Genes dictate just about everything about our bodies, from the color of our eyes to the shape of our ear lobes. But sometimes they get damaged or mixed up in ways that cause illnesses from birth. Is it because of what mothers eat, drink, or other conditions in the womb? No. "Most of them are just random errors that accumulate over time as the DNA divides," explains Ellen Simpson, Ph.D., a genetics counselor in the prenatal diagnostic center at the University of California, San Francisco Medical Center. "Every time our cells divide, they have to make a copy of our DNA and these mistakes accumulate. Most genetic disorders are due to ancient mutations that first happened a long time ago."
Many of us are walking around with between 5 and 10 recessive mutations, meaning we don't get sick because we only have one copy of the mutated gene and are just carriers. But, if we make a baby with someone who is also a carrier of that mutation, there's a 25 percent chance the child will get the mutation from both of you and officially have the disorder. That's the case in illnesses like cystic fibrosis, sickle cell anemia, and Tay-Sachs. There are other genetic disorders, such as Marfan syndrome, achondroplasia, and Huntington Disease, in which a child only needs to inherit the genetic mutation from one parent.
Chromosomal disorders, like Down syndrome, aren't inherited through genes -- but they happen when the baby receives either too many or too few copies of a gene or chromosome. Parents who know they carry a gene mutation, or are concerned they might, can have preconception genetic testing to determine their future children's risk for certain disorders. Pregnant women routinely have several different screening tests to determine whether their child is likely to have a genetic disorder.
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