Many thousands of hereditary human ailments and characteristics are now known to be due to changes in specific genes. Human genetics studies their causes and transmission. Prognosis is possible on the basis of diagnosis and family histories. Under certain conditions the probability of gene and chromosome distribution and thus of genetic sickness in the next generation may be calculated. During pregnancy prenatal diagnosis (e.g., by amniocentesis in the 16th to the 17th week, by analysis of the chorionic villi during the 8th to 11th weeks, or by other proce…