Question

In Drosophila, Dichaete ( \(D\) ) is a mutation on chromosome III with a dominant effect on wing shape. It is lethal when homozygous. The genes \(e\)bony body \((e)\) and pink eye (p) are recessive mutations on chromosome III. Flies from a Dichaete stock were crossed to homozygous ebony, pink flies, and the \(\mathrm{F}_{1}\) progeny with a Dichaete phenotype were backcrossed to the ebony, pink homozygotes. (a) Using the results of this backcross shown in the following table, diagram the cross, showing the genotypes of the parents and offspring of both crosses. (b) What is the sequence and interlocus distance between these three genes?

Short answer

Answer: The genotypes of the parent flies are (DdEePp) and (ddeepp), and the genotype of the F1 progeny flies is (D_EePp). To calculate the interlocus distance between the three genes, first calculate the recombination frequencies between ebony (e) and Dichaete (D), and Dichaete (D) and pink (p) using the offspring phenotype data. Next, arrange the genes in ascending order based on their recombination frequencies. Finally, multiply the recombination frequency by 100 to get the interlocus distance in centimorgans (cM).

Step-by-step solution

Identify the genotypes of the parents

The first stock flies are Dichaete \((D)\) crossed with homozygous ebony \((ee)\) and pink \((pp)\) flies. Since Dichaete \((D)\) is dominant and lethal when homozygous, these flies must be heterozygous for the Dichaete gene, having \((Dd)\) genotype. The genotypes of the parent flies are \((DdEePp)\) and \((ddeepp)\).

Identify the genotypes of the F1 progeny

In the first generation, the progeny with a Dichaete phenotype are selected for and backcrossed to the ebony, pink homozygotes (having \((ddeepp)\) genotype). Since Dichaete \((D)\) is dominant and only these flies are backcrossed, the genotype must be \((D_EePp)\).

Calculate the recombination frequencies

Using the backcross offspring results given in the table, we can calculate the recombination frequencies between ebony \((e)\) and Dichaete \((D)\), and Dichaete \((D)\) and pink \((p)\). Recombination frequency is equal to the sum of recombinant offspring divided by the total offspring. For example, the recombination frequency between ebony \((e)\) and Dichaete \((D)\) is the sum of ebony \((e)\) and Dichaete \((D)\) recombinant offspring divided by the total number of offspring. Let's denote the given number of offspring with each phenotype as follows: 1. \(N_{DEP}\) - Number of Dichaete, ebony, pink offspring 2. \(N_{Dep}\) - Number of Dichaete, ebony, non-pink offspring 3. \(N_{deP}\) - Number of non-Dichaete, ebony, pink offspring 4. \(N_{dep}\) - Number of non-Dichaete, ebony, non-pink offspring Then, based on the table provided, we calculate the recombination frequencies: Recombination frequency between ebony \((e)\) and Dichaete \((D)\): \(\frac{N_{Dep} + N_{deP}}{N_{DEP} + N_{Dep} + N_{deP} + N_{dep}}\) Recombination frequency between Dichaete \((D)\) and pink \((p)\): \(\frac{N_{DEP} + N_{deP}}{N_{DEP} + N_{Dep} + N_{deP} + N_{dep}}\)

Calculate the sequence and distance between the three genes

To determine the sequence and distance between the three genes, we first find the order of the genes. Since the recombination frequencies are known, the lowest recombination frequency corresponds to genes that are closest together. In this case, we use the recombination frequencies to arrange genes in ascending order. Next, we calculate the interlocus distance between the genes by multiplying the recombination frequency by 100 (since 1% recombination frequency corresponds to 1 map unit, or centimorgan (cM)). We can then use this map distance to get the sequence and interlocus distance between the three genes.

Key concepts

Gene Mapping

Gene mapping is like creating a genetic map that shows the positions of genes on a chromosome. In Drosophila genetics, this is a crucial concept as it helps us understand the layout of genes and the distances between them. Gene mapping provides insights into how traits might be inherited. It essentially creates a guide for predicting the likelihood of traits appearing in future generations.
To perform gene mapping, scientists often use information such as recombination frequencies to determine the relative locations of genes. By determining which genes sit closer together, scientists can establish sequences and see how information flows during reproduction. Imagine mapping like creating a map of a city, where genes are the various landmarks you want to visit!
In this exercise, we are interested in the genes related to the Dichaete phenotype, ebony body, and pink eyes in Drosophila. Gene mapping helps us figure out their order on the chromosome and the distances between them, which is essential for understanding genetic linkage and inheritance patterns.

Recombination Frequency

Recombination frequency is a measure used to determine the distance between genes on a chromosome based on how often they recombine during meiosis. It gives an idea of how "tied" or "linked" two genes are. The idea is based on the fact that genes located closer together on a chromosome are less likely to be separated by recombination.
When crossing over occurs during meiosis, the genetic material is exchanged between homologous chromosomes, shuffling the genes. This is where recombination frequency comes into play. It is calculated by taking the number of recombinant offspring (those with new combinations of traits that are different from the parent) and dividing it by the total number of offspring. This results in a percentage that can be used to estimate genetic distances.

• High recombination frequency: Genes are further apart.
• Low recombination frequency: Genes are closer together.

By using recombination frequencies, we can establish the order and distance of genes. For instance, in the flies with Dichaete, ebony, and pink mutations, we calculate recombination frequencies to determine how these genes are aligned on chromosome III.

Dominant and Recessive Mutations

Understanding dominant and recessive mutations is key to interpreting genetic inheritance. Dominant mutations are those that express their trait even if only one allele is present. Recessive mutations require both alleles to display the trait fully.
In our exercise with Drosophila, Dichaete \(D\) is a dominant mutation. This means if a fly has a single allele of Dichaete \(D\), the phenotype will be characterized by the Dichaete trait. Conversely, the mutations for the ebony body \(e\) and pink eyes \(p\) are recessive. These traits will be expressed only when the fly has two copies of each recessive allele (homozygous recessive).
Understanding whether a mutation is dominant or recessive helps in predicting the inheritance and expression of traits. It also plays a significant role in planning genetic crosses and analyzing their outcomes. Knowing how dominant and recessive traits work allows scientists to predict phenotypes of offspring based on the genetic makeup of the parents.