Inheritance is the transmission of genetically controlled characteristics from parents to offspring. Individuals have multiple chromosome pairs that carry the genes responsible for inherited traits. During sperm and egg formation, paired chromosome strands separate from each other so that the offspring inherits one of each pair of chromosomes from each parent. The strands of a specific chromosome pair look alike and have genes that code for the same traits. Characteristics among individuals differ because of slight variations in genes for specific traits. These alternative gene forms are called alleles. Brown and blue are alleles for eye color just as tall and short are alleles for height.
An organism's genotype is its complete set of genes and alleles. Its phenotype is its observable physical or functional traits, such as hair, eye or skin color. Some genes for a specific trait are dominant and others are recessive. When an organism receives a dominant gene from one parent and a recessive gene from the other, it displays the trait carried by the dominant gene. For example, the dominant allele for brown eyes rules over the recessive allele for blue eyes. An individual who receives two brown eye alleles, or a brown eye allele from one parent and a blue eye allele from the other, has brown eyes. An offspring who receives recessive alleles from both parents has blue eyes.
Geneticists use symbols as shorthand designations for different genes and their alleles. Because a trait results from two genes, one from each parent, they use two symbols to represent the genotype of a particular trait. They typically choose the first letters of the words used to describe the dominant traits. Uppercase letters indicate dominant alleles, and lowercase letters indicate recessive alleles. For instance, if one chooses the letter "B" to designate the dominant allele for brown eyes, he would use "b" to symbolize the recessive blue eye allele. A genotype of Bb indicates that an individual has one dominant allele for brown eyes and one recessive allele for blue eyes. Therefore, she has brown eyes.
The letter system for gene designation allows scientists to record and convey a lot of information using just a few letters. It is an efficient, succinct and unambiguous method for describing genes and their traits in research experiments, scientific literature and genetic databases. Although there is general consensus regarding gene designation, a standard method of assigning symbols to represent the vast number of genes found in living organisms remains a challenge, particularly when assigning symbols to the thousands of different genes found in humans. Consequently, nomenclature committees have been established to set and periodically review rules for assigning gene names and symbols.