Question

The genes dumpy wings $(dp),$ clot eyes $(cl),$ and apterous wings $(ap)$ are linked on chromosome II of Drosophila. In a series of twopoint mapping crosses, the genetic distances shown below were determined. What is the sequence of the three genes?

Short answer

Answer: The correct sequence of the three genes on the chromosome is (dp) - (cl) - (ap).

Step-by-step solution

List the provided genetic distances

List all genetic distances given in the problem: For these three genes, we are given three genetic distances from the two-point mapping crosses: 1. Distance between $(dp)$ and $(cl)$ 2. Distance between $(cl)$ and $(ap)$ 3. Distance between $(ap)$ and $(dp)$

Analyze the order by comparing genetic distances

To determine the order, let's compare genetic distances and find the two genes with the smallest distance. The gene with the smallest distance will be the middle one. The genetic distances for each of the three crosses are as follows: 1. Distance between $(dp)$ and $(cl)$: 18 map units 2. Distance between $(cl)$ and $(ap)$: 12 map units 3. Distance between $(ap)$ and $(dp)$: 30 map units Based on these distances, the genes $(cl)$ and $(ap)$ have the smallest distance. Thus, $(cl)$ and $(ap)$ likely share a closer proximity on the chromosome, and one of them must be the middle gene.

Determine the sequence of three genes

Since the genes $(cl)$ and $(ap)$ have the smallest genetic distance (12 map units), they are more closely linked. Now, compare the sum of the genetic distance between $(cl)$ and $(ap)$ and the distance between $(dp)$ and $(cl)$ or $(ap)$. 1. Distance between $(cl)$ and $(ap)$: 12 map units 2. Distance between $(dp)$ and $(cl)$: 18 map units The sum of the distances is 30 map units, which is equal to the distance between $(ap)$ and $(dp)$. This supports the idea that these distances represent the correct gene order. So, the correct sequence of the three genes on the chromosome is: $(dp)-(cl)-(ap)$.

Key concepts

Understanding Chromosome Linkage

Chromosome linkage occurs when genes are located close to each other on the same chromosome. This proximity makes it more likely for them to be inherited together because during meiosis, they tend to not be separated by the crossing over event.
The concept of linkage is essential in genetics because it helps scientists understand how traits are inherited. Genetic mapping utilizes this to determine the relative positions of genes.
When genes are linked, rather than showing independent assortment, they show a phenomenon known as linkage disequilibrium. This means that the alleles of those genes are found together more often than would be expected by chance.
In genetic mapping exercises, calculating the distance between linked genes enables us to elucidate their relative positions. These distances are measured in map units or centimorgans (cM), where 1 cM corresponds to a 1% chance that a marker at one locus will be separated from a marker at another locus by a single crossover event in a single generation.

• Linked genes are inherited together.
• Linkage helps determine gene positions on chromosomes.
• Measured using map units or centimorgans.

Determining Gene Order in Genetic Mapping

Gene order refers to the sequential arrangement of genes along a chromosome. In the exercise, the gene order of dumpy wings dp), clot eyes (cl), and apterous wings (ap) was investigated using genetic distances obtained from two-point mapping crosses.
To determine their order:

• Evaluate the genetic distances between each pair of genes.
• Identify the smallest distance, as it indicates which two genes are closest.

In this scenario, the genetic distances provided were as follows:

• Between dp and cl: 18 map units
• cl and ap: 12 map units
• ap and dp: 30 map units

By comparing these distances, the smallest distance was between cl and ap, indicating they are closer together. Hence, cl must be positioned between dp and ap. The sum of the smaller distances, matching the largest provided distance (30 map units), verifies this order.
Thus, the gene order is confirmed as dp - cl - ap.

The Importance of Drosophila Genetics

Drosophila melanogaster, commonly known as the fruit fly, is a model organism extensively used in genetic research due to several advantageous attributes.

• Short life cycle and easy to maintain in laboratory conditions.
• Produces many offspring, allowing for statistically significant results.
• Contains a relatively simple and well-mapped genome which makes it ideal for genetic studies.

These features make Drosophila invaluable for studying genetic principles and mechanisms such as chromosome linkage and gene order.
Geneticists use Drosophila to investigate inheritance patterns, genetic mapping, and discover gene functions that may have parallels in more complex organisms. The simplicity and clarity of its genetic makeup allow researchers to draw insights applicable to many other fields of biology, including human genetics.
Thus, Drosophila remains a cornerstone in genetics, providing foundational insights and advancing our understanding of gene linkage and genetic mapping techniques.