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Lecture 8

HMB265 - Lecture 8.docx

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Human Biology
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Stephen Wright

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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 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 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 oc
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