An Asymmetrical Relationship

"Tug of War Pits Genes of Parents in the Fetus"

According to Mendelian genetics, offspring inherit half of each parent's genome. There is growing evidence, however, which suggests an asymmetrical relationship between certain maternal and paternal genes. The genetic mechanism responsible for this is called imprinting, and it may explain "differences in a mother's and father's contributions to social behavior." The idea behind imprinting is that either the mother's or father's copy of a specific gene is inactivated.

A prime example of this involves the insulin-like growth factor 2 gene, which helps a fetus grow. In the paternal genome this gene is active, but is not in the maternal genome. Harvard evolutionary biologist Dave Haig's theory helps to explain this. His theory proposes a conflict of interests between the fetus and the mother. The fetus wants as much nutrition as possible, while the mother wants to distribute her resources evenly to all her future children. This particular gene is imprinted in humans, mice, and various other mammals. 1300 imprinted genes have been discovered in mice. It is estimated that 1% of the human genome is imprinted.

Another example of imprinted genes involves the sex chromosomes. Female offspring receive an X chromosome from both parents. One copy, in theory, is chosen at random to be switched off. There is a much greater chance that the paternal X will be switched off in the cortex. Imprinting may be responsible for some diseases. Since one copy of the gene is switched off, a mutation to the other could be extremely harmful.