Scientists at Massachusetts General Hospital in Boston found out that cells often randomly deactivate of alleles.
Alleles are gene copies- every human gets one from their mom and one from dad.
This new developmet might explain why some children in a family may have certain heritage disorders, while others do not. This disruption might take place in as many as over 1,000 of our genes (except from sex cells) which leads to different structures and levels in the proteins that are coded by these genes.
Researchers do not know the mechanism by which alleles are silenced yet but the discovery that these events (called random monoallelic expression) occur so frequently suggests that epigenetic effects may play a much more significant role in the development of some human diseases than believed before (Epigenetic Effects are influences on the activity of a gene that are not due directly to DNA mutations).
The research team looked at the activity of certain gene variants in human B lymphocytes which are white blood cells that fight infections. The especially focused on a sample of almost 4,000 genes and found out that nearly 400 of those genes undergo monoallelic expression. A several of those 400 genes that have been found, such as APP, were being linked to human disorders such as Alzheimer's. APP is a gene that is believed to increase the risk of getting Alzheimer's.
Source:
http://www.scientificamerican.com/article.cfm?id=is-disease-inheritance-more-random&page=1