Question

Another cross in Drosophila involved the recessive, X-linked genes yellow \((y),\) white \((w),\) and cut \((c t) .\) A yellow-bodied, white-eyed female with normal wings was crossed to a male whose eyes and body were normal but whose wings were cut. The \(F_{1}\) females were wild type for all three traits, while the \(F_{1}\) males expressed the yellow-body and white-eye traits. The cross was carried to an \(\mathrm{F}_{2}\) progeny, and only male offspring were tallied. On the basis of the data shown here, a genetic map was constructed. (a) Diagram the genotypes of the \(F_{1}\) parents. (b) Construct a map, assuming that white is at locus 1.5 on the X chromosome. (c) Were any double-crossover offspring expected? (d) Could the \(F_{2}\) female offspring be used to construct the map? Why or why not?

Short answer

Answer: The genotype of the F1 female parent is yw+ct, and the genotype of the F1 male parent is +wct. The genetic map looks like this: ct - (3.15) - y - (26.58) - w.

Step-by-step solution

(Step 1: Determine the F1 Parents' Genotypes)

Since we know that the female parent has a yellow body, white eyes, and normal wings, we can conclude her genotype is yw+ct. The male parent has normal eyes and body but cut wings, so his genotype must be +wct.

(Step 2: Determine the F1 Progeny's Genotypes)

The fact that the F1 females have a wild-type phenotype (normal body, eyes, and wings) implies that they must have at least one dominant allele for each trait. Since we know that the traits are X-linked, we can deduce their genotypes to be YwCt. The F1 males, on the other hand, express the yellow body and white eye traits, which indicates that they have inherited the recessive alleles from their mother. Their genotype is yw+ct.

(Step 3: Map Construction)

As we are asked to assume that the white gene is located at position 1.5 on the X chromosome, we will use this information for map construction. The map shows the order of the genes and the distances between them in terms of map units, which corresponds to the frequency of recombination between the genes. 1. Take the given gene order: white (w), yellow(y), and cut (ct). 2. Calculate the recombinations frequencies: - w and y: (ywct + YwCt) / total = 615 / 2314 = 0.2658 - w and ct: (ywCt + Ywct) / total = 297 / 2314 = 0.1283 - y and ct: (yWCt + Ywct) / total = 73 / 2314 = 0.0315 3. Calculate the distances: - w and y: 0.2658 * 100 = 26.58 map units - w and ct: 0.1283 * 100 = 12.83 map units - y and ct: 0.0315 * 100 = 3.15 map units The final genetic map should look like the following: ct - (3.15) - y - (26.58) - w

(Step 4: Double-crossover Offspring)

Double-crossovers occur when two recombination events happen within the same chromosome pairing during meiosis. In this case, it would mean that both the y and ct genes and the w gene would undergo recombination within a single chromosome. This can be determined by looking at the product of the recombination frequencies between y and ct and w and ct: (0.0315) * (0.1283) = 0.00404 Since this is a non-zero probability, the double-crossover offspring are expected.

(Step 5: Using F2 Female Offspring for Map Construction)

Since the traits are X-linked recessive, F2 female offspring will only express the phenotypes associated with these traits if they are homozygous recessive for the respective genes. In cases when the female offspring inherit one dominant allele, they will have a wild-type phenotype, making it difficult to determine the gene combination. Additionally, since genetic maps are based on recombination frequencies, and the information about female offspring recombination is unavailable, it is not possible to use F2 female offspring to construct the genetic map.

Key concepts

X-linked Inheritance

Understanding X-linked inheritance is crucial when studying genetics, especially within the context of Drosophila fruit flies. This mode of inheritance refers to genes that are located on the X chromosome, one of the two sex chromosomes in many organisms. Typically, males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).

Since males inherit only one X chromosome, they are more likely to express X-linked traits, even if those traits are recessive, because they do not have a second X chromosome that could carry a dominant allele to mask the recessive one. Conversely, females must inherit two copies of the recessive allele, one on each X chromosome, to exhibit recessive X-linked traits. In the exercise given, because F1 males are displaying the recessive traits, we know they must have inherited their single X chromosome with these traits from their mother.

Recombination Frequency

The concept of recombination frequency relates to the likelihood of two alleles being separated during crossing over in meiosis. It's a cornerstone of genetic map construction, indicating the distance between genes. The higher the recombination frequency, the further apart the genes are likely to be on the chromosome.

These frequencies are often expressed as percentages, with 1% equating to one map unit (m.u.) or centimorgan (cM), a unit of measure for genetic linkage. The calculation involves dividing the number of recombinant offspring by the total number of offspring, providing insight into positional relationships between genes. In our exercise, recombination frequencies between the X-linked genes are used to construct the genetic map of the Drosophila.

Drosophila Genetics

The fruit fly, Drosophila, is a model organism in genetic research due to its simple genetic structure and quick life cycle. Studies on Drosophila genetics have significantly contributed to our understanding of genetic principles, including linkage and gene mapping.

The organism's chromosomes are easy to observe under a microscope, and their phenotypes, like eye color and wing shape, are easy to identify. When constructing genetic maps using Drosophila, one can exploit the clear-cut phenotypic differences that correspond to genotypic variations, an advantage exemplified in the exercise involving X-linked genes and their associated traits.

Double-Crossover

A double-crossover event is a more complex recombination event that happens when two separate crossovers occur within the same chromosome segment in meiosis. This can result in the exchange of genetic material between homologous chromosomes in a way that can be especially informative for mapping.

In our Drosophila example, the possibility of double-crossovers is calculated by multiplying the recombination frequencies of each pair of genes that could potentially be involved in such an event. Although double-crossovers are less common due to their lower probability, they can provide valuable information for fine-tuning genetic maps and identifying gene order.

Phenotype and Genotype Correlation

The phenotype and genotype correlation in genetics is a fundamental concept that helps researchers understand how the genetic makeup (genotype) of an organism influences its observable traits (phenotype). Geneticists can often predict the genotype of an individual by examining their phenotype, particularly when well-established rules of inheritance, like Mendelian genetics, apply.

In our exercise focusing on Drosophila's X-linked genes, the F1 and F2 generations' phenotypes directly reflect their genotypes. This is apparent as phenotypic traits like body color and wing shape are indicative of the specific alleles on the Drosophila’s X chromosome. Careful observation of offspring phenotypes is vital for determining genotypes and, consequently, for accurate genetic map construction.