Lecture 8 - Linkage and Recombination
How linkage and recombination was first observed
When does crossing over occur?
What outcomes can occur when looking at the molecular model of recombination?
1) William Bateson and Punnett were studying two genes in sweetpeas (flour colour and
pollen shape- purple and long pollen is dominant). They did a self cross with the F1
generation and predicted a 9:3:3:1 ration is they were independent. But they observed
very different numbers. They believed that the two genes were somehow connected to
2) Morgan also investigated two traits in drosophila
3) Obtained an F1 dihybrid, and performed an F1 dihybrid test cross. If they were
independently assorting, you would expect a 1:1:1:1 ratio. But his observed ratios were
significantly different from what he predicted. Some combinations appeared more often
then predicted (phenotypes of F2 progeny directly reflect the genotype/alleles contributed
by the F1 generation being test crossed.
4) He made his predictions about linkage. Recombinant gametes have different combination
than the original alleles contributed by the parents
6) His first prediction: the parental combinations were held together in the majority of the
progeny because they were held together by the DNAbetween them.
7) Genetic linkage means that genes are located/ linked on the same chromosome
8) Two equally frequent parental types that totalled more than 50% of the progeny. Two
recombinant progeny with
Formed by the crossing over of two chromosomes
9) Chiasmata - physical exchange of non-sister chromatids occurring during meiosis.
Necessary so that homologous chromosomes stay together and properly segregate in
10)Cytogenetics - genetics at the cellular level
11) Sometimes there is a breakage between chromosomes and there is an exchange in genetic
information. The crossing over between non-sister chromatids of homologous
chromosomes occurs thanks to chiasmata
12)His second prediction: the parental type oc