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The word "allele" is a short form of "allelomorph" ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders) in the 1900s, [7] [8] which was used in the early days of genetics to describe variant forms of a gene detected in different phenotypes and identified to cause the differences between them.
Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. [2] The number of alleles an individual can have in a specific gene depends on the number of copies of each chromosome found in that species, also referred to as ploidy. In diploid species like humans, two full sets of ...
The definition normally excludes regions of the genome that control transcription but are not themselves transcribed. We will encounter some exceptions to our definition of a gene - surprisingly, there is no definition that is entirely satisfactory. [16] A gene is a DNA sequence that codes for a diffusible product.
In this way new combinations of genes can occur in the offspring of a mating pair. Genes on the same chromosome would theoretically never recombine. However, they do, via the cellular process of chromosomal crossover. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. [70]
DNA usually occurs as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. The set of chromosomes in a cell makes up its genome; the human genome has approximately 3 billion base pairs of DNA arranged into 46 chromosomes. [96] The information carried by DNA is held in the sequence of pieces of DNA called genes.
Genes are often considered the fundamental units of heredity and are typically encoded in DNA. A particular gene can have multiple different versions, or alleles, and a single gene may influence many different phenotypes. gene dosage The number of copies of a particular gene present in a genome.
The words homozygous, heterozygous, and hemizygous are used to describe the genotype of a diploid organism at a single locus on the DNA. Homozygous describes a genotype consisting of two identical alleles at a given locus, heterozygous describes a genotype consisting of two different alleles at a locus, hemizygous describes a genotype consisting of only a single copy of a particular gene in an ...
A haplotype (haploid genotype) is a group of alleles in an organism that are inherited together from a single parent. [1] [2] Many organisms contain genetic material which is inherited from two parents. Normally these organisms have their DNA organized in two sets of pairwise similar chromosomes. The offspring gets one chromosome in each pair ...