Laws of Heredity

The Laws of Heredity are few; their implications for life are vast. The simplest genetic characteristics are those whose presence depends on the genotype at a single locus; i.e., one gene controls the expression of one characteristic. Such characters are known as Mendelian, after their original discoverer, the Austrian botanist Gregor Mendel. Over 10,000 Mendelian characters have been identified in humans. In sum, Mendel's Laws of Heredity state that:

• Each physical characteristic corresponds to a single gene
• Genes come in pairs
• Only one gene of the pair is passed on to the next generation by each parent
• It is equally probable that either gene will be passed on
• Some characteristics are "dominant" while others are "recessive."
  

A trait (character) is dominant if it is expressed in the heterozygote (only one of the chromosome pair carries the gene) and recessive if it is only expressed in a homozygote (both chromosomes carry the gene). Dominant and recessive are properties of traits, not of genes themselves.

These pedigree patterns are not always as evident in humans as in the pea plants that Mendel originally studied to define these concepts. This is due to a number of confounding factors, chief among them being incomplete penetrance. The penetrance of a character is the probability that a person with the genotype will manifest the dominant character. Other confounders include delayed onset of late-age genetic disorders, multi-gene effects, and variable expression of genes (different features of a single genetic syndrome will appear in similarly affected individuals). In addition, spontaneous mutations can occur where no pedigree association exists.

Mendelian inheritance patterns were the first evidence to unlock the mysteries of heredity. While 10,000 traits are known to be Mendelian, at least as many traits are non-Mendelian. Height, intelligence, personality, and a thousand more characteristics of creatures are multifactorial - controlled by the interaction of numerous genes, each independently assorted. Furthermore, the same confounders for simple Mendelian inheritance (incomplete penetrance, environmental influences, spontaneous mutations) also occur in characteristics determined by multiple genes-but their effects are exponentially multiplied.

Still, the Laws of Heredity have taught us much. They form the basis from which we can begin to understand dynamic interactions between genes within the genome and between the genome and the environment.