DNA prediction of categorical eye and hair colour on the individual level. Examples of applying the HIrisPlex system to four European individuals (A–D). The actual eye and hair colours are displayed on the left side by photographs. The HIrisPlex prediction results, in terms of the probabilities belonging to certain colour categories, are shown on the right side, where the colour categories with the highest probabilities are highlighted.
Human hair, eye, and skin colour are very complex and difficult to predict, because each of these traits is controlled by more than one gene. It’s not really a matter of a child taking after the father or mother’s side; genes don’t work that way. What matters is which parent has the dominant versions of the various genes that affect the traits in question, because these are the ones most likely to be expressed by the child - though not always.
Every animal (including humans) carries two copies of every gene. Scientists now estimate that a human has about 30,000 genes in his/her genome, and every human has two copies of that genome: one from mom, and one from dad. The two versions of each gene (called alleles) may be the same in a single person, or they may be different. This means that the different versions can combine and interact in unpredictable ways to produce a wide range of phenotypes (physical appearance).
A trait that is controlled by several genes is called a polygenic trait. A polygenic trait is the expression of a single phenotypic trait that is affected by the action of more than one gene. There are too many examples to list, since most traits are - at least to some degree - polygenic. But human hair, eye and skin colour are among them.
Hair colour is a result of interaction between several genes that not only control the colour of the hair pigmentation (one gene controls the expression of brown -eumelanin- pigment and a different gene controls expression of red -phaeomelanin- pigment), but also how much pigment is deposited in the hair shaft. The darker the hair, the greater the melanin deposition, but one can’t really predict how dark a baby’s hair will be, since s/he may inherit a wide variety of “darkness level” genes from both parents, and they can recombine in various ways to produce hair that ranges in colour from very light to very dark.
If a person expresses both the eumelanin (brown) and phaeomelanin (red) genes, the hair will be reddish brown. Dark to light brown hair with no trace of red occurs when only eumelanin is expressed, but in varying concentrations. Blonde hair with no trace of red occurs when there is weak eumelanin expression and no phaeomelanin. Red hair occurs when there is strong expression of phaeomelanin and weak expression of eumelanin. Not all people express both genes, but in dark-haired people that do express both, you can sometimes see a reddish sheen in the hair in certain light. But the darker eumelanin pigment often makes it difficult to see the red pigment, if it’s present.
Light colored eyes (blue, green, hazel, grey, etc.) are usually considered recessive to dark-colored eyes. But this trait is controlled by at least five different genes. There are genes that control whether or not melanin is deposited in the iris (the dominant B allele codes for brown, and the recessive b allele, coding for no melanin, will result in pale irises. These will be blue in the absence of other pigments), the amount of pigment deposited (several genes that can combine to generate eyes that are very dark, almost black to relatively light brown), as well as overlying carotenoid pigments that can change a blue iris to green, aqua, grey, or any number of variations.
And to make things even more complicated, eye colour, like hair colour, can change with age.
Still, one can predict, to some degree, whether a child will have light-colored or brown eyes. The allele coding for light eyes (i.e. lack of melanin in the iris) is recessive to the allele coding for dark eyes (i.e. melanin deposited in the iris). For a person to have light eyes, s/he must inherit two copies of the b allele (genotype bb). A person needs only one copy of the B gene to have dark (brown) eyes, so can be either BB or Bb.
Skin colour is probably the most complex of all the traits. The shade of the skin in humans may be controlled by several genes, each with several alleles, and this makes the prediction of skin tone in a baby a nearly impossible task. x