Determining if two loci are linked
To determine whether two loci are linked on the same chromosome, perform a test cross of a double heterozygote with a double recessive.
If two loci are unlinked (on different chromosomes), there are four possible combinations of chromosomes that could be present in the gametes of the heterozygous parent. Because of random assortment, each combination is equally likely. There is only one combination of chromosomes in gametes of the recessive parent. When the gametes combine, they form equal numbers of offspring of the four possible phenotypes:
If the two loci are linked on the same chromosome, there are also four possible combinations of alleles on the chromosomes in the gametes of the heterozygous parent. (The recessive parent can have only one combination.) But most chromosomes have the same combination of alleles as in the heterozygous parent, because the loci are linked together. Only in the rare case where crossing-over takes place to the other two combinations of alleles occur in the gametes. By chance, most offspring get a chromosome where alleles are arranged as in the heterozygous parent, while few get the other combination. This results in many offspring having two of the combinations of phenotypes and few offspring having the other two combinations.
The recombination frequency is the fraction of individuals in which crossing-over occurred. Since the individuals in the reduced frequency categories represent cases of crossing over, the sum of those two categories over the total number of flies gives the recombination frequency as a fraction. This fraction can also be interpreted as a map distance, since loci that are farther apart tend to cross over more frequently. The recombination frequency expressed as a percent is the map distance in centimorgans (cM).
