Rest of Chapter 5
• Human recombination studies
• Mapping by tetrad analysis in fungi
• Analysis of ordered tetrads
• Other features of recombination.
Human pedigrees and mapping
• In humans, progeny numbers are small.
• Matings cannot be arranged for experiments.
• Coupling (AB/ab) vs. repulsion (Ab/aB)
heterozygotes cannot be distinguished most of
the time.
• Pedigrees can be pooled.
Lod scores
Fungi in genetic studies
• Haploid organisms can have genetic maps
made by using spore analysis.
• No testcrosses are needed.
• (review of fungal biology)
The life cycle of the baker’s yeast
(Saccharomyces cerevisiae)
Types of tetrads
PD = parental ditype
NPD = Nonparental ditype
(no linkage, PD=NPD)
(with linkage, PD>NPD)
TT = Tetratype tetrad
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.
Mapping with unordered tetrads:
• Map distance=(½)[TT]2[NPD]+4[NPD]/total
• ={(½)[TT]+3[NPD]/total # of tetrads }x 100
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.
Other types of recombination studies
• Recombination within genes
• Mitotic recombination
Fine structure mapping allows mapping
the internal structure of a locus.
Cistron – defines one genetic function
= mutations that fail to complement
The term comes from cis and trans.
Lozenge gene in Drosophila:
X + lzBS+ +
+
+lzg
v X
X ___________ x _______________> Y
ct +lzg
ct-lz=7.7 cM
lz-v=5.3 cM
134 out of 16,000 progeny had normal eyes
Frequency of recombination=0.008
What happened?
X ct ++ + in all nonlozenge females and
X + lzBSlzg v in 5 males with ‘new’ lz phenotype.
Somatic recombination (mitotic crossover)
can lead to twin spots