Chapter 5When two traits have the same pattern of inheritance they are said to be linked.Linkage arises from the two genes being on the same chromosome.In this situation Mendel’s Law of Independent Assortment does not apply.2
3Suppose gene A is linked to B, A is linked to C, and Bis linked to C. This is what we expect if A, B and C are all on the same chromosome.We say that A, B and C are all in the same linkage group.For any species if a large number of mutations is known, the maximum number of linkage groups is the same as the haploid number.
Genetic Recombination and GeneMapping
We will start with a pair of mutations on the samechromosome that have will have no recombinationbetween them.The probability that recombination occurs betweentwo loci is directly proportional to the distance between them. So we start with two genes that are very close toeach other, hvfor heavy wing vein and bwforbrown eye in Drosophila.
9Notice that we set up a cross with two heterozygous genes(in the F1 generation) and got a 1:2:1 ratio, not 9:3:3:1. The reason is linkage; hvand bware on the samechromosome and very close to each other. They segregatetogether, rather than randomly, as they would if they wereon different chromosomes.