Lecture 8Feb. 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 genesin 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 genesin 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