Birth defects have a variety of causes. The causes can be divided in 1) Genetics (that can be: a) caused by a defective gene; b) caused by a chromossomal anomaly, or c) mitochondrial anomaly) , 2) Multifactorials, and 3) Environmental. The diseases caused by a defective gene can be inherited as dominant, recessive or X-linked traits, or it can be caused by a "de novo" gene mutation. An individual has two copies of almost every gene and they are called alleles. An allele is dominant when it is expressed I.e, the gene is manifested is present. A recessive allele has to be in double to be expressed, i.e. the two copies of the gene has to be the same. In dominant herance both sexes can be affected, and the risk of having an affected child is about 50%, but many of the dominant diseases have minor manifestations. Examples are: Familial hypercholesterolemia, Huntington disease, Neurofibromatosis, Marfan Syndrome. In recessive diseases the risk of having an affected child is about 25%, both sexes are affected, and many diseses have serious manifestation. The parents of a child with a recessive trait are normal but they carry the gene. It is more frequent in consanguineous mating. Exemples are: Cystic Fibrosis, Tay Sachs , and Phenylketonuria. In X-linked diseases, the defect is transmited by the maternal chromossome to only the males. The females are carriers but they are not affected, or have a minor problem. There is a 50% of chance that the female carrier will transmit the gene to her sons or daughters. Example: Duchenne muscular dystrophy, and Hemophilia A. The mitochondrial inheritance is less common. Only the mother can transmit the gene. Both sexes can be affected. Exemples are: Leber Hereditary Optic Neuropathy, and Mitochondrial Myopathy. The chromossomal anomalies can be numeric or structurals. The human cells have 46 chromossomes, and any anomaly can cause a defect. One of the best known examples is the Down Syndrome (47 chromossomes instead of 46). Multifactorial refer when the cause is a mixture of a genetic susceptibility and environmental factors. Examples: Cleft palate, Congenital cardiac anomalies, pyloric stenosis. Environmental causes refer to birth defects caused by exposition of the fetus to a causative environmental factor. Examples: Alcohol Fetal Syndrome (consumption of alcohol during pregnancy), Fetal Talidomide Syndrome (use of talidomide during pregnancy), fetal rubeolla syndrome. It is important to say that the pre-natal care can prevent some birth defects, specially avoiding the use of alcohol or drugs. The use of folic acid is reccomended by the pregnant women, because they lower the incidence of some multifactorial defects like neural tube defects (like the bifid spine, and anencephalia). Sonograms during pregnancy are very sensitive, and can detect many structural fetal anomalies.