137 Key terms • For genes on the same chromosome (known as linked genes ), can new combinations of alleles be detected in the progeny of a dihybrid? Yes, new allele combinations (recombinants) regularly arise from such dihybrids. These are routinely detected in testcrosses to homozygous recessive testers. Their fre-quency is variable and depends on which genes are being studied. For any two genes a consistent value is obtained. • If new combinations of alleles arise, by what cellular mechanism does this happen? Crossovers are the cellular mechanism responsible. Crossovers occur more or less randomly along the chro-mosome at the four-chromatid stage of meiosis and can result in recombinants. Any one region may experience a crossover in one meiocyte and none in another meio-cyte. Any pair of nonsister chromatids can take part in a crossover. However, some double crossovers result in nonrecombinants. • Can the frequency of new combinations of alleles for linked genes be related to their distance apart on the chromosome? Yes, in general if we let the frequency of new allele com-
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