Life is often not as simple as it looks at first sight. It happens quite often that characteristics are determined by more than one gene. These genes also influence each other through all kinds of dominance relationships, which makes things even more complex.
Let's assume the presence of two genes A and B, which both have a dominant allele (A and B) and a recessive allele (a and b)
Normally we would see these phenotypes:
|A-B- (9/16)||aaB- (3/16)|
|A-bb (3/16)||aabb (1/16)|
There are cases in which the (dominant) B-allele masks the effect of the A-gene completely. The B-gene is called epistatic:
|A-bb (3/16)||aabb (1/16)|
An example is according to some researchers the coat type.
S - smooth coated
s - long hair
R - rough hair
r - no rough hair
If the allele R is present the influence of the S-locus is lost. Whether the dog would be longhaired or smooth coated cannot be seen; the R-gene is epistatic in relation to the S-gene.
It also happens that a double (recessive) a-allele (aa) masks the effect of the B-locus entirely. The a-gene is called cryptomeric:
|A-B- (9/16)||aa-- (4/16)|
An example of this is the influence of the extension factor (the red-gene on the E-locus) on the B-locus (black/brown). When a dog is homozygous for the extension gene (ee) the effect of the B-gene is hidden.
A third effect is seen when one homozygous recessive locus (either aa or bb) results in one phenotype and two homozygous recessive loci (aabb) results in a different phenotype:
|A-B- (9/16)||aa-- (3/16)|
|--bb (3/16)||aa-- (1/16)|
These effects can also be combined:
This can be demonstrated by crossing two short-legged breeds such as the Dachshund and the Scottish Terrier. Both breeds are homozygous for short legs. The dominant gene that causes this however is different for both breeds. The offspring of this cross is heterozygous for both genes and is short-legged. Crossing the offspring results in 1/16 of the pups being homozygous recessive for both genes. Those pups have normal length legs.
A nice example is the group of genes that cause a white coat colour. The combination below was investigated during research at the University of Utrecht into deafness among white Bull Terriers.
A homozygous sw-dog has a white coat; the (dominant) W-gene also results in a white coat. Only S-ww dogs have coloured coats. The W-allele (dominant white) masks the s-locus and the swsw combination masks the w-locus.
In this example we ignored the fact that certain combination are often deaf and the fact that swswww-dogs often show some black markings (extreme white), so that these dogs actually have a different phenotype.
This combination can be illustrated by the red coat colour; it can either be caused by the homozygous recessive ee (Extension locus) or by the homozygous recessive ayay(Agouti locus). If these two genotypes are crossed they produce heterozygous offspring: AayEe. If these dogs are crossed these two genes are mutually cryptomeric; A-ee, ayayE- and ayayee result in a red coat.
Mendel's laws are often mentioned and most of the time referred to by their names. Knowing the names of the laws is not very useful, but for the sake of a complete overview (and the fact that they are mentioned in several publications) they will be briefly discussed.
The first law
The monohybrid cross. Crossing a homozygous dominant animal with a homozygous recessive animal results in offspring that has the phenotype of the dominant characteristic. Crossing BB x bb results in 100% Bb, which are phenotypically black (B=black, b=brown).
When crossing the offspring of the first law, the next generation shows a split of properties; 75% shows the property of one grandparent and 25% the property of the other grandparent. Crossing Bb x Bb produces 25% BB, 50% Bb and 25% bb; since BB and Bb are phenotypically equal, the ratio appears as 75% black and 25% brown.
Dihybrid cross. When crossing animals that differ in two or more properties, these properties are inherited separately. We've already seen this in the diagram of the "No interactions" section.