Genetics can be a difficult thing to understand. Though both parents contribute an equal portion of genes to each child, the actual phenotypes, or physical result, of the genetic code can favor one parent highly over the other. Though sci-fi movies have hinted at the idea of choosing the looks and attributes of each child though selective gene choices, currently this isn't something that is practical or easily done. Unfortunately, there is no way to guarantee that a child with two parents of different hair color will end up with a certain kind of hair. However, there are a few hard and fast rules to help you figure out what color or colors of hair that your child is likely to have, if you can't wait the nine full months to discover the answer for yourself.
Two or More Genes:
There are at least two different genes that determine hair color, that control a black/brown pigment in hair called eumelanin, and a red/yellow pigment called pheomelanin. Blondes are low in both, black haired people are high in eumelanin and low in pheomelanin, and auburn hair has higher levels of both. These aren't just on/off genes, and the colors can be expressed in varying degrees, resulting in hair color from white-blonde to black.
Four Choices From Each Parent:
So each gene has two matching alleles, one that is contributed by each parent. However, each parent has two different alleles that they could have passed on to you. Which means that for each gene, you have four possible combinations of alleles that you could pass on to your baby. To further complicate things, some of these alleles count more than others when it comes to the physical result. Dominant alleles overpower recessive alleles, and when they are present in the gene, they're the only ones you see. Black hair is a dominant allele, which explains why most of the children of black and dark-brown haired parents will be dark haired. However, this is not completely true, because if both parents are carrying the recessive light-haired versions (low eumelanin), then a surprise fair-haired baby may appear.
Baby Color Isn't Always Adult Color
Just because your baby comes out of the womb a platinum blonde doesn't mean he or she will stay that way. Hair color can change several times before adulthood, most often from baby to toddler, and in adolescence.