Homozygosity

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Homozygosity (from ancient Greek ζυγωτός zygotós , German , connected ' ) is a term used in the art of genetics and means homozygosity .

A diploid organism has from every gene that z. B. the blood group or hair color encoded, two copies, usually one from each parent. These are on the homologous chromosomes . Different variants of a gene are called alleles . If both alleles of an individual are the same for a certain trait, the genetic make-up for this trait is inherited or homozygous. If, on the other hand, there are two different alleles, this is referred to as mixed inheritance or heterozygosity .

The differences between these alleles do not necessarily have to become clear in the appearance ( phenotype ) of an individual, so that the genotype can not always be inferred exactly from the phenotype .

A special case of purity can be present in sex chromosomes , for example , if genes occur only once in the otherwise diploid individual. This is called hemizygosity rather than homozygosity . An example is the X chromosome (apart from the pseudoautosomal region ) in male mammals .

Examples

If a person has blood group A, then different genetic variants are possible:

  • "AA" (pure inheritance): Both parents inherited the genetic information for blood group A, so the gene for "A" is on both chromosomes.
  • "A0" (mixed inheritance): One parent inherits blood group A, the other has blood group 0. Since blood group A is dominant over blood group 0, the person still has blood group A. Nevertheless, the genotype of this person now contains a recessive characteristic for the formation of blood group 0.

The genes for blood groups “A”, “0” or “B” are different versions of the same trait, they are alleles. If there are more than two alleles, one speaks of multiple alleles (compare: polymorphism ).

If the difference between pure and mixed herbalism cannot be determined from the outside, one can often make it

  • through genetic testing
  • based on the hereditary characteristics of the parents
  • based on the characteristics inherited from the children

determine. If both parents have inherited traits for a gene, the offspring are not necessarily inherited. For example, if one parent has the genotype "AA" for the blood group and the other parent has "00", all offspring have "A0".

Homozygosity is aimed at in animal and plant breeding in order to achieve pure-bred parent generations which, through hybrid breeding , should lead to the greatest possible heterozygosity and thus to the greatest possible heterosis effect .

Homozygosity can lead to a genetic problem if both parents are heterozygous about an inherited recessive disorder. This does not occur in the parents, but according to Mendel's rules, the probability that this disease breaks out is 25 percent for every child .

literature

  • Lexicon of Biology . 7th volume. Spektrum Akademischer Verlag, Heidelberg 2004, ISBN 3-8274-0332-4 .