1. Unit C: Cell Division, Genetics and Molecular Biology
Chapter 19
Section
Gene Linkage and Crossover

2. Morgan and Gene Linkage
Besides dealing with sex linked traits, Morgan also worked with the idea of linked genes.
While carrying out his experiments, Morgan observed that in some of the crosses, instead of having the predicted Mendelian ratios, almost all of the offspring had the same combination of traits as the parents.

3. In Morgan’s experiments, he crossed a fruit fly that was homozygous for the wild-type body colour (AA) and straight wings (BB), with a fruit fly that was homozygous for the black body colour (aa) and curved wings (bb).
The resulting F1 generation were heterozygous for each gene, which was expected.
Next he crossed two F1 flies and predicted a 9:3:3:1 phenotypic ratio.
However, what he observed was that all the individuals with the wild-type body colour had straight wings and nearly all the individuals with black bodies had curved wings.

4. In order for this to occur, Morgan concluded that the alleles for the two genes must not have undergone independent assortment.
Which means, that he believed the genes were on the same chromosome = linked genes.
He found that the ‘A’ allele was linked to the ‘B’ allele, and the ‘a’ allele was linked to the ‘b’ allele.

5. Linked Genes b B B b a A A a B
The only two possible sex cells. b A a

6. If his theory was to be believed the F1 cross would look like this:AB ab
AABB
AaBb
aabb
Due to the linkage, the only two possible sex cells that could be made were: ‘AB’ and ‘ab’.

7. Besides body colour and wing shape, Morgan found other linked genes:
Trait
Dominant/Recessive
Location
Wingless
Recessive lethal
Chromosome 2
Curly wings
Dominant
Purple eyes
Recessive
Stubble Bristles
Chromosome 3
Ebony body
Miniature wings
Sex linked recessive
Chromosome 4
Cut wings
White eyes
Vermillion eyes

8. Linked Genes and Decreasing Variability
As we saw in the example earlier, when genes are linked, only a limited number of combinations for sex cells can be made.
This causes a decrease in the variability of the offspring and the entire population.
Variability is needed among natural populations for the survival of the species and in order for evolution to continue.

9. Crossing Over
By the Punnett square analysis of the F1 cross earlier, it was predicted that the two possible sex cells from each parent would produce a 3:1 ratio of wild-type body with straight wings over black body with curved wings.
However, once again this is not what Morgan saw.

10. Out of 400 offspring this was his result:
Phenotype
Number
Wild –type body,
Straight wings
290
Black body,
Curved wings 92
Wild-type body, 9
He was expecting 75% to have a wild-type with straight wings and 25 % to have a black body with curved wings.
Instead he sees all four possible phenotypes show up.
How can this be?

11. It was later found that the new combinations were arising from the phenomenon known as crossing over.
Crossing over occurs when the homologous chromosomes intertwine and exchange genetic information.
The exchange of the genetic information was creating new possibilities for sex cells and therefore, new unexpected phenotypes for the offspring.

12. Crossing Over b a B A b a B A Original two sex cells. B A b a A b B a
Two new sex cells due to crossing over.

13. Crossing Over and Increasing Variability
While linked genes decrease variability among populations, crossing over attempts to counter it by increasing the variability.