Lecture 8 Feb. 20, 2006 Genetic linkage Human pedigrees Tetrad analysis

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