Inherited defects resulting from mutant genes or chromosome abnormalities tend to occur in patterns of inheritance. Such patterns include dominant (in which the defect will occur if either parent supplies an abnormal gene to its offspring), recessive (in which both parents must supply an abnormal gene) or others, such as sex-linked (in which the gene is nearly always associated with the X chromosome and not the Y chromosome).
Some common diseases or disorders caused by genetic defects include deficiencies of particular enzymes that lead to the body’s inability to perform normal metabolic functions, and chromosome abnormalities that can result in sterility, abnormal growth or development, or fetal death. Viruses, certain drugs, and radiation are common causes of chromosomal damage.
The complex interaction between genetic and environmental factors is being studied and is becoming better understood.
Factors that tend to produce abnormalities in body formation include toxic plants (such as poison hemlock, which may contribute to contracted tendons in foals), viral infections that occur during pregnancy, drugs, trace elements, nutritional deficiencies, and physical agents such as radiation, abnormally high body temperature, uterine positioning, and twisting of the umbilical cord. These factors may be difficult to identify. They often follow seasonal patterns and stress and may be linked to maternal disease. They do not follow the pattern of family inheritance that is shown by genetic changes.