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 Genetics and Genes
Ⅱ The Inheritance of Genes
 
Ⅲ  Recombination of Genes
 
Ⅳ  Gene Interaction
 
V Gene Mutations
 
Ⅵ  Genomics
 

  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).

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