The theories of Mendel, based on his work with the garden pea, are still the basis for introductory study in the field of genetics. The basic unit of hereditary expression, the gene, is a structural unit. Genes on chromosomes determine the characteristics of the individual, such as hair coat color, size, metabolic function, reproduction, the occurence of genetic disease, and other traits that we can see or measure. Many traits are also influenced by non-genetic factors, referred to as environmental effects. Animals that reproduce sexually produce sex cells, or gametes.
Females produce eggs and males produce sperm.
The sex cells contain a sample one-half of the chromosomes, or genetic information, of the individual. When the sperm of the male parent fertilizes the egg of the female parent, the complete genetic makeup of the progeny, or offspring, is determined. This pairing of genetic information equally from each parent means that individuals contain the genetic information passed on from their ancestors. Genes are usually represented with letters and symbols. This is an example of heterozygous genotype.