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L26 (Slide 21-49) - Linkage Recombination and Mapping Be able to define…
L26 (Slide 21-49) - Linkage Recombination and Mapping
- Be able to define and calculate recombination frequencies, and understand how this differs for linked and unlinked genes*
- Be able to use nomenclature for the configuration of multiple genes relative to their chromosomal location, including the terms coupling and repulsion.
- Be able to calculate outcomes of crosses with linked genes
- Define and use the X 2 test
- Understand the we can estimate gene distance by using recombination frequencies
- Understand that map distances are additive
- Understand and be able to calculate outcomes, including gene order, from a three point testcross
- Understand that multiple crossovers affect estimation of
genetic distance and be able to calculate map distance from a three point testcross
Linked Genes Segregate Together and Crossing over produces Recombination between them
=> Recombination Frequency = (No. Recombinant progeny) / (Total No. of Progeny) X 100
=> Tells us what proportion of gametes will be recombinant
Recombination due to cross-over is largely influenced by the arrangement of the alleles on the homologous chromosomes
Repulsion
Repulsion* (trans configuration)I.e. Pair of alleles on one chromosome consists of a dominant allele and a recessive allele and this arrangement is true on the other homologous chromosome as well*
If In Repulsion Arrangement
Majority of Progeny =
- Express combination ( green thorax, black pup); p+ b
or
- Express alternate combination (purple thorax, brown pup)
p b+
Coupling
*Coupling ( cis configuration): The dominant alleles are paired together on one chromosome and on the other homologous chromosome are paired the recessive alleles.*
If in Coupling Arrangement;
Majority of Progeny =
- Express dominant phenotypes for both alleles
i.e. (green thorax, brown puparium) p+ b+
or
- Express homozygous recessive
i.e. (& purple thorax, black puparium) p b
-
Expected Progeny from Linked Genes with Some Cross-Over/Recombination in Meiosis
1. No. Non-Recombinant Progeny > 50%
2. No. Recombinant Progeny < 50%
Example;
- No. Non-recombinant = 108
- No. Recombinant = 15
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-
-
Dihybrid
- Locus 1 = Colour of thorax: purple (p) is recessive to normal green (p+)
- Locus 2 = Colour of puparium: black (b) recessive to normal brown (b+)