Crossing Over in Meiosis
AP Biology
Ch. 13
Ms. Haut
Crossing Over
• Process by which parts of homologous
chromosomes are interchanged
• Crossing over and independent
assortment are mechanisms that produce
new combinations of genes
Important Features
• Loci (position of genes) on chromosome
are arranged in a linear sequence
• 2 alleles of a gene occupy corresponding
positions in homologous chromosomes
Homologous
chromosomes prior
to meiosis
Homologous
chromosomes after
DNA replication
Chiasma
• Occurs after synapsis
of homologues has
occurred in Prophase I
of meiosis
• Crossover involves
the breakage of each
of the 2 homologues
(actually chromatids)
and the exchange of
parts
Notice the crossing over.
y
x
x
+
+
z
y
z
+
+
+
Where did the cross-over
event take place?
x
+
+
+
y
+
A
B
+
x
y
+
+
+
+
C
D
Stop
Crossing Over
and
Genetic Linkage
Genetic recombination
• The order of loci remains the same, but the
alleles that reside along the chromosome are
now re-shuffled.
• The “crossing-over” of the DNA between the
two chromosomes has been observed
microscopically, as well as inferred
genetically.
• As long as the two genes being monitored
are far enough apart on a chromosome so
that a cross-over is likely to occur,
independent assortment is observed.
• Chromosomes with recombinant
combinations of linked genes are formed
by the occurrence of crossover in the
region between the 2 loci
• The probability that crossover will occur
between 2 loci increases proportionally as
the distance between those loci increases
The mean numbers of crossovers per meiosis depends
on the distance separating genes
Genetic map distance = mean number of crossovers
between two genes
Genetic linkage
• If no breakage occurs between them, the 2 loci will
appear to be linked together.
• The frequency of recombination can be determined
experimentally by counting the number of
recombinants and “non-recombinants”.
• Larger numbers of observations improve the
resolution of the frequency value.
• Unlinked genes will segregate independently and
have a recombination frequency equal to or greater
than 50%.
• Linked genes (on the same chromosome and close
together) will have recombination frequencies of less
than 50%.
Genetic linkage
• Accumulation of inheritance patterns of many gene
pairs will lead (eventually) to gene maps of each
chromosome.
• Pair-wise and three-locus linkage associations can
be formed.
• The frequencies of recombination can also be used
to estimate the physical distance between loci along
a chromosome.
• The values for recombination frequency can be
considered as “map distances” on the genetic map
• One unit of genetic map distance is defined as the
length of chromosome in which, on average, one
crossover is formed in every 50 meiotic events.
Genetic linkage
• Genetic map order of genes is usually
conserved when the DNA is examined
directly (physical order).
• Genetic map distance between genes
varies considerably from the physical
DNA distance.
Recombination Distance
• Distance between two genes =
# Recombinants / Total
Crossing Over during Meiosis in Sordaria
• Sordaria fimicola – ascomycetes fungus
Strains of Sordaria that produce black and tan spores can be purchased.
The strains can be used to inoculate a Petri plate containing an agar. At
first, the strain grows as a mycelial mat and then it produces fruiting
(reproductive) bodies.
Hyphae
growing from the
points of the
inoculation are seen
in the magnified
view of the agar
surface to the right.
If a strain producing tan
spores is inoculated on one
half of the plate and a strain
producing black spores is
placed on the other half,
hyphae grow from both points
and eventually meet at the
center of the plate where they
fuse in the equivalent of
mating. Since the hyphae of
both strains are haploid, the
fusion product is diploid. The
diploid hyphae start to
differentiate into a fruiting body
called a perithecium as seen
to the right. You can see the
perithecia forming in first
picture of the Petri plate; they
are the dark line down the
center of the plate.
Perithecium –sac structure that
contains ascospores
In the perithecium, diploid cells divide
first by meiosis and then by mitosis to
produce 8 haploid spores. The spores
are contained in a translucent saclike
structure called an ascus (pl. asci). To
the right , you can see a ruptured mature
perithecium releasing several
asci. Normally the asci would break
open and release haploid spores (seen
below) which would be air-carried to new
locations where they would germinate
and divide by mitosis to produce new
hyphae.
You can view the asci by taking a
perithecium from a culture and placing it
in a drop of water on a slide. If you
gently press on a coverslip covering the
drop, the perithecium will burst open and
release the asci. Too much pressure and
you will burst the asci as well, ruining the
preparation.
To the right is a photo of asci that
resulted from a cross between two
black strains. All of the spores are
black.
To the right is a photo of asci that
resulted from a cross between two
tan strains. All of the spores are
tan.
To the right is a photo of asci that
resulted from a cross between black
and tan strains. Look at a single
ascus and note that it contains both
black and tan spores. Those on
which the pattern of spore
distribution in the ascus is 4 tan to 4
black were produced from cells in
which no crossing over occurred.
Such asci are called nonrecombinants. Other asci contain
black and tan spores that are
distributed in 2:4:2 patterns or
2:2:2:2 patterns. These asci only
result from cells in which crossing
over has occurred and are called
recombinants. Because the
recombinant patterns result only
from crossing over, the frequency of
occurrence of recombinants is a
measure of how often crossing over
occurs.