Download Linkage, Recombination, and Crossing Over

Linkage, Recombination, and
Crossing Over
Genes that are on the same chromosome
travel through meiosis together; however,
alleles of chromosomally linked genes can
be recombined by crossing over.
A Chromosome
Map
Heterozygous
for two traits
GgRr
Gametes:
GR;
Gr;
gR;
gr;
Phenotypes:
YeRo-YeWr-GrRo-GrWr
9 - 3 - 3 - 1
3
Linked Genes Do Not
Assort Independently
Linkage Phases:
Coupling and Repulsion
Crossing Over
MAX 50% recombination
Multiple
Crossovers
Crossing Over Occurs in
Prophase of Meiosis I
Key Points
• Linkage between genes is detected as a deviation
from expectations based on Mendel’s Principle of
Independent Assortment.
• The frequency of recombination measures the
intensity of linkage. In the absence of linkage, this
frequency is 50 percent; for very tight linkage, it is
close to zero.
Key Points
• Recombination is caused by a physical exchange between
paired homologous chromosomes early in prophase of the
first meiotic division after chromosomes have duplicated.
• At any one point along a chromosome, the process of
exchange (crossing over) involves only two of the four
chromatids in a meiotic tetrad.
• Late in prophase I, crossovers become visible as
chiasmata.
Chromosome Mapping
Linked genes can be mapped on a
chromosome by studying how often
their alleles recombine.
Observable Outcomes of
Crossing Over
• Formation of chiasmata in late prophase.
• Recombination between genes on opposites
sides of the crossover point.
Genetic Map Distances
• The distance between
two points on the
genetic map of a
chromosome is the
average number of
crossovers between
them.
Recombination
Mapping with a
Two-Point
Testcross
• The Recombination Frequency between vg
and b is 18%
• This is equal to 18 map units, or 18
centiMorgans (cM) on the genetic map.
Recombination Mapping with a
Three-Point Testcross
Determining the Gene Order
•
There are 3 possible gene orders
1.
2.
3.
•
•
sc - ec - cv
ec - sc - cv
ec - cv - sc
The two most common classes are the parentals.
Among the recombinant classes, the 2 rare classes
represent the double crossovers.
•
The gene that is “switched” in the double crossover
classes compared to the parental is the middle gene (in
this case, ec).
Calculation of Map Distances
Interference and the Coefficient
of Coincidence
• Assuming independence, the expected
frequency of double crossovers is 0.091 ×
0.105 = 0.0095.
• The observed frequency of double
crossovers was 2/3248 = 0.0006.
• A crossover in one region inhibited a
crossover nearby.
Recombination Frequency and
Genetic Map Distance
Recombination Frequency and
Genetic Map Distance
Chiasma
Frequency and
Genetic Map
Distance
Key Points
• The genetic maps of chromosomes are based on
the average number of crossovers that occur
during meiosis.
• Genetic map distances are estimated by
calculating the frequency of recombination
between genes in experimental crosses.
Key Points
• Recombination frequencies less than 20 percent
estimate map distance directly; however,
recombination frequencies greater than 20 percent
underestimate map distance because multiple
crossover events do not always produce
recombinant chromosomes.
• An average of one chiasma during meiosis is
equivalent to 50 centiMorgans of genetic map
distance.
Genetic Distance
and Physical Distance
Key Points
• In Drosophila, genes can be localized on maps of the
polytene chromosomes by combining recessive mutations
with cytologically defined deletions and duplications.
• A deletion will reveal the phenotype of a recessive
mutation located between its endpoints, whereas a
duplication will conceal the mutant phenotype.
• Genetic and cytological maps are colinear; however,
genetic distances are not proportional to cytological
distances.
Linkage Analysis in Humans
Pedigree analysis provides ways or
localizing genes on human
chromosomes.
Linkage Between the ABO and
Nail-Patella Loci
Calculation of the Recombination
Frequency
• 4/13 offspring (31%) in
this pedigree are
recombinant.
• Combining data from
many pedigrees, the
genetic distance between
the ABO and NPS1 loci is
10 cM.
• Molecular markers can
also be mapped.
Key Points
• Linkage between human genes can be detected by
analyzing pedigrees.
• Pedigree analysis also provides estimates of
recombination frequencies to map genes on human
chromosomes.
• The coefficient of coincidence (c) is the
ratio of observed double crossovers to
expected double crossovers.
c = 0.0006 / 0.0095 = 0.063
• Interference (I) = 1 - c
I = 1 - 0.063 = 0.937
Recombination and Evolution
Recombination—or the lack of it—
plays a key role in evolution.
Evolutionary Significance of
Recombination
• Meiotic recombination is a way of shuffling
genetic variation to potentiate evolutionary
change.
• In sexually reproducing species,
recombination can allow favorable alleles of
different genes to come together in the same
organism.
Inversions Suppress Recombination
in Heterozygotes
Double Crossovers in Inversion
Heterozygotes
The Human X and Y
Chromosomes
Genetic Control of
Recombination
• The products of many genes are involved in
recombination.
• Crossing over does not occur in Drosophila
males.
• The amount of recombination varies among
species.
Key Points
• Recombination can bring favorable
mutations together.
• Chromosome rearrangements, especially
inversions, can suppress recombination.
• Recombination is under genetic control.
Chromosome 9 in Maize
Evidence that Crossing Over
Causes Recombination