In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.


Do you expect any of these gene pair(s) to assort independently? Explain your reasoning.

Verified step by step guidance

1. Understand the concept of genetic linkage and independent assortment: Genes that are located close to each other on the same chromosome are said to be linked and tend to be inherited together. Independent assortment occurs when genes are located far apart on the same chromosome or on different chromosomes, allowing them to segregate independently during meiosis.
2. Review the map positions of the genes provided: The body color gene (y⁺/y) is at position 0.0, the eye color gene (w⁺/w) is at position 1.5, and the bristle form gene (f⁺/f) is at position 56.7. These positions are measured in map units (m.u.), where 1 m.u. corresponds to a 1% recombination frequency.
3. Determine the distances between the genes: Calculate the genetic distances between each pair of genes. For example, the distance between the body color gene and the eye color gene is |1.5 - 0.0| = 1.5 m.u., and the distance between the body color gene and the bristle form gene is |56.7 - 0.0| = 56.7 m.u. Similarly, calculate the distance between the eye color gene and the bristle form gene.
4. Evaluate the likelihood of independent assortment: Genes that are more than 50 m.u. apart are expected to assort independently because the recombination frequency approaches 50%, which is equivalent to random assortment. Compare the calculated distances to this threshold to determine whether any gene pairs are likely to assort independently.
5. Conclude based on the distances: Based on the distances calculated, determine which gene pairs are close enough to be considered linked (less than 50 m.u.) and which are far enough apart to assort independently (greater than or equal to 50 m.u.). Provide reasoning for your conclusion, referencing the principles of genetic linkage and recombination.

Key Concepts

Gene Linkage

Gene linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together during meiosis. This occurs because linked genes do not assort independently, as they are physically connected on the chromosome. In the case of Drosophila, the proximity of the body color gene (0.0 m.u.) and the eye color gene (1.5 m.u.) suggests they are likely linked, while the bristle form gene (56.7 m.u.) is farther away, indicating a potential for independent assortment.

Map Units and Recombination Frequency

Map units, or centimorgans (cM), are a measure of genetic distance based on the frequency of recombination between two loci during meiosis. One map unit corresponds to a 1% chance of recombination occurring between two genes. The closer two genes are on a chromosome, the lower the likelihood of recombination, which affects their inheritance patterns. In this scenario, the small distance between the body color and eye color genes suggests they are less likely to assort independently.

Dominance and Alleles

Dominance refers to the relationship between alleles, where the presence of a dominant allele masks the expression of a recessive allele. In the context of the Drosophila genes mentioned, the wild-type alleles (y⁺, w⁺, f⁺) are dominant over their respective mutant alleles (y, w, f). Understanding dominance is crucial for predicting phenotypic outcomes in offspring, especially when considering how linked genes may influence traits together rather than independently.