Thus a male need have only one gene for baldness for the trait to be expressed, but a female must have two. These should be distinguished from congenital birth defects. A congenital defect is one that the infant is born with, such as a cleft lip, a birthmark, or congenital syphilis, but the defect can arise during conception or pregnancy and not be related to heredity. Hereditary diseases, on the other hand, are passed from generation to generation by genes. Some diseases, such as cystic fibrosis, are transmitted by recessive genes.
Mutation is the term used for a spontaneous change in a chromosome or gene.
Normally chromosomes duplicate themselves exactly during cell division. Occasionally, however, the new cells contain an altered gene or chromosome. If the mutation occurs in an ovum or sperm involved in reproduction, the new trait will be expressed in the offspring. Many mutations are so minor that they have no visible effect. A mutation that is very harmful will usually result in the death of the fetus and spontaneous abortion.