Allele is the most important term used in genetics. It is a variation of the gene. Alleles are not genes; they are not physical structures, but rather “abstract” entities that determine how and when genes are expressed (they don’t make proteins).
Alleles can be defined as allelic variations or loci or loci variants and these terms are used interchangeably. An allele is one of two or more forms produced by alternative or alternate forms of genetic material. It is distinguished from another form which produces an alternative form of genetic material; i.e., two alleles can produce one organism with two different genotypes (phenotypes).
|A variation of the gene.||A part of DNA that codes for a specific trait or function in an organism|
|Giving specific traits to living organisms (e.g. blue eyes, brown skin).||A code for the specific trait (e.g. eye color, skin color).|
When this happens, it produces different results on the organism’s phenotype because it uses different parts of DNA for its expression in nature. A single allele can produce many different phenotypes, depending on environmental conditions and even the surrounding organisms in the ecosystem, since there are many different possible combinations of alleles that can be expressed.
Alleles do not have to be the same size; they can vary in size from one another. For example, a particular allele might be small, and a particular allele might be large.
A gene is a part of DNA that codes for a specific trait or function in an organism (e.g., a gene for skin color). Genes are the units of heredity (the inheritance of traits) and also specify how an organism’s characteristics are passed on to its offspring.
Genes come in pairs, one inherited from each parent. Each gene has its own unique sequence of nucleotides (i.e., DNA molecules). Genes contain information that specifies how an organism’s characteristics are passed on to its offspring (e.g., the instructions on how to make skin pigment).
Genes are organized into chromosomes within the nucleus of cells and they consist of long chains of deoxyribonucleic acid (DNA) molecules arranged in structures called genes or chromosomes (see figure below).
Humans have 46 chromosomes. Each chromosome is a tiny piece of DNA that contains all the information needed to build and maintain an organism. Chromosomes are arranged in pairs, one inherited from each parent.
Humans have 23 pairs of chromosomes, one from each parent. The chromosome on which a gene is located is called the “first” or “p” chromosome, and the other is called the “second” or “q” chromosome
The number of genes on each chromosome varies from species to species. For example, humans have about 20,000 genes on their 46 chromosomes (see below). However, it is possible for an organism to have more than one gene on a single chromosome; for example, some fish have two copies of a particular gene on their two chromosomes.