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Plants differing in character

Linked autosomal traits Traits that occur on the same chromosome are not assorted independently, like they would if they occurred on different chromosomes. The dominant, wrinkled wings (W) allele and rosy eyes (ro) are linked autosomal traits. If P1 homozygous wrinkled-winged and rosy-eyed males were paired with wild type females, the Punnett square of this cross would be: The F1 generation resulting from this cross would all be heterozygous for the two traits and have red (wild type) eyes and wrinkled wings. So far this is no different than if the traits occurred on separate chromosomes. What is the expected phenotype ratio in the F2 generation? If females of these F1 individuals were back-crossed to their P1 mutant genotype the resulting offspring would be expected to have the following genotypes: As seen in the above table, a 1 wrinkled-winged: 1 wrinkled-winged rosy-eyed phenotype ratio is expected from this cross. However, the observed results from this cross differ significantly from what is expected. If one were to make this cross with real flies, they would find that a small percentage of the resulting offspring would have either just rosy eyes or be completely wild type. This is the result of alleles being recombined due to crossing over of homologous chromosomes during metaphase I of meiosis. In fruit flies, crossing over occurs only in female gametogenesis. The figure below shows where crossing over occurs and its effect on linked traits. There is also an excellent discussion of crossing over in Campbell (1996; pp. 266–269) along with how back crosses are used with linked traits for gene mapping. Please read over that discussion carefully