Cinnabar eyes (cn) and reduced bristles ( r d ) are...

Cinnabar eyes (cn) and reduced bristles ( $r d$ ) are autosomal recessive characters in Drosophila. A homozygous wild-type female was crossed to a reduced, cinnabar male, and the $\mathrm{F}_1$ males were then crossed to the $\mathrm{F}_1$ females to obtain the $\mathrm{F}_2$. Of the 400 $\mathrm{F}_2$ offspring obtained, 292 were wild type, 9 were cinnabar, 7 were reduced, and 92 were reduced, cinnabar. Explain these results and estimate the distance between the $c n$ and $r d$ loci.

Key Concepts

Autosomal Recessive Inheritance

This concept involves traits that are expressed only when an individual inherits two copies of a recessive allele. In genetic crosses involving autosomal recessive characters, the mutant phenotype appears only in homozygous recessive individuals, while heterozygotes display the dominant wild-type phenotype. This pattern is key to understanding how carriers can pass such traits along without exhibiting them themselves.

Genetic Linkage

Genetic linkage refers to the tendency of loci that are located close together on the same chromosome to be inherited together during meiosis. When genes are linked, their alleles do not assort independently, leading to offspring ratios that deviate from those predicted by Mendel’s law of independent assortment. The non-parental phenotypes observed in the cross indicate recombination between linked genes.

Recombination Frequency

Recombination frequency is a measure of how often a crossover event occurs between two linked loci, resulting in new allele combinations. It is calculated by dividing the number of recombinant offspring by the total number of offspring. This frequency is directly proportional to the physical distance between the two genes on the chromosome, providing a method of mapping their relative positions.

Genetic Map Distance

Genetic map distance is the estimated separation between two loci, usually expressed in map units or centimorgans (cM), where 1 map unit corresponds to a 1% recombination frequency. By determining the proportion of recombinant progeny in a cross, one can estimate the distance between the loci. This concept is fundamental in genetic mapping and helps to create linkage maps that reflect the order and relative distances of genes on a chromosome.

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