Definitions

Although scientists commonly refer to "the" human genome, each of us, with the exception of identical twins, is a unique genetic mosaic, one that never existed before and never will again. Our uniqueness is due to the way inheritance operates. When the body cells that produce sperm cells and egg cells divide, one member of each original chromosome pair goes to one new cell and the other member goes to the other new cell. That is why sperm cells and egg cells contain only twenty-three unpaired chromosomes.

Chance alone decides which member of each chromosome pair goes to a particular sperm or egg. When you work out the mathematics, you find that each sperm- or egg-producing cell has the potential to produce more than 8 million different chromosome combinations in each new sperm or egg. But the actual diversity is even greater, because genes can spontaneously change, or mutate, during formation of a sperm or an egg, and because small segments of genetic material are apt to "cross over" (exchange places) between members of a chromosome pair before the final division.

The upshot is that each of us is the potential parent (given time and energy) of billions of genetically different offspring. And because it takes two to make a baby, and each parent separately contributes one of billions of possible combinations to each child, the potential number of genetic combinations from any given set of parents is staggering. Thus siblings, even though they share, on average, half their genes and are apt to be more alike in some ways than two biologically unrelated people, can also be unlike one another in many ways. And children, who share, on average, half their genes with each parent, can be unlike their parents, in purely genetic terms.

Psychology in Perspective. Carol Tavris, Carole Wade. (HarperCollins 1995). Pages 70-71.