Polygenic Inheritance

Assessment Statements (objectives)

10.3.1 Define polygenic inheritance.10.3.2 Explain that polygenic inheritance can contribute to

continuous variation using two examples, one of which must be human skin color.

Polygenic Inheritance

• http://www.youtube.com/watch?v=gouqTq5p168

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

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

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

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.

Continuous Variation

Discontinuous Variation

Continuous and discontinuous variation

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

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

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

Eye Color• 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

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

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

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

Melanin Protects

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