Genetic Recombination
in Eukaryotes
Message
Recombinants are those
products of meiosis with
allelic combinations
different from those of
the haploid cells that
formed the meiotic diploid.
In meiosis, recombinant products with new combinations of parental alleles are
generated by:
1. independent assortment (segregation) of alleles on nonhomologous
chromosomes.
2. crossing-over in meiotic synaptonemal complexes between nonsister
homologs.
Meiosis in a diploid dihybrid cell.
Genotype A/a; B/b
The detection of recombination in diploid organisms.
The advantage of a testcross (homozygote recessive tester)
Independent assortment
Two unlinked genes produce
always a recombinant frequency
of 50%.
(Testcross of a dihybrid)
Self of a dihybrid
Prunett square showing the
genotypic and phenotypic
ratios.
Genes
Phenotypes
Genotypes
1
2
3
2
4
9
3
8
27
.
.
.
n
2n
3n
Dihybrid selfing
Cross between two A/a ; B/b dihybrids
– recombination occurs in both members of cross
– recombination frequency is 50%
A;B
A;b
a;B
a;b
A;B
A/A ; B/B
A/A ; B/b
A/a ; B/B
A/a ; B/b
A;b
A/A ; B/b
A/A ; b/b
A/a ; B/b
A/a ; b/b
a;B
A/a ; B/B
A/a ; B/b
a/a ; B/B
a/a ; B/b
a;b
A/a ; B/b
A/a ; b/b
a/a ; B/b
a/a ; b/b
9 A/– ; B/–
3 A/– ; b/b
Ratio:
3 a/a ; B/–
1 a/a ; b/b
9:3:3:1 segregation in maize
Crossing-over
Chiasmata at meiosis.
Each line represents a chromatid of a pair of synapsed
chromosomes
In dihybrids for linked genes, recombinants arise from meioses in
which nonsister chromatids cross over between the genes under study.
Recombinants produced by
crossing-over
Linkage Symbolism:
A
A
B
B
a
A
b
B
Genetic Maps (linkage maps)
Message
Recombination between linked genes can be used to map their
relative distance on the chromosome. The map unit (1m.u. or 1cM)
is defined as a recombinant frequency of 1%.
In a dihybrid of linked genes the RF will be between 0% and 50%.
Recombination frequency (RF)
• Experimentally determined from frequency of
recombinant phenotypes in testcrosses
• Roughly proportional to physical length of DNA
between loci
• Greater physical distance between two loci,
greater chance of recombination by crossingover
• 1% recombinants = 1 map unit (m.u.)
• 1 m.u. = 1 centiMorgan (cM)
Linkage maps
# observed
140
50
60
150
• RF is (60+50)/400=27.5%, clearly less than 50%
• Map is given by:
A
27.5 m.u.
B
Mapping
• RF analysis determines relative gene
order
• RF between same two loci may be
different in different strains or sexes
• RF values are roughly additive up to 50%
– multiple crossovers essentially uncouple loci,
mimicking independent assortment
• Maps based on RF can be combined with
molecular and cytological analyses to
provide more precise locations of genes
Genetic maps
• Useful in understanding and
experimenting with the genome of
organisms
• Available for many organisms in the
literature and at Web sites
• Maps based on RF are supplemented with
maps based on molecular markers,
segments of chromosomes with different
nucleotide sequences
Comparison of physical and
genetic maps
The yeast chromosome 1 is shown.
A) indicates a region where the
genetic map is contracted owing to
decreased frequency of crossingover.
B) indicates a region where the
genetic map is expanded owing to
increased frequency of crossingover.
The mechanism of crossing-over
Three types of DNA recombination:
1. Homologous recombination
2. Site-specific recombination
3. Illegitimate recombination
The mechanism of crossing-over
Two types of homologous recombination.
Crossover between two dsDNA molecules results in the reciprocal exchange of
DNA. Gene conversion involves a nonreciprocal transfer. The donor sequence
remains unchanged, while the recipient sequence is changed.
The Meselson-Radding
heteroduplex model.
a)
b)
c)
d)
single stranded nick
DNA polymerase
ssDNA displaces its
counterpart in the
homologue
displaced ssDNA is
digested
The Meselson-Radding
heteroduplex model.
e)
f)
ligation completes the
formation of a Holliday
junction
resolution according to
Holliday model in two
alternative ways creats either
a crossover chromatid (V) or a
non-crossover chromatid (H).
Repair of mismatched nucleotides in heteroduplex DNA
fungal tetrads for segregation analysis
An A/a meiocyte undergoes
meiosis, resulting in an equal
number of A and a products.
The abberant 5:3 octad is explained by a
heteroduplex formed during meiosis. In this case the
nucleotide differences in the heteroduplex are not
repaired.
The role of RecA in strand transfer.
The E. coli RecA protein binds to ssDNA.
The resulting nucleoprotein complex
aggregates with dsDNA in a triplestranded DNA complex in which the
bases do not pair. This complex
facilitates invasion of the ssDNA.
Strands are subsequently exchanged and
a heteroduplex can be formed.
Site-specific recombination
involves defined DNA sites, is independent of RecA, and requires specific
enzymes. (Examples: bacteriphage λ)
Integration of λ DNA into the E. coli chromosome
involves site-specific recombination between the attP
sequence of the phage and the bacterial attB
sequence. The recombination is catalysed by an
integrase.
Illegitimate recombination
does not require segments of homologous DNA.
(Examples: transposable elements, T-DNA)
1
T
T
2
3
T
T = transposable element
Mitotic Crossing-over
Message
A mitotic crossover generates homozygosity of
alleles of heterozygous loci distal to the crossover.
Mitotic Recombination in Drosophila
Cross: y+ sn / y+ sn X y sn+ / y sn+
Mitotic Recombination in Drosophila
y sn+ / y sn
y sn+ / y sn+
y+ sn / y+ sn