POLYGENIC INHERITANCE INTERACTIONS BETWEEN GENES Polygenic inheritance When

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 • May be due to an environmental influence such as diet OR it may be due to the interaction of several genes.

The interaction of two genes

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 (pp. RR) with Peas (PPrr) type gives Walnut (Pp. Rr) as the F 1

Comb shape in chickens An incestuous cross of the Walnut F 1 produces all four types of comb in the F 2

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

Comb shape in chickens Walnut Pea Rose Single P_R_ P_rr pp. R_ pprr

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.

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

Coat colour in mice

Coat colour in mice

Coat colour in mice Phenotype Proportions Mousy Black Albino (white) 9/16 3/16 4/16

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.

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.

Mouse coat colour revisited • Crossing pure breeding Wild Type (AABBCC) to Albino (aabbcc) gives Wild type offspring in the F 1 (Aa. Bb. Cc) • The F 2 produced from these gives 5 types of coat colour: Wild Type (“mousy” or agouti), Black, Chocolate, Cinnamon, Albino.

Mouse coat colour revisited

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

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.

Human skin pigmentation • Initially this was thought to be controlled by two genes P 1 and P 2 • These control the production of the protein melanin • P 1 P 1 P 2 P 2 gives black skin • p 1 p 1 p 2 p 2 gives white skin • Crossing these would lead to brown • Crossing two brown skinned people would lead to a range of possible skin colours.

Human skin pigmentation

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.

Human height

Nature and nurture

A polygenic character typically shows: • a continuous variation • a normal (Gaussian) distribution It may also be influenced by the environment • E. g. Skin colour and UV light • E. g. Height and diet