Inheritance Patterns

An organism's observable characteristics, such as height, hair texture, skin color, or ear shape, are known as the phenotype of that organism. The phenotype is determined partly by the environment and partly by the set of genes that the organism inherited from its parents. Adult height, for instance, is due partly to nutrition (an environmental influence), and partly to a set of genes governing things such as rates of bone growth, sensitivity to specific hormones, and the like. Phenotype includes not only large-scale characteristics such as height, but every expressed trait, including the types and amounts of all the proteins produced in each cell in the body.

The set of genes an organism inherits is known as its genotype. Genes are carried on chromosomes in the cell nucleus. Animals and most other multicellular organisms possess two sets of chromosomes in each cell, one set inherited from the mother, and one from the father. Such an organism is said to be diploid. In humans, the maternal and paternal sets each include 23 chromosomes, so humans have 46 chromosomes in each cell. Analysis shows that the maternal and paternal chromosome sets are virtually identical, and they can be matched up to form 23 pairs. One pair, however, may not be a pair at all. These are the sex chromosomes, so called because they determine the sex of the organism. In humans, the female carries two identical sex chromosomes, called X chromosomes, while the male carries two dissimilar chromosomes, one X and one Y. The other 22 pairs of chromosomes are called autosomes.