Question

The genes dumpy\((d p),\) clot \((c l),\) and apterous \((a p)\) are linked on chromosome II of Drosophila. In a series of two-point mapping crosses, the following genetic distances were determined. What is the sequence of the three genes?

Short answer

Based on the provided genetic distances, determine the sequence of the three genes on chromosome II of Drosophila using the two-step process described in the solution.

Step-by-step solution

Identify possible gene orders

There are three possible gene orders for these genes: 1. dumpy -> clot -> apterous 2. dumpy -> apterous -> clot 3. clot -> dumpy -> apterous

Compare genetic distances to find the correct gene order

We will now examine each of the gene orders and their corresponding genetic distances to determine the correct gene order: 1. dumpy -> clot -> apterous - In this order, the distance between dumpy and clot will be 'x', and the distance between clot and apterous will be 'y'. The distance between dumpy and apterous should be the sum of x and y: x + y = z 2. dumpy -> apterous -> clot - In this order, the distance between dumpy and apterous will be 'z', and the distance between apterous and clot will be 'y'. The distance between dumpy and clot should be the sum of z and y: z + y = x 3. clot -> dumpy -> apterous - In this order, the distance between clot and dumpy will be 'x', and the distance between dumpy and apterous will be 'z'. The distance between clot and apterous should be the sum of x and z: x + z = y Now, using the genetic distances provided in the exercise, check which of the above conditions hold true. The gene order corresponding to the condition that holds true is the correct sequence of the three genes.

Key concepts

Gene Linkage

Gene linkage refers to the tendency of genes to be inherited together because they are located on the same chromosome. The closer two genes are to each other on a chromosome, the less likely they are to be separated during crossing over in meiosis, which means they are more likely to be inherited together. This concept is a cornerstone of genetic mapping.

In Drosophila, or fruit flies, which are commonly used in genetic studies, genes that exhibit linkage can be mapped in relation to each other through a series of crosses. Understanding gene linkage allows researchers and geneticists to predict genetic ratios and inheritance patterns more accurately, which can be vital for studying genetic diseases, evolutionary biology, and for breeding purposes in agriculture.

Chromosome Mapping

Chromosome mapping, often known as genetic mapping, is a technique used to determine the position of genes on a chromosome. The fundamental principle is to calculate genetic distances between different genes based on the likelihood that they will be separated during chromosomal crossover events. These distances are typically expressed in units called centimorgans (cM), indicating the frequency of recombination between genes.

Genetic maps provide a visual representation of gene order and distances on a chromosome, which can be crucial for understanding the genetic architecture of organisms like Drosophila. These maps are constructed from experimental data gathered through various types of genetic crosses, including two-point and three-point crosses. The precision of the map depends on numerous factors, such as the number of individuals in the study and the amount of recombination observed.

Two-Point Mapping Crosses

Two-point mapping crosses are experiments designed to determine the distance between two genetic markers—such as mutations or genes—on the same chromosome. In these crosses, organisms that are heterozygous for the genetic markers are bred, and the progeny are observed to see which combinations of markers are present. The frequency with which the two markers are inherited together versus apart is used to estimate the genetic distance between them.

For example, in the exercise with Drosophila, the genetic distances observed between pairs of genes allow us to infer their order on the chromosome. The underlying assumption is that if two genes are closer to one another, there will be fewer recombinations between them, resulting in a lower recombination frequency and a smaller genetic distance.

Genetic Distances

Genetic distances measure the relative positions of genes on a chromosome. These distances are based on the frequency of recombination events that occur between them, with one centimorgan representing a 1% chance that a segment of DNA will cross over between homologous chromosomes during meiosis.

In practical terms, understanding genetic distances helps with predicting the likelihood that certain alleles will be transferred together during reproduction. It also provides context for the mapping exercise mentioned earlier, where the observed recombination frequencies of the genes 'dumpy', 'clot', and 'apterous' were analyzed to deduce their order on the fruit fly's chromosome. By combining these measures with the logical deductions from the sequence of genes, we finalize a genetic map that reflects the physical reality of their placement within the genome.