which genes cause hereditary diseases? and why do some families carry these genes?

Question: which genes cause hereditary diseases? and why do some families carry these genes?
- Keywords:
  disease,
  gene
Asked by tom73 to Asif, Laura, Lena, Sean, Viv on 19 Mar 2012.
- Lena Ciric answered on 19 Mar 2012:
  
  There are many genes that cause many different diseases. Usually it’s a small mutation in genes that results in a deficiency of some sort. Most diseases are a result of a number of mutations in a number of genes rather than just one gene. These mutations came about by accident and have then been passed down through generations. The diseases they caused weren’t serious enough to stop the people from having children and so they have been passed down.
- Sean Murphy answered on 19 Mar 2012:
  
  Some genetic diseases result from what we call recessive genes. We have two copies of each gene, and often one is “stronger” than the other. The weak gene is called recessive and the strong gene “dominant”. Usually the dominant gene is strong enough to correct for any mutations in the recessive gene. However, in some families, the chance of receiving two recessive genes (one from each parent) is higher. When someone inherits two recessive genes their chances of getting genetic diseases is increased.

Question: which genes cause hereditary diseases? and why do some families carry these genes?