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Lecture 8

HMB265H1 Lecture Notes - Lecture 8: William Bateson, Genetic Recombination, Genetic Linkage


Department
Human Biology
Course Code
HMB265H1
Professor
Stephen Wright
Lecture
8

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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
each other
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
5)
6) His first prediction: the parental combinations were held together in the majority of the
progeny because they were held together by the DNA between 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
Anaphase II
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 occurs more than 50% because the genes are
linked
13) The change in chromosome can be detected using physical/genetic markers. Can also be
used to see if progeny are the result of recombination
14)
15) Studied two genes that were linked (in corn) - the C gene. Looked at the heterozygote
where there was a knob and part of chromosome 8 on chromosome 9. Recombinant
chromosomes had either a knob or part of chromosome 9, but not both. The only way this
could have occurred is if there was crossing over
16) This was visual confirmation that chromosomes cross over. Morgan devised the theory
but could not prove it. This was verified by Stern, McClintock and Creighton
17) Meiosis contributes to genetic diversity in 2 ways: independent assortment and crossing
over between homologous chromosomes
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