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 Phenotype Mousy Black Albino (white) Proportions
9/16
3/16
4/16

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 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