Post on 20-Jan-2016
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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