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Human Body

9 8 The egg bobbed at anchor, with a thousand others, made in your mothers ovary before her own birth. A generation later it set sail, and met your fathers sperm swimming into port, spawned just two moons earlier, as one of that days batch of 300 million. Apart from the fusion of gametes sperm and egg at the moment of conception, all cells are made by cell division mitosis p.14, or in the rare case of gametes, meiosis. A gamete has 23 chromosomes, the long strands of DNA that carry genes, so when two gametes unite, the conceptus has 46, which are copied into every cell in your body. The chromosomes in the gametes that came together to make you were remixes of your grandparental chromosomes, made in prophase I of meiosis, when each of your parents parents pairs swapped parts of their limbs. So you are a meeting of your parents, but a proper mix of your grandparents. Girls 23rd chromosomes are Xs from both parents, but boys inherit their fathers Y and one of their mothers Xs. Of their two Xs, girls inactivate one in every cell to form a Barr body. Vanguard sperm batter the giant ovums shield before one lucky straggler gets in, bringing its 23 chromosomes to join the 23 that the ovum doesnt discard at this point she waited for ages in late anaphase II, to make a unique new set of 46 chromosomes. PrEconcEPtIon two generation gaps
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