Did you know that your ethnicity can impact your baby’s risks for developing certain genetic conditions and diseases?
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.
What Are Genes?
Genes play a huge role in who we are. Be it from the color of our eyes to the seasonal allergies that make us sneeze. Our parent’s genes predispose us to behave, appear, and feel in ways that are sometimes beyond our control. And because our genetic makeup is passed from generation to generation, there is sometimes the risk that we will inherit negative characteristics as well. Sometimes these negative traits come in the form of a genetic disorder or disease.
Aside from the everyday cold or flu, many illnesses do have a genetic component. Our genes may not necessarily cause an illness, but they might influence its development, or make it more likely that we will develop a disease. Our genes also regulate and modulate our body’s response to an illness or a medication. Genes are units of DNA which produce individual proteins that keep our body’s machinery working. Each of us has 30,000-40,000 genes in each one of our cells, and half of these are inherited from each parent. Remarkably, we differ from one another only by 0.1 percent of our DNA! But a defect, or mutation, in a single gene can cause a serious (or worse, fatal) genetic disorder that can be transmitted through a family.
Dominant Genes
We inherit our genes in pairs, one from each parent. If one of the gene pair has a stronger effect than its mate, it is called a dominant gene. If there is a defect in that dominant gene, then the parent likely has a genetic disorder, and there is a 50 percent chance that this affected parent will pass this mutated gene onto each of her offspring. There are a few thousand dominant genetic disorders. Examples include, Huntington’s disease (a degenerative disease of the brain), Adult Polycystic kidney disease, Neurofibromatosis, and Marfan’s syndrome.
If one parent has a defective dominant gene:
- There is a 50 percent risk that each child will inherit the gene
- These is an equal chance of transmitting to sons and daughters and of receiving the gene from either parent
- There will be a total absence of the disease from those who do not inherited the harmful gene
- The genetic disorder will appear in every generation (even though signs may be subtle).
The idea that a disease skips a generation is mostly untrue. Most likely, the disease manifested itself in such a way that it was missed because the symptoms were so slight, not looked for, or not recognized. From time to time, a new dominant gene disease will occur for the first time in a family. Afterwards, the person affected, with his new mutation, will transmit the disorder to half of his children. For some of these disorders, especially those that affect the nervous system, the disease will present itself earlier and earlier over each succeeding generation. The result can be as dramatic as a child less than five years of age presenting symptoms and signs of Huntington’s disease.
Recessive Genes
When defects occur in each of the matching genes from both parents, a recessive genetic disease may occur. Examples of recessive genetic diseases include, cystic fibrosis, sickle cell disease, and Tay-Sachs disease. Typical characteristics of recessive inheritance include:
- Parents who may be related (e.g., cousins)
- These is an equal chance of transmitting to sons and daughters and of receiving the gene from either parent
- There is a one in four risk in every pregnancy that the child will be affected.
Usually (but not always) neither parent nor other relatives (other than the child’s siblings) are affected by the gene. Also, some recessive conditions are higher amongst certain ethnic groups as the members of that group may share, as cousins do, some of their genes. There are well-recognized recessively inherited disorders in every ethnic or racial group, and most of these do have a bearing on pregnancy risks.
X-linked Disorders
When a female transmits a single defective gene from one of her two X chromosomes, an X-linked disease may occur. The disease will affect half of her sons, and half her daughters will be carriers. Well-known examples include Duchenne Muscular Dystrophy and the bleeding disorder, Hemophilia A. Typical characteristics of an X-linked type disorder include:
- The disorder is never transmitted from father to son
- But the gene is passed from an affected man to all his daughters and then through them to, on average, half his grandsons.
Not infrequently, carrier females may show signs of the disorder. For example, mothers of boys with Duchenne Muscular Dystrophy often develop heart problems and should therefore have regular cardiac evaluations.
Disorders of the Mitochondria
Floating in the fluid that surrounds the nucleus of each cell are several hundred small circular bits of DNA called mitochondria. A defective gene in these mitochondria can be passed from a mother to all of her children of both sexes. However, not all offspring will show signs of the disorder. And, affected and carrier males will not transmit the mutation to any of their children!
One example of maternal mitochondrial inheritance is one form of deafness. In this case, a specific mitochondrial gene mutation predisposes a person to develop deafness after taking an antibiotic such as Gentamycin. This antibiotic is commonly used in tiny, premature infants and could cause deafness in genetically susceptible infants.
Y Chromosome Disorders
Genes that pass from father to son do so on the male (Y) chromosome. Mutations or missing genes on the Y chromosome may lead to infertility due to absent sperm or a low sperm count. These disorders will be passed (if a father is able to sire children) to all his sons, but to none of his daughters.
Over 8,500 genetic disorders and traits are transmitted through single genes. As emphasized in my column on the Family Tree, knowledge of the family history is critical in recognizing a pattern of gene transmission through the family. And once you know, you can deal with the condition accordingly with the help of a doctor.
