Law of Segregation
Modern genetics owes a tremendous debt to the work done by the 19th century Austrian monk Gregor Mendel. In his work cultivating pea plants, Mendel discovered three laws: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance. The Law of Segregation is best defined by stating that during the formation of a gamete, the alleles segregate so that each gamete carries only one allele for each gene. Mendel’s observation on this law was later proved by German botanist Oscar Hertwig in 1876 and Belgian zoologist Edouard Van Beneden in 1883.
For every trait passed on from parent to child (animal or plant), a pair of alleles for that trait, dominant or recessive, exists. For example, in humans, eye color is an allele. Each parent then passes on randomly one of the alleles to the child. A child receives its own paired alleles for a particular trait from both parents. Under the law of segregation, the two alleles of a trait separate and end up in different gametes in the child.
A gamete is a cell that fuses with another during fertilization. In humans, the egg (ovum) and sperm are two gametes. Human eggs carry only X chromosomes, while sperm can be either X or Y, thus determining the sex of the child. Under the law of segregation, a gamete will receive one of the two alleles for any trait, either the dominant or recessive trait.