So far we have looked at situations where one pair of alleles controls one characteristic.
There are occasions where a number of genes interact together.
This is where one gene affects several characteristics. For example, a disease caused by one pair of alleles may have several or many symptoms.
This is where one characteristic is affected by two or more genes (e.g, skin colour).
Several genes control skin colour, we will look at just two to make it a little simpler.
The alleles will be called A and B, and each of these has one alternative allele, a and b.
A and B cause the skin to be dark, a and b cause it to be light
A person who has the genotype AABB will have very dark skin;
A person who has the genotype aabb will have white (albino) skin.
A person with genotype AAbb, aaBB or AaBb will have medium colour skin.
Thus if two people of genotype AaBb have children, they may have any colour skin:
This is where one gene interferes with the expression of another gene.
Mouse coat colour is controlled by two pairs of alleles: B and C
B = black coat colour, b = brown coat colour
C = pigment production, c = no pigment production
Therefore, if a mouse has cc, it will be an albino, if it has Cc or CC it will be black or brown.
We have assumed in all the examples covered so far that the genotype always has exactly the same effect on the phenotype.
For example: people with the alleles AaBb for skin colour will have medium dark coloured skin.
This is not always true. If this person is exposed to a lot of sunlight, it is very likely that their skin will be darker as they tan.
In the case of height, the alleles determine the potential height that someone could achieve but with a poor diet, they may be well short of their maximum height. Thus environment can have a large bearing on the extent to which genes are expressed.