The DNA at a gene locus , i.e. at a specific location on a specific chromosome , can vary between individuals of a species. Such variants of a gene are called alleles of the gene ( ancient Greek αλλήλων allélon “each other, mutually”). Different alleles often cause different expressions of the trait corresponding to the gene in the phenotype of the individual. For example, for a gene that determines flower color, there may be one allele that makes the flowers red and another allele that makes them white. The existence of alleles and their transmission during reproduction explains the inheritance of individual traits and in particular the Mendelian rules discovered by Gregor Mendel in 1865 and now named after him . Harmful alleles cause hereditary diseases.– The phenomenon itself is called allelia. If a gene has more than two alleles, one speaks of multiple alleles. Alleles can be inherited unchanged over a long period of time or can arise from mutation .
The effects of allelia on the inheritance of genotypic and especially phenotypic traits are so diverse that this article can only describe the basic pattern. The living beings considered here are graduates . This means that they produce germ cells, gametes , with a single set of chromosomes, and all the other, the 'somatic' cells, have two sets of chromosomes, one each from the germ cells combined to form the zygote during fertilization . In particular, the following applies to man because he is a graduate.
Alleles in the phenotype
All somatic cells emerge from the zygote by cell division and therefore have identical chromosomes. The observed gene locus shows the alleles originating from the two germ cells on the two homologous chromosomes. If the two alleles are identical, the individual is called homozygous , pure hereditary, if they are different, heterozygous or mixed hereditary.
Alleles and genotype
If the individual is homozygous with an allele A at the gene locus under consideration, A is a gene like any other and causes the expression (or characteristics) assigned to it. Heterozygosity, unequal alleles A and B in the homologous chromosomes, can affect the phenotype in different ways.
Dominant / recessive inheritance
One possibility is that one allele, for example A, permeates its corresponding expression over that of B. The phenotype with the AB gene combination is identical to the inherited phenotype with the AA combination. One then says A dominates B. In the combination AB, A is dominant, B is recessive . One speaks of dominant / recessive inheritance. When Mendel crossed pure red-flowered peas with pure white-flowered peas, RR with WW, he only found red-blooded offspring in the first generation: RW had the same phenotype as RR. Red was dominant, white was recessive.
The Japanese wonder flower also has alleles R and W for red or white flowers. However, neither of the two genes is dominant. Mixed-inheritance individuals show here with the color pink a mixed form of both forms. Such an inheritance is called intermediate.
The simultaneous expression of both variants of the characteristic corresponding to the two alleles is also possible. In the AB0 blood group system , A and B produce two different antigens on the surface of red blood cells. In the phenotype, the combination of AB has the effect that the surface of the blood cells shows both antigens. An inheritance in which each allele unhindered by the other generates the characteristic in its own form is called codominant.
There are many deviations from the above illustration. Some examples. The term 'dominant' is to be understood relatively. The recessive allele can also generate its shape of the trait to a small extent. - Many useful plants and some wild plants are tetra- or octoploid , so no diplonts. The observed gene locus can be occupied with different alleles on the multiplied chromosomes, which results in many different genotypes. - If the protein encoded by a gene is required in large quantities in the cell, this gene can be found on several, even many, gene loci, and on different chromosomes. All of these gene loci can each be occupied by different alleles. - The cell of the man does not contain a homologue to the sex chromosome Y (men have XY, women XX) and one of the two X chromosomes in women is 'muted', i.e. not active.
William Bateson , in his Mendel's Principles of Heredity of 1902, defended Mendel's assumption that there are two variants of the hereditary factors in every diploid cell. He named the copy of the hereditary factor after the Greek word for "other" allelomorph . This term was later shortened to allele .
- Ernst Peter Fischer: History of the gene. Fischer, Frankfurt 2003, ISBN 3-596-15363-8
- Wolfgang Hennig: Genetics. Springer, Berlin 2002. ISBN 3-540-42958-1
- Thomas Junker: History of Biology. Beck, Munich 2004, ISBN 3-406-50834-0
- David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum: Purves Biologie . German Translation ed. by Jürgen Markl. 9th edition 2011. p. 1687.
- Wilfried Janning, Elisabeth Knust: Genetics: General Genetics - Molecular Genetics - Developmental Genetics . 2nd Edition. Georg Thieme, Stuttgart 2008, ISBN 978-3-13-151422-6 , p. 12 f .
- Wolfgang Hennig: Genetics. Springer, Berlin 2002. p. 33.
- Elisabeth Günther: Outline of Genetics, Gustav Fischer, Stuttgart 1971, p. 163
- Ernst Peter Fischer: History of the gene. Fischer Taschenbuch Verlag, Frankfurt 2003, p. 10f.
- Mendel's Principles of Heredity by William Bateson in facsimile
- Jürgen Groth: My molecules. Your molecules. On molecular individuality (online book 2009)
- ALFRED: The ALlele FREquency Database - website on the frequency of alleles in defined human populations