The following example refers to humans. The same general principles apply for all animals, but the numbers of chromosomes and sex determination are different in many cases.

All the DNA in our cells is contained in 46 chromosomes. A chromosome is basically another way of saying "long piece of DNA." Just like encyclopedias are contained in volumes, DNA is contained in chromosomes. We get 23 chromosomes from our mother, via the egg, and 23 chromosomes from our father, via the sperm. Most of the chromosomes are numbered based on their size, chromosome 1 being the largest and 22 being the smallest. Each cell in our body contains two chromosome 1's, two chromosome 2's, two chromosome 3's, etc. The X and Y chromosomes are given unique names because they are sex-specific. Of the 46 chromosomes in each of a woman's cells there are two X chromosomes. Men have one X and one Y chromosome in each cell. When egg and sperm are formed, each gets only half of the chromosomes. Thus, an egg will have one each of chromosomes 1 through 22 and an X chromosome (women don't have Y chromosomes). A sperm will have one each of chromosomes 1 through 22 and either an X or a Y chromosome. If a sperm containing an X chromosome fertilizes an egg, then the resulting child will be female (two X chromosomes). If a sperm containing a Y chromosome fertilizes an egg, then the resulting child will be male (XY).

Okay... so what does this have to do with purple mice? Lets say a mouse has one copy of the purple hair gene and it is on chromosome 7. There is a 50% chance of the specific chromosome 7 with the purple hair gene getting into a given egg/sperm. If a mouse is born with one copy of the purple hair gene, the issue of dominant/recessive traits arises. Quite simply, a dominant gene is one for which the effect is noticed if there is a single copy and a recessive gene is one for which the effect is noticed only if there are no other dominant genes present (therefore recessive genes must usually be present in two copies). So imagine we crossed a purple haired mouse with a black mouse and the child got one purple hair gene and one black hair gene, you wouldn't notice the purple for the black. If we crossed a purple haired and white haired mouse and the child got one purple hair gene and one white hair gene, you would be able to see the purple hair. By repeatedly breeding purple haired mice, you could eventually get a mouse with two copies of the purple hair gene.

On the next page, we will discuss how DNA works.