1. Polygenic inheritance
Topic 10.3
Polygenic inheritance

2. 10.3.1 Define Polygenic inheritance
Polygenic Inheritance – When a single trait is influenced by many genes.
Each allele of a polygenic character often contributes only a small amount to the over all phenotype.
This makes studying the individual alleles difficult.
In addition environmental effects smooth out the genotypic variation to give continuous distribution curves.
Occurs when there are more than one
gene that controls the phenotype.
• The more chromosomes involved
contributes to more variation.
• Human skin color seems to be controlled
by three or four chromosomes.
• Human hair color is also controlled by
various genes.

3. Polygenic Inheritance of skin color
At least four loci and possibly more genes are involved.
Example 3 loci: each has two possible alleles: A,a B,b C,c,
each capital allele adds one unit of darkness
each lower case allele adds nothing
Parents with
intermediate tone
aabbcc
AABBCC
Offspring can have tone darker or lighter than either parent

4. Polygenic Inheritance of skin color
Most examples of polygenic inheritance involve more than two genes with codominant alleles. As the number of genes involved increases, the number of posible phenotypes increases. Eventually, it becomes impossible to divide individuals into discrete groups-the variation is continuous. 1 2 3 4 5 6
Number of ‘darker’ alleles