Probably 3 %. a mother could give birth to a child with Trisomy 21 at any age because Trisomy 21 is a mutation and mutations happen all the time. Not all mutations are as Down Syndrome. A child born to two homozygous brown eyed parents could be born with blue eyes even if no one in the family had blue eyes. The child would develop blue eyes due to mutation.
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First thing is Male gametes are called sperm and Female gametes are called eggs. So some physical differences are that the eggs are spherical in shape, much larger then sperm, and are immobile (they stay in the female reproductive organ during fertilization). Sperm are smaller, are in a cone shape, and have tails so the could move to the egg during fertilization.