1. Polygenic Inheritance

2. Assessment Statements (objectives)
Define polygenic inheritance Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin color.

3. Polygenic Inheritance

4. Polygenic inheritance defined
Involves 2 or more genes influencing the expression of one trait
With 2 or more allelic pairs found at different loci, the number of possible genotypes is greatly increased
Most human traits are too complex and show too many combinations to be determined by one gene

5. Polygenic inheritance defined
This makes it difficult to determine which genes are responsible for traits whose genetic components are poorly understood like:
Mathematical aptitude
Musical talent
Susceptibility to certain illnesses or cancer

6. Continuous and discontinuous variation
With dominate and recessive alleles of a single gene, the number of possible genotypes is limited
Ex. Either a person has an attached earlobe or doesn’t
When multiple alleles are introduced, the number of possibilities for a single trait increases accordingly
Ex. ABO blood type has 4 alleles and 4 possible phenotypes

7. Continuous and discontinuous variation
When a second gene is introduced, the number of possible genotypes increases dramatically
With 3, 4, or 5 genes determining the phenotype, the number of possibilities is so big it is impossible to see in the phenotype the difference between certain genotypes
When an array of possible phenotypes can be produced it is called continuous variation.

8. Continuous Variation

9. Discontinuous Variation

10. Continuous and discontinuous variation

11. Continuous and discontinuous variation
The color of skin in humans is an example of continuous variation
It is believed that the intensity of pigment in skin is due to the interaction of multiple genes
Continuous variation can also be seen in:
Height
Body shape
Intelligence
Each of these is also influenced by environment
Height depends on genes and nutrition

12. Continuous and discontinuous variation
A trait is not continuous (discontinuous) if you can check “yes” or “no” for a trait
Example: attached earlobes?
The answer is either yes or no so it doesn’t show continuous variation
The same is true for blood type because there are few choices: A, B, AB, O
Can show discontinuous variation of graph

13. Continuous and discontinuous variation
When there are many possibilities, then the trait shows continuous variation
The graph produces a bell-shaped curve with smooth transitions between the groups of frequencies

14. Although it is possible to classify eye color into groups such as Brown, Green, Blue, and Hazel
Close examination of Brown eyes would reveal a wide variety of colors
Even though nature shows a considerable amount of variety in eye color, societies and governments have imposed a small number of categories
Eye Color

15. Skin Color
Even though there is a boundless variety of shades of human skin color, many societies are still fixed on labeling people with categories such as “white” or “black” or “mixed”
This is an oversimplification that serves more of an administrative purpose than a biological one

16. Melanin Protects
There is a valid biological reason why humans have varying skin color:
Protection from the Sun’s harmful UV radiation
When sunlight is very intense, as in the tropics, people need protection from the sunburn which can lead to melanoma, a type of skin cancer
The best protection from UV radiation is increased melanin in the skin
People with dark skin have more melanin while people with light skin have little melanin

17. Melanin Protects
Best protection from UV radiation is increased melanin
Apart from albinos, all people have continuous melanin pigmentation
Although is it mostly determined by genetics, it is possible to increase the melanin level in your skin by exposing your body to sunlight.
Process called tanning

18. Melanin Protects

19. UV light and calciferol (vitamin D)
Regions far from the equator has much less intense sunlight
One beneficial aspects of UV light is that it helps the skin to make calciferol, which is essential for proper growth and bone formation
The type of UV radiation which does this is UVB
It is essential for good health to allow moderate amount of UV radiation onto the skin
In regions of low Sun exposure, people need light colored skin
If they had intense concentrations of melanin, not enough UV rays would get through to allow calciferol production