In Drosophila, the yellow (y) gene is near the end of the...

In Drosophila, the yellow (y) gene is near the end of the acrocentric X chromosome, while the singed (sn) gene is located near the middle of the X chromosome. On the wings of female flies of genotype $y \mathrm{sn} / y^* s \mathrm{sn}^{+}$, you can very rarely find patches of yellow tissue within which a small subset of cells also have singed bristles. a. How can you explain this phenomenon? b. Would you find similar patches on the wings of females having the genotype $y^{+} s n / y s n^{+}$?

In Drosophila, the yellow (y) gene is near the end of the acrocentric X chromosome, while the singed (sn) gene is located near the middle of the X chromosome. On the wings of female flies of genotype $y \mathrm{sn} / y^* s \mathrm{sn}^{+}$, you can very rarely find patches of yellow tissue within which a small subset of cells also have singed bristles.
a. How can you explain this phenomenon?
b. Would you find similar patches on the wings of females having the genotype $y^{+} s n / y s n^{+}$?

Key Concepts

Loss of Heterozygosity

Loss of heterozygosity occurs when a cell that was originally heterozygous (carrying two different alleles) becomes homozygous for one allele due to mitotic recombination. This change can unmask recessive traits in the cell lineage, resulting in patches of tissue that display a phenotype corresponding to the previously hidden recessive allele.

Genetic Linkage and Gene Order

Genetic linkage describes the tendency of genes located near each other on the same chromosome to be inherited together. The physical distance between genes, as well as their positions relative to the centromere or telomere, affects the probability of recombination between them. This concept is central in understanding why recombination events can separate or bring together particular alleles, thereby creating mosaic patches with recombinant phenotypes.

Mosaicism

Mosaicism is the presence of two or more genetically distinct cell populations within a single organism, derived from a single fertilized egg. When a mitotic recombination event occurs in a developing tissue, the altered cell propagates through subsequent cell divisions, leading to visible patches with a phenotype different from surrounding cells.

Mitotic Recombination

Mitotic recombination refers to a crossover event occurring during the mitotic division of somatic cells. In this process, parts of homologous chromosomes exchange segments, leading to daughter cells that are homozygous for certain gene regions. This can result in patches of tissue (clones) with a distinct genotype and phenotype, even though the organism is genetically mosaic overall.

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