Paired patches of tissue genetically different from each other and from the background tissue, produced by mitotic crossing over in an individual of heterozygous genotype during development of the organism.
In the Drosophila example diagrammed above, twin spots occur on a female of genotype y +/+ sn. Most of the thorax and abdomen show wild-type pigment and long, black bristles, but there are adjacent patches of yellow and singed tissue. These twin spots are clones of cells derived from the reciprocal products of an exchange between the singed gene and the centromere.
Subjects: Genetics and Genomics.