HMB265H1 Lecture Notes - Lecture 10: Chromosome, Centromere, Genetic Distance

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HMB265 Lecture 9 Linkage Mapping
Genes on the same chromosome are linked
- Genes can generate excess of parental gametes or generate recombinant gametes
- Normally, the parental configuration occurs at a high frequency whereas the
recombinant configuration is infrequent
- What do these frequencies imply?
Morgan’s hypothesis and Sturtevant’s extension
- Genes are located in a linear order along the chromosomes
- Genes which are further apart have a higher frequency of recombination (crossing-over)
- Distance between chromosomes can be calculated using recombination frequency
- Recombination frequency can be used to map genes on a chromosome
o We can say that crossover frequency is a function of the distance between loci
Genes closer on the chromosome have a lower recombination frequency
Constructing linkage maps
- Frequency of crossovers between two genes is a function of their distance apart on the
- Recombination frequency (RF) = (number of recombinants) / (total number of progeny)
- 1% recombination = 1RF = 1 map unit (m.u.) = 1 centriMorgan (cM)
Sturtevant’s cross
- He crossed two pure-breeding lines of Drosophila
- One line had Red eyes (v+v+) and cut edges (ctct)
- The other line had vermillion eyes (vv) and uncut edges (ct+ct+)
- Both genes are located on the same chromosomes
- He crossed the two lines and obtained an F1 generation with the following phenotypes
o Red eyes and uncut edges
v+v ct+ct
o vermillion eyes and cu edges
vv ctct
- He then crossed the F1 progeny and received the following frequencies for each
Vermillion, uncut
Vermillion, cut
Red, uncut
Red, cut
- RF = (92 + 99) / total = 0.132, or 13.2%
- v and ct are 13.2 m.u. away from each other
- We can see that most flies had the parental recombination parental gametes most
A third linked gene
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