Identifying Linked Genes

Linkage is usually identified from the results of a dihybrid test cross, in which a double heterozygote (AaBb) is crossed with a double homozygous recessive (aabb).If the genes are unlinked, the four phenotypes appear in a 1 : 1 : 1 : 1 ratio. If they are linked, the offspring show two large classes and two small classes:• the two large classes have the parental combinations of characteristics (no crossing over between the genes)
• the two small classes are the recombinants, produced when crossing over separated the allelesA chi-squared test against 1 : 1 : 1 : 1 gives a value above the critical value of 7.82, so the null hypothesis of independent assortment is rejected.The amount of linkage is measured by the recombination frequency:recombination frequency = (number of recombinant offspring / total offspring) × 100%Worked example: a test cross in Drosophila gives 1000 offspring: 420 grey body normal wings, 410 black body vestigial wings, 90 grey vestigial and 80 black normal. The first two are the parental classes. The recombinants total 90 + 80 = 170.recombination frequency = 170 / 1000 × 100% = 17%A recombination frequency of 50% means the genes are behaving as if unlinked (every gamete type equally likely). A value less than 50% shows the genes are linked. The lower the value, the closer the genes are on the chromosome, which is the basis of chromosome mapping.