LINKAGE AND CROSSING OVER
žGenes
that are on the same chromosome travel through
meiosis together. This is called linkage.
žAlleles of
chromosomally linked genes can be recombined by crossing over.
žDuring meiosis,
homologous chromosomes
pair, and physical exchange of
chromosome segments recombine genes. This is supported by the cytological observation that homologous chromosomes pair during
meiosis.
žAt
the switch
points, the two homologues are crossed
over, as if each has
been broken and then reattached to its partner.
žA
crossover point is called a
chiasma
(plural, chiasmata). Crossing over describes the
process that creates
the chiasmata—that is, the actual process of exchange between paired chromosomes.
žRecombination—the
separation of linked
genes and the formation of new gene combinations—is
a result of
the physical event of crossing over.
Linkage
žWhen
two or more characters of
parents are transmitted together to the offsprings of few generations such as F1 , F2 , F3
etc. without any recombination,
they are called linked characters and the phenomenon is called linkage.
žThis is a deviation from the Mendelian principle of independent assortment.
žMendel’s
law of independent assortment is
applicable to the genes that are situated in separate chromosomes. When genes
for different characters are located in the same chromosome, they are tied to
one another and are said to be linked.
žThey
are inherited together by the offspring
and will not be assorted independently.
žThus, the tendency of two or more genes of
the same chromosome to remain together in the process of inheritance is called
linkage.
Coupling
vs.
Repulsion
žThe
condition of having the dominant alleles for both genes on the same parental chromosome
and both
recessive alleles on the other parental chromosome is
called “coupling”.
žThe
opposite condition, having one dominant and one recessive on each parental
chromosome is called “repulsion”.
EARLY
EVIDENCE FOR LINKAGE AND RECOMBINATION: W.
Bateson and R. C. Punnett
Experiment
žPlants
with red flowers and long pollen grains were
crossed to plants with white flowers and short pollen grains.
žAll
the F1 plants had red flowers and long pollen grains, indicating that the alleles for these two phenotypes
were dominant.
žWhen
the F1
plants were self-fertilized,
instead of the 9:3:3:1 ratio expected for two independently assorting genes, they obtained a peculiar ratio of 24.3:1.1:1:7.1.
žAmong
the F2
plants, the classes
that resembled the original parents (called
the parental classes)
are significantly overrepresented and
the two other (non-parental) classes are significantly underrepresented.
žBateson
and Punnet
could not provide the correct explanation for this observation.
žThe
correct explanation for
the lack of independent assortment
in the data is that the genes for flower color and pollen length are located on the same
chromosome—that is, they are linked.
EARLY
EVIDENCE FOR LINKAGE AND RECOMBINATION:
MORGAN’s
Experiment
žLater,
Thomas Hunt Morgan found a similar deviation from Mendel’s second law while
studying two autosomal genes
in Drosophila.
žOne
of the genes affected eye color:
pr: purple (recessive)
pr+: red (dominant)
žThe
other gene affected wing length:
vg: vestigial (recessive)
Vg+: normal (dominant)
žMorgan
performed a cross to obtain dihybrids, then followed with a testcross:
žMorgan’s
testcross results were as follows (listed as
the gametic
classes from the dihybrid):
žThese
numbers deviate drastically from the Mendelian prediction
of a 1:1:1:1 ratio.
Parental classes are overrepresented while the non-parental classes are
underrepresented.
žThe
ratio of two parental classes is 1:1 while the ratio of two non-parental
classes is also 1:1.
žMorgan
performed another cross
to obtain dihybrids,
then followed with a testcross,
where the alleles were in different combinations:
žMorgan’s
testcross results were as follows (listed as
the gametic
classes from the dihybrid):
žAgain, these numbers
deviate drastically from the
Mendelian
prediction of a 1:1:1:1 ratio. Parental classes are
overrepresented while the non-parental classes are underrepresented.
žThe
ratio of two parental classes is 1:1 while the ratio of two non-parental
classes is also 1:1.
