I have a freind whose father is enormous (Olympic weightlifter) and whose mother is tiny tiny tiny. Neither however would be diagnosed with any congenital abnormalities. My question is, How is it that such people with such vastly different genetic traits bear normal looking children? How come you rarely get cases of a huge head and small feet for example? How does the body reign in to an average? And i do understand there are tons of genes for each attribute, we can’t deny that often a child gets “his mother’s eyes and his father’s nose”. Why can’t he get his father’s cranium and his mother’s feet?
But I do know that in some breeds of dog, like the American Pit Bull Terrier, they can range in size from 25# to 80#. A breeder I know once bred a 60# male with a 30# female and the litter came out about half 30-35# and half 55-60# with none in between. I think with humans you usually get more of the “in between”.
This is a phenomenon known as “regression towards the mean.” For most characteristics that are determined by a large series of genes, such as body size, hair color, general physique, etc., if the parents are far from the population mean, the offspring will be closer to it. This goes both for like and unlike parents: for example, two extremely tall people are likely to have tall children, but the kids won’t be quite as tall as the parents.
As for not having a huge head and small feet, the general size of body appendages is apt to be correlated, because they are likely controlled by a series of developmental genes for overall body size - there are no separate genes that control head size completely independant of foot size.