A trans dihybrid should express
the dominant phenotypes for both genes in cells all over the body. This indeed
will be the case for most cells. However, if a mitotic crossover occurs, this
can result in the production of homozygous allele combinations in all heterozygous
genes that are located on that chromosome arm distal to the crossover (that
is, on the portion of the arm beyond the crossover in a direction away from
the centromere). Therefore, recessive alleles will be expressed unexpectedly
in the phenotype. Because the cells that are products of a mitotic crossover
will produce a group of descendent cells that tend to remain together during
development, the recessive phenotypes will be expressed as patches or spots
on the body. In the example in Figure 15-7, a twin spot of recessive phenotypes
is produced by a crossover between the centromere and the nearest locus, and
a single yellow spot is produced by a crossover between the loci (Figure 15-8).
Figure 15-8 Unexpected
sectors of body surface phenotypes, produced in Drosophila by mitotic crossovers.
The fly has a genotype of y sn+/y+ sn (where y represents yellow body and
sn represents singed bristles).