Through each generation, genes are shuffled and re-shuffled. Half of a baby's genes come from its father and half from its mother. The baby's parents in turn inherited half each of their genes from each of their own parents. One quarter of each person's genes therefore come from the grandparents. So how does this happen? Instead of containing the full complement of 46 chromosomes, each egg and each sperm has just half, or 23, chromosomes each. When they meet at conception the chromosomes pair up to again make up the full complement that now forms the genetic blueprint for the new individual. One of the 23 pairs of chromosomes are sex chromosomes, so gender is also determined at conception. Each egg carries an X chromosome and each sperm either an X or Y. If two X chromosomes combine, the baby will be a girl; if an X and Y chromosome combine, the baby will be a boy (see How gender is determined).
Excerpted from Pregnancy Day by Day.
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