Expert: Dana Krempels, Ph.D. - 10/22/2008

Question
me and my boyfriend are white but my boyfriends father is black. what colour baby will we have

Answer
Dear Tara,

Inheritance of skin color in humans is controlled by approximately ten different genes.  The more genes controlling a single trait, the harder it is to predict the outcome for a child with mixed heritage.

Human hair, eye, and skin color 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.  

A brief explanation:
Every human carries two copies of every gene.  Scientists now estimate that a human has about 30,000 genes in the 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.

Let's say that there's a human gene that codes for the shape of the forehead hairline.  There are two versions of the gene.  One, which we'll call "W" codes for a small "V" of hair to point down onto the forehead (Widow's Peak).  The other version, which we'll call "w", codes for a straight hairline.  In this case, the W allele of the gene masks the expression of the w allele.  The W is dominant, and the w is recessive.  So if every person has two copies of this gene, then the possible combinations are:

WW - Widow's peak
Ww - Widow's peak
ww - straight

Human hair, skin and eye color are not that simple.  Instead of being controlled by only one gene, these traits are each controlled by *several* different genes, each with two or more versions (alleles).
This means that the different versions can combine 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 color, eye color, and skin color are among them.

One cute, easy-to-see example of a polygenic trait is the inheritance of fruit color in bell peppers, and it is a bit analogous to the human traits just named. There are at least three genes involved here, which we'll abbreviate as:

  * Y - timing of chlorophyll elimination (Y - early; y - normal)
  * R - color of carotenoids (R - red; r - yellow)
  * C - regulation of carotenoid deposition
   (C - normal; c1, c2 - lowered concentration)

(The capital letters indicate the dominant alleles; the lower case indicate various versions of recessive alleles.)

This leads to a few possible genotypes producing interesting phenotypes:
        o Y- rr c1c2 - pale yellow
        o Y- rr Cc2 - darker yellow
        o yy rr CC - green
        o Y- R- CC - red
        o yy Rr CC - purple
        o Y- Rr Cc2 - pale yellow

You can see what these look like here:

http://www.bio.miami.edu/dana/pix/bellpeppers.jpg

See?  It is a little bit like human color, but in this case there are only *three* genes involved.  Imagine how complicated things get when there are more than three genes, as there are in human hair, eye, and skin color!

The more genes involved in the expression/appearance of a trait, the more possible variations there are, and the more impossible it becomes to guess what a baby will look like, especially if you don't know the exact genetics of the parents.  (Knowing the grandparents' phenotypes can help, but usually not very much.)

Hair color is a result of interaction between several genes that not only control the *color* of the hair pigmentation (brown eumelanin pigment or 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.

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 (1) 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), (2) 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 (3) 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 color--like hair color--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).  We usually abbreviate the light eye allele as "b" and the dark-eye allele as "B"--they are different versions of the same gene.

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 color 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.  That said, light-colored skin is generally recessive to darker colored skin.  Since both you and your boyfriend have light-colored skin, the various alleles of the skin color genes you pass on to your child will *most* likely recombine to produce a child with lighter skin--though this isn't a guarantee.  If your boyfriend has any genes coding for dark pigmentation (and if his father is black, then this is likely), they could recombine and produce a child with darker skin.

I know this doesn't give you the answer you were seeking, but I hope it at least gives you an idea of how truly complicated (if not impossible) it is to predict something like this.  

Whatever color the baby, I hope s/he will be happy and healthy!  Good luck!

Dana