At the level of the gametes

As a diploid individual, you have two copies of DNA: one that you got from your father and one that you got from your mother (refer to Chapter 3). Your children will also have two copies of DNA: one copy that they got from you and one copy that they got from the other parent. That means that each of your offspring got only half of your genes and half of their other parent's genes. You're diploid, your partner is diploid, and the only way to make sure your offspring are diploid is for your gametes (the egg and sperm) to be haploid — that is, to contain only half of your DNA.

Which half? This is where genetic randomness kicks in. At some locations of your genome, both copies are the same. Or in science-speak, both alleles at that locus are identical. In this case each of your gametes (be that sperm or egg) will have the same allele at that locus; there's only one to choose from. But other times, you have two different alleles at that locus because the one you got from your mother and the one you got from your father weren't the same. Any given gamete you produce will get one or the other.

You've been successful: You've survived (you looked both ways before you crossed the street), you found food (maybe that was why you crossed the street), and now you're about to pass your genes on to the next generation, but it's a matter of chance which of the two different alleles you carry will make it into any particular gamete. On average, each allele will end up in half of your gametes. While you produce a huge number of gametes and the two alleles are equally represented, only a paltry number of offspring are likely to result from them.

Women have about 400,000 eggs; men produce even more sperm. But women don't have 400,000 offspring. For that reason, which particular genes your offspring have are up for grabs. In other words, the production of any one child involves an element of chance, and the kids you end up with are the result of random pairings, as it were.

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