Transcript Document

TABUK UNIVERSITY
Faculty of Applied Medical Sciences
Department of Laboratory Technologies
Linkage genes and genetic
recombination
AY: 1433-1434/2012-2013
Lecture 5
Mendel’s law of independent
assortment
• Knowledge of which allele has been inherited at one locus gives no
information on the allele has been inherited at the other locus
S/s
SY
25%
Y/y
Sy
sY
sy
25%
25%
25%
Mendel’s law of independent assortment: Most pairs of
genes assort independently
S
Y
s
y
Segregation
S
Y
Parental types
Gametes
s
y
S
y
Non parental types
s
Y
BUT……
• Genes that are located close to each other on the
same chromosome are linked and usually travel
(segregate) together during meiosis (not
independently=dependently).
Genes linkage
• Makes an exception to Mendel’s law of
independent assortment.
• Linkage ≠ independent assortment
Discovery of Linkage
• In 1900, Mendel’s work was re-discovered. Bateson
and Punnett were testing his theories with as many
different genes and organisms as possible.
• Individuals with new combinations of genes(non
parental) are called recombinants.
• Recombination is a result of Crossing Over
Crossing over occurs during Prophase 1 of meiosis.
Crossing Over: a random event
• When two homologous chromosomes were
positioned side by side, they “synapse”, which
means to pair closely with each other. DNA
strands from the two chromosomes are matched
with each other.
• Thus, sections of the two chromosomes might
cross, break, and reattach.
Mechanism of crossing-over (in PROPHASE I of meiosis)
Crossing over (exchange) occurs between two non sisters chromatids (each
chromatid from one chromosome)
Recombination Process
The Process of Recombination, animated version
(Attached youtube video)
What is Linkage?
• Linkage occurs when two genes are close to each
other on the same chromosome.
• Genes far apart on the same chromosome assort
independently: they are not linked.
• Linkage is based on the frequency of crossing over
between the two genes.
Frequency of Crossing Over= Recombination frequency
= nombre of recombinant gametes/ total nombre of gametes
The recombinant gametes are those gametes that differ from both of the
haploid gametes that made up the original diploid cell (so differ to the parental
gametes).
Linkage genes
Otherwise, recombination frequency indicate a genetic distance
between two genes on the same chromosomes
Dg-g= RF
Shorter the distance between two loci, higher the frequency of
recombinations (chance to occurs) .
• A genetic distance unit, or centiMorgan (cM), is equal to crossing
over between 2 genes in 1% of the gametes.
• 1 cM is the distance between genes for which the recombination
frequency is 1%.
Detecting Linkage through Testcrosses
• Linked genes are found by looking for deviation from
the frequencies expected from independent assortment
(different to 25,25,25,25% for each gamete).
• A testcross (one parent is homozygous recessive) works
well for analyzing linkage:
– If the alleles are not linked (independant), and the second
parent is heterozygous, all four possible combinations of traits
will be present in equal numbers in the progeny (25% for
each).
– A significant deviation in this ratio (more parental and fewer
recombinant types) indicates linkage.
Testcross to show that two genes are linked
Testcross to show that two genes are linked
Demonstration that the recombination frequency between two genes
far apart on the same chromosome cannot exceed 50 %
Demonstration that the recombination frequency between two genes
located far apart on the same chromosome cannot exceed 50 %
offspring of single and double crossovers
LINKED or NON-LINKED?
• A recombination frequency of 50% means that
genes are unlinked. There are two ways in
which genes may be unlinked:
– They may be on separate chromosomes.
– They may be far apart on the same
chromosome.
• 0 < recombination frequency ≤50%
Test cross Results
•
•
•
•
PP LL x pp ll
F1: 100% Pp Ll
F2 results in table
Very significant deviation
from expected Mendelian
ratio: chi-square = 97.4, with
3 d.f. Critical chi square value
= 7.815.
• The null hypothesis for chi
square test with 2 genes is
that the genes assort
independently. These genes
do not assort independently.
phenotype obs
exp
ratio
exp
num
P_ L_
284
9/16
215
P_ ll
21
3/16
71
pp L_
21
3/16
71
pp ll
55
1/16
24
Coupling vs. Repulsion
• The original test cross we did was PL/pl x p l. Among
the offspring, PL and pl were parental types, and pL
and Pl were the recombinant types. There was
24.3% recombination between the genes.
• When the dominant alleles for both genes were on
the same parental chromosome (PL), with both
recessives on the other parental chromosome (pl),
they called “coupling”: the P and L genes are “in
coupling phase”.
• When one dominant and one recessive on each
parental chromosome (Pl or pL), is called “repulsion”.
Example: Test Cross in Repulsion
• Now do the test cross in repulsion: Pl /
pL x p l
• Here, the parental types are P l and p L,
and the recombinant types are P L and
p l.
• However, the percentage of
recombinants is 24.3%.
• 123 P L + 120 p l = 243 recombinant
offspring.
243/ 1000 total offspring = 24.3 %
• The percentage of recombination
depends on the distance between the
genes on the chromosome, and NOT on
which alleles are on which
chromosome.
phenotyp obs
e
PL
123
Pl
372
pL
385
pl
120
total
1000