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Gene Expression

Gene Expression. Sex-linked Genes and Crossing Over Frequencies. Sex-Linked Genes. In Mammals part of the X chromosome is homologous with the Y chromosome, but the Y is much smaller than the X so there are parts of the chromosome that have no matching part of the Y Chromosome.

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Gene Expression

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  1. Gene Expression Sex-linked Genes and Crossing Over Frequencies

  2. Sex-Linked Genes • In Mammals part of the X chromosome is homologous with the Y chromosome, but the Y is much smaller than the X so there are parts of the chromosome that have no matching part of the Y Chromosome. • Any gene carried on the non-homologous part of the X chromosome is called Sex linked, and the Y chromosome is genetically empty.

  3. Sex-linked Genes • Examples • Red/green colour blindness in humans • Red/white eye colour in fruit flies • All tortoise-shell cats are females

  4. Recombination Frequencies and Chromosome Mapping • The frequency of recombination between two loci (position on the chromosome) is a measure of the distance between them. • Distances between loci are expressed as the cross-over value (COV).

  5. Cross-over Values • COV = %age recombination = Number of recombinants X 100 Total number of offspring 1

  6. COV’s in Fruit Flies • P= normal eyes p= purple eyes • B= Normal body b=black body colour colour • A back cross of BbPp X bbpp = 151 normal eyes and body 8 purple eyes and normal body 10 normal eyes and black body 131 purple eyes and black body Total 300 offspring

  7. Crossover value = 18 X 100 = 6% 300 This is the %age of offspring that show the new combination of alleles from crossing over.

  8. Chromosome mapping • Cov’s can be used to construct a chromosome map. If the cov = 10% the loci are said to be 10 map units apart. Double crossovers • With loci that are some way apart it becomes more likely that 2 crossovers will occur between them.

  9. The effect of the 2nd crossever is to countact the effect of the first. • As a result, the further apart 2 loci are, the more the frequency of recombination underestimates the frequency of crossing over. • For this reason chromosome maps should be made using loci with low recombinant frequencies (5%).

  10. Chromosome maps • The distance between loci that are far apart is determined by adding the closer distances. 6 9 3 a b o s Therefore a-s = 6+9+3 = 18 units

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