Lecture 6: Feb. 13, 2006 Linkage and crossovers Linkage and crossovers Genetic mapping Genetic mapping Three-point cross Three-point cross

Two genes on nonhomologous chromosomes (unlinked genes) yield 4 kinds of gametes in equal proportions

Two genes on the same chromosome (linked genes) can exist in two possible configurations

Recombinant gametes are created by recombination (crossing over) between homologous chromosomes

The genetic distance between 2 genes is expressed in map units (% recombination)

Crossing over must occur between 2 genes to produce recombinant gametes Here the crossing over did not occur between the 2 genes. As a result, all four gametes are nonrecombinant (parental combinations)

Two recombinations between a pair of genes result in 4 nonrecombinant gametes

Additivity of map distances can be used for creating genetic maps For the 3 genes rb, y and cv with the map distances between rb-y and rb-cv shown in (A) above, there are 2 possible genetic maps. The distance y-cv of 1.3 map units yields map (B) and y-cv distance of 13.7 map units yields map (C).

A genetic map of chromosome 10 of corn

Use of cytologically marked chromosomes shows that crossing over involves breakage and reunion of chromosomes

Unusual inheritance of X-linked genes in crosses involving female Drosophila with attached X-chromosomes

The gametes generated by the 3 kinds of double crossovers

Consequences of a 2-strand double crossover in a cross involving 3 genes

A mapping function corrects for the loss of detectable recombinants due to multiple crossovers