10.2 Inheritance
An example of a cross between linked genes
An example of linkage that actually led to the discovery of this idea was the classic experiment by Bateson,W., Saunders, E.R. and Punnett, R.C. In 1904.
They experimented on genes for flower colour in this case P for purple and p for Red and the gene influencing the pollen grain shape L for long and l for round.
They crossed PPLL and ppll and then self crossed the resulting PpLl
|
Phenotype and genotype |
Observed |
Expected from 9:3:3:1 ratio |
|
Purple, long (PpLl) |
284 |
216 |
|
Purple, round (Ppll) |
21 |
72 |
|
Red, long (ppLl) |
21 |
72 |
|
Red, round (ppll) |
55 |
24 |
Source http://en.wikipedia.org/wiki/Genetic_linkage
The discrepancy between the results and prediction was clear to the Authors and they suggested some sort of explanation. Bateson, W., et al. p89
“The peculiarities of this distribution are without doubt significant ………………….. There is, therefore some coupling of pollen-shape and colours.”
Their experimental results would be explained now as revealing linkage between the P and L alleles and the p and l alleles.
Parental Phenotypes
Purple Long pollenRed short pollen
Parental Genotypes
Gamete 
F1 Genotype
F1 test crossed with a homzygous recessive Red short pollen
Gametes without crossing over
Possible offspring
Gametes with crossing over (notice that crossing over cannot change the alleles for the homozygous recessive)
Possible offspring with crossing over