žMorgan
suggested that the two genes in
his analyses are located on the same pair of homologous chromosomes, i.e. they are linked.
New
combinations
of
alleles
arise from crossovers
žThe
linkage hypothesis explains why allele combinations
from the parental generations remain together—because they are physically attached by the segment of chromosome between them.
žBut
how do we explain the appearance of the minority class of non-parental combinations?
žWhen homologous
chromosomes pair in
meiosis, the chromosomes occasionally
break and exchange parts in a process called crossing over. The two new combinations are called
crossover products.
Cis-
and trans conformation
žWhen
the two dominant or wild-type alleles are present on the same homolog,
the arrangement is called
a cis conformation.
Cis: RL/rl
žWhen
the two dominant or wild-type alleles are
present on the different
homologs, the arrangement is called a trans conformation.
Trans: Rl/rL
CROSSING
OVER AS THE PHYSICAL BASIS
OF
RECOMBINATION
žRecombinant
gametes are produced as a result of crossing over between homologous chromosomes.
žThis
process involves a physical exchange
between the chromosomes.
žThe
exchange event occurs during the prophase of
the first
meiotic division.
žAlthough
four homologous
chromatids are present, forming a tetrad,
only two chromatids cross over at any one point.
žEach
of these chromatids breaks at the site of
the crossover, and the resulting pieces reattach to produce the recombinants.
žThe
other two chromatids are not recombinant at this site.
žThere
is a possibility for multiple exchanges
in a tetrad of chromatids.
žThere
may, for example, be two, three, or even four separate
exchanges—customarily called
double, triple, or quadruple crossovers.
žHowever,
that exchange between sister chromatids
does not produce genetic recombinants because
the sister chromatids
are identical.
EVIDENCE
THAT CROSSING OVER
CAUSES RECOMBINATION
žIn
1931 Harriet Creighton and Barbara McClintock
obtained evidence that genetic recombination was associated with a material exchange between chromosomes.
žCreighton
and McClintock studied homologous
chromosomes in maize
that were morphologically distinguishable.
žTwo
forms of chromosome 9 were available for
analysis:
one was normal
and the other had cytological aberrations at each end—a heterochromatic knob at one end
and a piece of a different chromosome at the
other.
žThese
two forms of chromosome
9 were also genetically marked to detect
recombination.
žOne
marker gene controlled kernel color (C, colored; c, colorless),
and the other controlled kernel texture
(Wx, starchy;
wx, waxy).
žCreighton
and McClintock performed the following testcross:
žTheir
results showed that the C Wx and
c wx recombinants
carried a chromosome with only one of the
abnormal cytological markers.
žThe
other abnormal marker had evidently been lost through an exchange with the
normal chromosome 9 in the previous generation.
žThese findings suggest that recombination was caused by a physical exchange between paired chromosomes.
CHIASMATA
AND CROSSING
OVER
žThe
cytological evidence for crossing over can be seen during late prophase of the first meiotic division when the chiasmata
become clearly visible.
žAt
this time paired chromosomes repel each other slightly, maintaining close contact only at the
centromere and at each chiasma.
žAs
we might expect, large
chromosomes typically
have more chiasmata
than small chromosomes.
žThus, the number of chiasmata is roughly proportional to
chromosome length.
Recombination
frequency
žLet’s assume that genes
A and
B are
on different chromosomes and that an AABB individual
is crossed to an aabb individual.
žFrom
this cross the AaBb offspring
are then testcrossed to the double
recessive parent (aabb).
žBecause
the A and
B genes
assort independently, the F2 will consist of two classes
(AaBb and
aabb) that are phenotypically like the
parents in the original
cross and two classes (Aabb and
aaBb) that are phenotypically recombinant.
žFurthermore,
each F2 class will occur with a frequency of 25 percent.
Thus, the total frequency of recombinant progeny from a testcross involving two
genes on different chromosomes will be 50 percent.
žA frequency of recombination
less than 50 percent implies that the genes are linked on the same chromosome.
