Final answer:
Linkage describes the nonrandom association between alleles that are located closely on the same chromosome, leading to those alleles being inherited together more often than alleles that are unlinked.
Step-by-step explanation:
The nonrandom association between alleles in a haplotype is described as linkage. During the formation of gametes, alleles that are close to each other on the same chromosome tend to be inherited together more frequently than not. This phenomenon can be observed through genetic experiments, such as those conducted by Morgan and his colleagues, which involved test crosses between heterozygous individuals and homozygous recessive parents. In cases of linked genes, most offspring had parental allele combinations, but a minority showed nonparental types due to recombination.
For example, in a test cross between a heterozygous individual (AaBb) and a homozygous recessive individual (aabb), offspring mostly exhibited the expected parental genotypes (AaBb or aabb). However, there also were recombinant genotypes (Aabb or aaBb) among the offspring, albeit in smaller numbers, indicating that recombination events, while less frequent, still occurred. These recombination events result in gametes with new combinations of alleles, thus demonstrating the concept of homologous recombination during meiosis.