Each organism has at least two forms of each gene, called alleles, one from the father and one from the mother. They may have the same or different information. Their position on the chromosome is called the locus (from the Latin locus, place, plural loci). For example, for determining the color of eyes, one allele contains the information necessary for blue and the other for brown. When both alleles contain the same information the individual is homozygous, and when they contain different information the individual is heterozygous, for that particular trait. When the two alleles contain different information, normally only one is expressed. This one is known as the "dominant" allele, while the other is called "recessive." The degree of allelic variability in a species is known as heterozygosity.
With no knowledge of the structure of the genetic code, the Austrian naturalist and Augustinian monk Gregory Mendel (1822-1884) described the behavior of the alleles by analyzing crosses of peas with different characteristics. His descriptions of the laws of heredity, published in 1866, are now known as Mendel's laws.
The structure of DNA was deciphered by the British biophysicist Rosalind Franklin (1920-1958), New Zealand physicist Maurice Wilkins (1916-2004), zoologist and biologist James Watson (1928) and English physicist and biologist Francis Crick (1916-2004). The latter three received the Nobel Prize for Medicine in 1962.
In organisms which reproduce sexually, half of the chromosomes originate from each parent. During the formation of sex cells (gametes) in both parents, the chromosome number is reduced by half (in a process known as meiosis). These cells are called haploid(from the Greek haploos, single). During fertilization, the chromosomes from each parent re-assemble and the cells are then called diploid(from the Greek, diploos, double). This means that each organism has two copies of each gene. Some organisms may have three (triploids) or four copies (tetraploids).
