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POLYGENIC INHERITANCE

POLYGENIC INHERITANCE. INTERACTIONS BETWEEN GENES. Polygenic inheritance. When single genes affect on a character  discontinous variation (categories) Most characters such as height or mass and even eye colour, show continuous variation

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POLYGENIC INHERITANCE

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  1. POLYGENIC INHERITANCE INTERACTIONS BETWEEN GENES

  2. Polygenic inheritance • When single genes affect on a character discontinous variation (categories) • Most characters such as height or mass and even eye colour, show continuous variation • May be due to an environmental influence such as diet OR it may be due to the interaction of several genes.

  3. The interaction of two genes

  4. Comb shape in chickens • These can come in four shapes which are controlled by two non-linked genes P and R • Each gene has two alleles, one dominant (P and R) and one recessive (p and r) • Crossing Pure breeding Rose type (ppRR) with Peas (PPrr) type gives Walnut (PpRr) as the F1

  5. Comb shape in chickens An incestuous cross of the Walnut F1 produces all four types of comb in the F2

  6. Comb shape in chickens • What must be the genotypes for each of these phenotypes? Walnut …………………….. Pea …………………….. Rose …………………….. Single ……………………..

  7. Comb shape in chickens Walnut P_R_ Pea P_rr Rose ppR_ Single pprr

  8. Unusual ratios • Coat colour in mice is controlled by at least two genes • A, the Agouti or Mousy gene and C the Expression of coat colour gene • The recessive a allele give Black and the recessive c allele gives Albino • The cc genotype is said to be epistatic to the A gene • cc switches off the A gene.

  9. Coat colour in mice Crossing two mice AaCc what ratio of offspring are produced? Do the Mendelian rations of 9:3:3:1 result?

  10. Coat colour in mice

  11. Coat colour in mice

  12. Coat colour in mice

  13. The interaction of three or more genes More genes interacting = more variety in the offspring • In the coat colour of mice the are several genes interacting to produce a range of different coat colours • Three of these genes are A, B and C.

  14. Mouse coat colour revisited • The A gene controls the production of a small yellow band near the end of each hair which gives rise Agouti (mousy) coat • The recessive allele gives Non-agouti • The B gene give the ground colour Black • The recessive allele gives Brown colour • The C gene controls the expression of the coat colour genes as a whole • The recessive allele gives Albino.

  15. Mouse coat colour revisited • Crossing pure breeding Wild Type (AABBCC) to Albino (aabbcc) gives Wild type offspring in the F1 (AaBbCc) • The F2 produced from these gives 5 types of coat colour:Wild Type (“mousy” or agouti), Black, Chocolate, Cinnamon, Albino.

  16. Mouse coat colour revisited

  17. Mouse coat colour revisited What must appear in the genotypes of : Wild type …………………. Black …………………. Chocolate …………………. Cinnamon …………………. Albino ………………….

  18. Mouse coat colour revisited Wild type A_ B_ C_ Black aa B_ C_ Chocolate aa bb C_ Cinnamon A_ bb C_ Albino _ _ _ _ cc The more genes that interact the greater the range of phenotypes produced This ultimately leads to a continuous distribution of traits where one blends into another.

  19. Human skin pigmentation • Initially this was thought to be controlled by two genes P1 and P2 • These control the production of the protein melanin • P1P1P2P2 gives black skin • p1p1p2p2 gives white skin • Crossing these would lead to brown • Crossing two brown skinned people would lead to a range of possible skin colours.

  20. Human skin pigmentation

  21. Human skin pigmentation • This would produces a range of 5 different shades with a normal distribution • That is to say a bell shaped curve • This does not fit with what is observed • There are far more shades than this observed in “black” and “white” • Observations on the human population suggest that it is controlled by four or more genes at different loci.

  22. Human height

  23. Nature and nurture

  24. A polygeniccharactertypically shows: • a continuous variation • a normal (Gaussian) distribution It mayalsobeinfluenced by the environment • E.g. Skin colour and UV light • E.g. Height and diet

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