Inheritance

In the biological field, inheritance is generally defined as the passing on of genes from one generation to the next. This transfer takes place at the genetic level.

Inheritances

The most important inheritances are:

• Autosomal dominant inheritance: The gene in question is located on an autosome and behaves dominantly. It is sufficient (dominant) to cause the disease. A typical example of autosomal dominant inheritance is neurofibromatosis generalisata type I (v. Recklinghausen). If both parents have the mutation, there is a homozygous autosomal dominant inheritance. Normally, autosomal-dominant inherited diseases affect structural proteins. If > 50% of a structural protein has an abnormal composition, the function is disturbed. The disease becomes clinically manifest.
• Autosomal recessive inheritance: The gene in question is located on an autosome and behaves recessively (non-dominant). In recessive inherited disease, both parents must have a mutated allele. They are therefore carriers of the trait, but are not affected. Mostly, these are enzymatic diseases in which the normal gene in the heterozygous state can compensate for the function.
• X-linked inheritance: The gene in question is located on the X chromosome. These inheritances are usually recessive. Only very rarely is X-linked dominant inheritance. Here, mutation on an X allele leads to disease.
• X inactivation: In the female organism, either the paternal or the maternal X chromosome is randomly inactivated in individual cells during early embryogenesis (Lyon effect). However, the daughter cells then retain their original activation.
• Y-chromosomal inheritance: the most obvious inheritance through the Y-chromosome is the male sex.
• Mitochondrial inheritance: Mitochondrial inheritance is extrachromosomal inheritance. The genes are not located on the chromosomes in the nucleus, but in the mitochondria. The mitochondrial genome consists of 14 protein-coding regions. In contrast to chromosomal inheritance, these genes are not passed on by both parents, but only by the mother (maternal inheritance). Known mitochondrial diseases mainly include muscular, neurological and ophthalmological diseases. Approximately 10% of children with mitochondrial diseases suffer from skin and hair diseases (trichothiodystrophy, hypertrichoses, exanthema and pigment disorders).

Tumor-associated genes: Certain mutated genes can predispose to tumor development. There are 3 main categories:

• Tumor suppressor genes
• Oncogenes
• DNA repair genes

The genetic information of humans, the entirety of which is called the genome, is encoded on the DNA(deoxyribonucleic acid). The special packaging form of DNA is called chromosomes. The human being has 46 chromosomes, whereby 44 autosomes and 2 gonosomes (sex chromosomes) are distinguished. The autosomes and the gonosomes of the woman (XX) are present twice (2n = 46). Men have the gonosomes XY.

The presence of a double set of chromosomes is called diploidy.

Certain sections of DNA that code for certain characteristics are called genes. In most cases, genes code for certain proteins, which can then take over very different functions in the human organism. The entirety of genetic information is called the genome, the entirety of all proteins is called the proteome. The human genome contains about 25,000 genes. On average, a gene codes for 3 proteins, which then take over different functions in the human organism. The special genetic make-up of an individual is called genotype, the special expression of the characteristics is called phenotype. In the genome, about 2000 phenotypes have been precisely assigned up to now.

A gene of the same gene locus (gene locus) can be changed in its nucleotide sequence by mutations. This results in different alleles (gene versions) of a gene locus. These then lead to different expressions of a characteristic in different individuals.

In diploid organisms (diploidy) there are 2 alleles of a gene, which can be the same (homozygous) or different (heterozygous). In heterozygosity, the expression of the corresponding characteristic is determined either by both alleles or by only one allele (dominant/recessive characteristic expression).

• The science that deals with heredity is called genetics. Human genetics deals with human heredity and with hereditary diseases that are related to it.
• The most comprehensive source of information on genetic diseases is the online database of Victor A McKusick(http://www.ncb.nlm.nih.gov/omim/)

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