II.6 A genetic linkage exists between genes on the same chromosome.
If two genes are located on the same chromosome, there is increased probability that the alleles at both loci will be transferred together during reduction division. Thus, if an individual has allele at a1 locus A on a certain chromosome and allele b1 at locus B and, alleles a2 and b2 on the other (homologous) chromosome, then it will most probably produce four types of gametes a1b1, a1b2, a2b1 and a2b2, however in a ratio other than 1:1:1:1. The closer the loci A andB on the chromosome, the smaller will be the probability that crossing-over will occur in the section between them and the less probable will be the occurrence of types with recombined halotypes between the gametes, i.e. gametes with allele combinations a1b2 and a2b1. The strength of the genetic linkage is calculated from the ratio between the gametes with unrecombined and recombined halotypes. The strength of the genetic linkage of two loci is expressed as the ratio of the number of individuals with recombined genotypes to the total number of individuals in the progeny. If there are two loci on the chromosome right next to one another, there is a negligible chance that crossing-over will occur between them during a single meiosis. In this case, the number of individuals with unrecombined genotype will be approximately equal to the total number of progeny and the coefficient of the strength of the genetic linkage will approach a value of 1 (the ratio will approach a value of 0). On the other hand, if the two loci lie on two different chromosomes, approximately half the descendants will have the recombinant genotype; in this case, the ratio will have a value of 0.5. The strength of the genetic linkage is usually measured in centimorgans. There is a genetic distance of one centimorgan between loci between which an average of 1% recombination occurs during meiosis.