Chapter 5 Genetic Linkage and Chromosome Mapping Jones and Bartlett Publishers © 2005

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

Genetic maps are based on % recombination. There is much less recombination in heterochromatin compared to euchromatin Genetic maps are based on % recombination. Physical maps are based on other methods such as gel electrophoresis or DNA sequencing

The 4 haploid cells generated by a meiosis are kept together in a sac by ascomycete fungi

The life cycle of the baker’s yeast (Saccharomyces cerevisiae)

Tetrad analysis of unlinked genes using unordered asci For unlinked genes, parental ditype (PD) (having 2 kinds of spores) and nonparental ditype (NPD) asci are produced in equal proportion For unlinked genes, recombination between one of the genes and its centromere produces tetratype asci (TT) having 4 kinds of spores

Tetrad analysis results for linked genes in unordered tetrads-1 No crossovers or 2-strand double crossovers result in parental ditype (PD) asci. One recombination between the genes results in tetratype (TT) asci

Tetrad analysis results for linked genes in unordered tetrads-2 Three-strand double crossovers give the same result as a single crossover, tetratype asci (TT). Four-strand double crossovers give non-parental ditype (NPD) asci. As a result, for linked genes, PD >> NPD.

A branch diagram for analyzing unordered tetrads data

The life cycle of an ascomycete fungus with ordered tetrads

Analysis of ordered tetrad data Ordered tetrads allow one to map the distance between a gene and its centromere. No crossover between a gene and its centromere gives first division segregation. A crossover between a gene and its centromere gives a second division segregation.

Somatic recombination (mitotic crossover) can lead to twin spots