Download linkage

linkage
Genetic Determination of Sex
Linkage
 Studies show that genes are arranged
in a linear order on the chromosome, so
one chromosome has many genes.
 Genes on the same chromosome are
called Linked Genes, and move
together during meiosis.
 These genes could be arranged with 2
dominant genes on one homologue and
2 recessive on the other, or one
dominant and one recessive on each.
Linkage
A
a
A
a
B
b
b
B
written
OR
written
AB
ab
Ab
aB
Linkage
 So far the crosses we have learnt about
have the following phenotypic ratios:
Monohybrid cross 3:1
 Test cross 1:1
 Dihybrid cross 9:3:3:1
 Test cross 1:1:1:1
These are due to the fact that genes are on
different chromosomes and they segregate
when the gametes are formed.

Linkage
 Early experiments showed that there
are some traits that did not obey
Mendel’s Law of Independent
Assortment.
 These are linked genes.
 Refer to pg 123/124 wkbk or 172/173
txtbk
Linked Genes with Crossover
 When the chromosomes line up as
tetrads in meiosis, crossing over may
occur.
 The point at which the chromatid breaks
is called the Chiasma.
 When the chromatids separate out in
independent assortment, the ones with
new combinations of alleles are called
recombinants.
 Pg 125 wkbk or pg 174 txtbk
Crossover Value
 This is the percentage of offspring
which show separation of the genes.
 This is also known as COV or
Crossover value.
Crossover = No of recombinants x 100
Value
Total no of offspring
Crossover Value
 E.g. refer to pg 126/127 wkbk or 174
txtbk
Chromosome Mapping
 Crossover value enables us to work out
the relative positions of genes on a
chromosome.
 This depends on the idea that the
further the genes are apart, the more
likely they are to break between alleles.
The closer they are, the less likely they
are to break between alleles.
Chromosome Mapping
 For convenience a COV of 1% is taken
to represent one unit on a chromosome.
 E.g. pg 127 wkbk or pg 175 txtbk
 Try problems pg 175/176
 Wkbk self check pg 128