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.

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 are less likely to be separated by recombination events. In the context 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 linked, while the bristle form gene (56.7 m.u.) is farther away, indicating a potential for independent assortment.

Independent Assortment

Independent assortment is a principle of genetics that states that alleles of different genes segregate independently of one another during gamete formation. This principle applies to genes located on different chromosomes or those that are far apart on the same chromosome. In the case of the Drosophila genes mentioned, the body color and eye color genes are close together, suggesting they may not assort independently, while the bristle form gene, being much farther away, may assort independently.

Map Units (m.u.)

Map units (m.u.) are a unit of measurement used in genetics to describe the distance between genes on a chromosome, with one map unit equivalent to a 1% chance of recombination occurring between two genes. In the Drosophila example, the distances between the body color and eye color genes (1.5 m.u.) and the bristle form gene (56.7 m.u.) indicate their relative positions and likelihood of being inherited together. Smaller distances suggest a higher likelihood of linkage and lower chances of independent assortment.