1. Polygenic Inheritance
Topic 10.3

2. Assessment Statements
Define polygenic inheritance.
Explain that polygenic inheritance can contribute to continuous variation using examples of: human skin colour.

3. Polygenic Inheritance
when two or more genes influence the expression of one trait
Eg Human Skin Colour,
Grain colour in wheat

4. Continuous and discontinuous variation
When an array of possible phenotypes can be produced, it is called continuous variation
Examples: skin color, height.
When only a small number of phenotypes can be produced, it is called discontinuous variation
Examples: earlobe attachment, blood group

5. Graphical representation
Continuous variation
Discontinuous variation

6. Eye Color Look closely at your iris. What color are your eyes, really?
Iris pigmantation is used in Biometric ID
Is this a trait controlled by multiple alleles and has continuous variation. ?

7. Skin color
How does the existence of multiple genes controlling skin colour result in the appearance of many different shades of skin colour in humans?
In skin colour - more than one gene controls one characteristic;
This can cause many different possible phenotypes;
The allele of each gene promotes melanin pigment in the skin
The more alleles promoting melanin pigment the darker the skin

8. Thoughts Should there be equal esteem for all humans?
Why is human diversity so often used to divide and discriminate, rather than be appreciated, respected, and celebrated?
What problems might a person with low melanin pigmentation have in a hot sunny climate?

9. Polygenic inheritance of color in wheat.
Kernal color in wheat is determined by two genes. A range of colors occur, from white to dark red, depending on the combinations of alleles. Dark red plants are homozygous AABB and white plants are homozygous aabb.

10. the F1 offspring are all double heterozygous AaBb.
Crossing individuals with the phenotype extremes yield offspring that are a 'blend' of the two parents.
When these homozygous phenotypes are crossed
AABB x aabb
Dark x white
the F1 offspring are all double heterozygous AaBb.

11. But what happens when the two double heterozygous genotypes are crossed?  
Parent Phenotypes: all brown
Genotypes: AaBb x AbBb
Punnet square:
There is no blending in the offspring. Offspring can be more extreme than either parent
There are grades of colour – evidence of polygenic inheritance. AB Ab aB ab
AABB
AABb
AaBB
AaBb
AAbb
Aabb
aaBB
aaBb
aabb

12. Skin color
How does the existence of multiple alleles controlling skin colour result in the appearance of many different shades of skin colour in humans?

13. Human skin colour is controlled by multiple alleles (and the environment)
At least three genes control skin color, let’s call them genes A, B, and C.
Someone who is AABBCC would have very dark skin color and someone who is aabbcc would have very light skin color.
If they got married and had children, their children would all be AaBbCc and have mid-brown skin.
If two of those people would get married and have children, the Punnett square would look like the one above.

14. Try this IB question (HL)
How does the existence of multiple genes controlling skin colour result in the appearance of many different shades of skin colour in humans?
1 ……………………………………………………………………………..
2 ……………………………………………………………………………..
3 …………………………………………………………………………….. 4
……………………………………………………………………………..

15. More IB Questions
A polygenic character is controlled by two genes each with two alleles. How many different possible genotypes are there for this character?
A. 2
B. 4
C. 9
D. 16
Which human trait shows a pattern of polygenic inheritance?
A. ABO blood type
B. Sickle cell anemia
C. Skin colour
D. Co-dominant alleles