(sb\ sb\ \ cu\ cu\) 2. Heterozygous for Stubble bristles and homozygous recessive for curled wings: \(Sb\ sb\ \ cu\ cu\) 3. Homozygous recessive for Stubble bristles and heterozygous for curled wings: \(sb\ sb\ \ Cu\ cu\)

Evaluating the potential outcomes

We want a male genotype that, when mated with a female, will help us detect recombinants among the offspring. Let's examine the potential offspring genotypes for each male option: 1. Homozygous recessive for both traits: \(sb\ sb\ \ cu\ cu\) Offspring genotypes would include: *Sb sb Cu cu (wild type with no curly wings) *sb sb Cu cu (recessive phenotype with no Stubble bristles) 2. Heterozygous for Stubble bristles and homozygous recessive for curled wings: \(Sb\ sb\ \ cu\ cu\) Offspring genotypes would include: *Sb sb Cu cu (recessive phenotype) *Sb sb cu cu (recessive phenotype for both traits) 3. Homozygous recessive for Stubble bristles and heterozygous for curled wings: \(sb\ sb\ \ Cu\ cu\) Offspring genotypes would include: *Sb sb Cu Cu (wild type) *Sb sb Cu cu (recessive phenotype for Stubble bristles)

Choosing the best male genotype

Considering the possible offspring genotypes from each potential male genotype, the best male genotype to detect recombinants would be the one that results in the most variety of phenotypes in the offspring. In this case, option 2 (Heterozygous for Stubble bristles and homozygous recessive for curled wings: \(Sb\ sb\ \ cu\ cu\)) would give the most variety of phenotypes. Therefore, the male genotype that should be chosen as the mate for the female is \(Sb\ sb\ \ cu\ cu\).

Key concepts

Linked Genes

In genetics, linked genes refer to genes located close together on the same chromosome, which tend to be inherited together during meiosis. In the case of Drosophila, the genes for Stubble bristles (Sb) and curled wings (\(cu\)) are examples of linked genes because they both reside on chromosome III.
Unlike unlinked genes that assort independently, linked genes do not follow the law of independent assortment. Instead, their close proximity reduces the chances of crossing over between them, meaning they often pass to the next generation as a pair.
Thus, understanding linked genes is vital when predicting offspring's genotypes, especially in exercises that require tracking mutant variations like in Drosophila.

Dominant and Recessive Alleles

Alleles can either be dominant or recessive. Dominant alleles, like the Stubble bristles (Sb), express their traits even if only one copy is present. This means that any organism carrying even one Sb allele will show the characteristic Stubble bristles. However, in a homozygous state, this trait is lethal for Drosophila.
On the other hand, the curly wings gene \(cu\) is recessive. A recessive trait only manifests when an organism has two copies of the recessive allele. Here, this means a Drosophila will only have curly wings if it possesses two copies of the \(cu\) allele.
This dominance-recessive relationship is crucial for understanding how traits like these are passed on to offspring, helping in allele segregation prediction.

Genotype and Phenotype Analysis

The representation of an organism's genetic makeup is known as its genotype, which directly influences the organism's phenotype—the physical expression of traits.
For instance, the female Drosophila described in the exercise has the genotype \(\frac{Sb}{+}\). Her phenotype will be Stubble bristles because the Sb allele is dominant, masking the wild-type (+) phenotype.
Genotype analysis involves examining the combination of alleles an organism has, while phenotype analysis focuses on the visible traits. In genetic exercises, this analysis helps predict the outcome of genetic crosses, revealing the potential appearance of offspring based on certain genotypic pairings.

Recombinant Offspring

Recombinant offspring are a result of genetic recombination, where linked genes are shuffled during meiosis due to crossing over between homologous chromosomes.
This genetic reshuffling creates new allele combinations different from the parent organisms, allowing detectable variations in offspring traits, which is crucial when studying genetic linkage and mapping.
In experiments like the Drosophila genetics problem, choosing the right parental genotypes is essential to successfully observe recombinants. Here, selecting a male genotype like \(Sb\ sb\ \ cu\ cu\) helps maximize variety, facilitating the detection of new combinations of Stubble bristles and curled wings among their offspring.

Lethal Alleles

Lethal alleles are mutations that can result in the death of an organism if present in a certain zygotic combination. In Drosophila, the Sb allele is an example of a dominant lethal allele. If an individual is homozygous for the Sb allele (\(Sb\ Sb\)), it does not survive.
The presence of lethal alleles necessitates careful consideration when predicting offspring viability from genetic crosses. While these alleles can offer interesting insights into genetics, they also serve as natural constraints on the genotypic distributions within populations.
Therefore, geneticists must be mindful of such alleles when analyzing breeding strategies to avoid nonviable progeny.