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). Fourstrand 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