The Complex World Of Hereditary Diseases
It's a common belief that children are born with hereditary diseases when one or both parents suffer from the same diseases. The fact is, hereditary diseases are far more complicated than that. We may get blue eyes or brown hair from our mother or father, or from a grandparent or great-grandparent. That's a little more straightforward, but insofar as hereditary diseases are concerned, it's a whole different ball game. It's not that the parents aren't involved. It wouldn't be called a hereditary disease if they weren't. It's simply that the degree of involvement isn't always apparent, nor can it always be proven.
In addition some so-called hereditary diseases are not always hereditary, or at least not believed to be so. Down syndrome is a case in point. Down syndrome is caused by an excess chromosome, and this can sometimes be due to the genetic makeup of the mother, who may be a "carrier" of the disease, having a chromosome that is abnormal but does not act as such, and does not exhibit the symptoms herself. The father can also be a carrier, but this is rather rare. In such a case, Down syndrome can be hereditary, but this happens only in about 1% of all cases. In most instances, there is no evidence to show Down syndrome has been inherited.
Usually But Not Always The Mutation Of A Single Gene - Other hereditary diseases, such as cystic fibrosis, show a much stronger tie between parent and child, or appears more apt to run in families. The majority of hereditary diseases may be traced to the mutation of a single gene, however proving the mutation will or will not result in an inherited disease proves to be a very complex issue. It seems more likely that several genes may play a role in contributing to a specific disease, and the process isn't something that is spelled out in black and white. Diabetes, some types of cancer, asthma, and mental disorders often appear to be hereditary diseases, as they definitely run in some families. None of these of course is a purely hereditary disease, as they can strike anyone, whether there is a family history of the disease or not.
Multifactoral Inheritance Disorders - Many diseases are the result of a combination of mutations in one or more genes and environmental factors. These are called multifactoral inheritance disorders. Gene mutation, hence heredity, plays a role, but how much of a role is not always clear. Multifactoral inheritance disorders include Alzheimer's disease, heart disease and high blood pressure, diabetes, obesity, and certain types of arthritis. Anyone of these can come about purely as a result of environmental factors, but there are definitely instances where they could be rightfully called hereditary diseases, and gene mutation can play a significant role.
Hereditary Diseases Are More Probabilistic Than Deterministic - At times hereditary diseases can be caused by a gene mutation present in one parent, while in other instances both parents must have a gene mutation, not necessarily the same gene, for a child to inherit a particular disease. Even if both parents have the same gene mutation, it can only be said the child is more likely to inherit the same mutation, but that does not necessarily mean the child will suffer from a specific hereditary disease. As noted above, nothing is black and white. While a study of genetics provides many answers as to why we are what we are, there are still many more unknowns than knowns as to why a genetic mutation may result in a disease in one person and not in another. The wild card in all of this appears to be the constantly changing and different environments different people live in.