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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
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