We are all the products of our ancestors: traits such hair and eye color, skin tone, and even the timbre of our voices are passed on. But we don't just inherit physical traits. Genes, chromosomes, and DNA are passed from generation to generation as well. As such, we may each carry a number of harmful genes determined by our ancestors. These genes are often single recessive defective genes and their effects are, for the most part, inconsequential, so most of us remain completely unaware of them. Potential problems can arise, however, when both parents have the same defective gene.
We inherit our genes from our parents equally. The mode of transmission is discussed in How to Have the Healthiest Baby You Can: Understanding How Genes Impact Disease. As I wrote in that piece, those who carry a dominantly inherited disorder have a 50 percent risk of transmitting the harmful gene to each of his (or her) offspring. Recessive disorders are inherited when both parents carry the same harmful gene. There is then a 25 percent risk in each pregnancy of having an affected child. Sex-linked disorders are transmitted through harmful genes directly from a mother to half her male offspring, while half her female offspring will be carriers.
We have inherited almost all our genes from our very distant ancestors. When our forefathers moved from Africa to Asia and then to Europe, their genetic passport not only remained intact, but for the thousands of years after, mixed with their partners' genes to produce today's generations. During those population movements, ethnic, racial, and religious groups tended to stay together and find partners in their own group. As a result, if there was a harmful gene mutation, it spread among that particular group. This explains why there are specific disorders or "founder mutations,” that are characteristic, but not absolutely specific, of a particular ethnic group. This inbreeding also explains why marriages between first cousins or uncle-niece unions, have an increased risk of genetically defective offspring. But it is important to know that sometimes genes do mutate naturally, and single gene disorders can happen out of the blue, such as Duchenne muscular dystrophy, achondroplastic dwarfism, hemophilia A, tuberosclerosis, neurofibromatosis and many other disorders. In fact, about half of single gene dominant disorders are caused by a new mutation. With hemophilia A and Duchenne muscular dystrophy, for example, about one third of those affected were born with a new mutation.
When planning pregnancy, it is important to know your genetic family history and also your ethnic origin. Just knowing this information can help you avoid having a child with a serious or fatal genetic disorder.
Risks and Recommended Tests for Disorders Based on Ethnic Origin
If both parents carry one of the specific defective genes outlined below, there is a 25 percent risk of having an affected child. Testing for these disorders requires only a straightforward DNA analysis of a small blood sample.
If You Are White:
There is a one in 25 likelihood of carrying a gene mutation for cystic fibrosis (a serious and potentially lethal chronic disorder of the lungs and pancreas). Standard recommendations now (by the National Institutes of Health and the American College of Medical Genetics) are for all couples to have this test prior to starting a pregnancy or less optimally in the earliest weeks of a pregnancy. The carrier detection rate approximates 90 percent.
For Those of Mediterranean Origin: Italians, Greeks, Cypriots, Spaniards, Asians and Orientals
There is an approximate one in 12 likelihood of carrying a defective gene for the serious hemolytic anemia called beta-thalassemia. Incredibly, there are over 150 million people who are carriers of this disorder.
For Those of Armenian, Turkish, Arabic and Sephardic Jewish Origin:
The odds are approximately one in 8 for carrying a defective gene for familial Mediterranean fever. This disorder is characterized from childhood by unexplained fevers, abdominal, chest and joint pains, and later in life, by kidney failure. Early diagnosis by DNA analysis is important since very effective treatment is available for this disorder.
If You Are Black:
There is an approximate one in 12 likelihood for carrying the common gene mutation for the serious and potentially lethal sickle cell anemia.
For Those of Ashkenazi Jewish Origin:
Standard recommendations now include DNA carrier tests for Tay-Sachs disease (one in 27 to one in 30) and Canavan’s disease (one in 40) (both degenerative diseases of the brain that are invariably fatal) with optional testing available for Gaucher’s disease (odds one in 15), Niemann-Pick disease type A (one in 90), familial dysautonomia (one in 30), Fanconi anemia (one in 90), and deafness without any other features due to a mutation in the Connexin-26 gene (odds one in 26).
For Asians and Orientals
The risk of being a carrier of the common serious hemolytic anemia called alpha thalassemia varies, but is mostly higher than one in 10. There is also a major advantage for carriers of the thalassemias. Carriers are more resistant (protected) to malaria!
Don't ignore your ethnic origin. Straightforward DNA tests provide opportunities to avoid starting a pregnancy if your offspring are at high risk, and if pregnancy termination is not an option you would consider. If both parents are carriers of a specific genetic disorder, and prenatal diagnosis is not a choice you wish to make, other options include artificial insemination by donor, receiving a donated egg, preimplantation diagnosis, or adoption. Knowing and understanding your own genetics is the first step towards having a healthy family.