A large number of polymorphic blood proteins are found in the blood that are inherited in known Mendelian patterns. The best-known system of blood proteins is the ABO system, which consists of three basic blood antigens.

Over 10,000 inherited traits have now been recognized in humans. Phenylketonuria is an example of an abnormality that is inherited as a simple recessive; achondroplastic dwarfism, on the other hand, is inherited as a simple dominant. Not all patterns of inheritance follow these simple Mendelian rules. One distinctive pattern of inheritance occurs when the gene in question is located on the X chromosome. Since males inherit only one X chromosome from their mothers and one Y chromosome from their fathers, genes on the X chromosome are not paired as they are in females.

A number of abnormalities are not due to single genes but to errors in the number and structure of chromosomes. Finally, looking at the molecular level, we see that genetic abnormalities can be specific errors in the genetic code.

Today, a gene is seen as a section of DNA that has a specific function, such as the coding of a particular protein. One of the most ambitious ventures of the past decade is the Human Genome Project, the mapping of the entire human genome.