[Wells, Catherine]

Wells, Catherine  asked the Naked Scientists:

Hi,

I understand about dominant and recessive genes in such things as eye colour, but was wondering how it is that if a black and a white person have a child together it would be brown?  Also, what causes 'marbled' skin colour that you see quite rarely on some people?

Many thanks,

Cath

What do you think?

[Evie]

Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in a great variety of different skin tones.

Incomplete dominance
Discovered by Karl Correns, incomplete dominance (sometimes called partial dominance) is a heterozygous genotype that creates an intermediate phenotype. In this case, only one allele (usually the wild type) at the single locus is expressed in a doseage dependent manner, which results in an intermediate phenotype. A cross of two intermediate phenotypes (= monohybrid heterozygotes) will result in the reappearance of both parent phenotypes and the intermediate phenotype. There is a 1:2:1 phenotype ratio instead of the 3:1 phenotype ratio found when one allele is dominant and the other is recessive. This lets an organism's genotype be diagnosed from its phenotype without time-consuming breeding tests.

The classic example of this is the color of carnations.

     R    R'
R   RR   RR'
R'  RR'  R'R'

R is the allele for red pigment. R' is the allele for no pigment.

Thus, RR offspring make a lot of red pigment and appear red. R'R' offspring make no red pigment and appear white. Both RR' and R'R offspring make some pigment and therefore appear pink.

Another readily visible example of incomplete dominance is the color modifier Merle in dogs.

http://en.wikipedia.org/wiki/Human_skin_color
http://en.wikipedia.org/wiki/Incomplete_dominance#Incomplete_dominance