1. Genetic Determination of Sex
linkage
Genetic Determination of Sex

2. 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.

3. Linkage
A B written AB
a b a b OR
A b written Ab
a B aB

4. 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.

5. 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

6. 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

7. 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

8. Crossover Value
E.g. refer to pg 126/127 wkbk or 174 txtbk

9. 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.

10. 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