Many factors influence gene expression. Some cause the expression of features in deviation from Mendelian inheritance rules. Penetrance and expressivity: The penetrance says something about how often a gene is pronounced. It is defined as the percentage of people who have the gene and a corresponding phenotype develop (s. Penetrance and expressivity.). A gene with incomplete (lower) penetrance can not be marked, although the trait is dominant or recessive, and when it is the gene responsible for this characteristic is present on both chromosomes. The penetrance of the same gene can vary from person to person and depend on the age of a person. Sometimes no abnormality (Nichtpenetranz called.) Can be in carriers of the trait discover clinically; However, a self unaffected carriers can transmit the abnormal allele on his children, who then may show the full picture of this disorder. In such cases, the mode of inheritance seems to skip a generation of offspring. However Nichtpenetranz based in some cases, rather to the fact that an investigator not familiar with the present fault or does not recognize Minor forms. In the case of a less pronounced disturbance is pronounced “fruste forme” also from a. The phenotypic expression (expressivity) of genes is the extent to which a gene is expressed in a person. It can be expressed as a percentage, eg. For example, when a gene has 50% expressivity, only half of the features are present or the severity is only half of what you can expect at full expressivity. The expressiveness can be affected by environmental factors and other genes, so that people may differ in phenotype with the same gene. The expressivity may differ even within a family. Penetrance and expressivity. As the genotype is pronounced in the phenotype depends on the penetrance and expressivity. The penetrance determines whether the gene is pronounced or not. That is, it refers to how many people with the gene the property (disease) have associated with him. (Such., 50% if only half of the people have the property) the penetrance can fully (100%) or incomplete. The expressivity determines how much the property (disease) affects the person and how many symptoms occur. The expression, which can be expressed as a percentage, ranging from a full to a minimal expression, but it can also not be present. Various factors, including genetic predisposition, exposure to pollutants, other environmental factors and age can influence the expressivity. Both penetrance and expressivity may vary: People with the gene may have the feature or not, and in people with the trait, it may be less pronounced. Gender Limited inheritance If a feature only in one sex, it is called sex-linked. A sex-linked inheritance is different from the X-linked inheritance, which relates only to the characteristics that carries the X chromosome. With sex-linked – perhaps more accurately or, influenced by gender – Inheritance special cases are meant in which change by the sex hormones and other physiological differences between men and women due to the expressivity and penetrance of a gene. Early balding is z. As an autosomal dominant trait, but presumably it is up to the female sex hormones that baldness in women is rare, and only if, then after menopause. Genomic imprinting (imprinting) means genomic imprinting (imprinting) that genetic material otherwise comes to expression, depending on whether it is inherited from the mother or the father. For most autosomes both the paternal and maternal alleles are expressed. However, in <1% of the expression of alleles is possible only from the paternal or maternal allele from. For example, the gene for insulin-like growth factor 2 may be normally only expressed from the paternal allele. Genomic imprinting is usually determined by effects that normally occur during the development of gametes. Changes such as the methylation of DNA can lead to certain maternal or paternal alleles are differentially expressed. A fault can apparently skip a generation when the genomic imprinting prevents the causative allele is expressed. Faulty imprinting, such as the abnormal activation or inactivation of alleles can lead to disturbances (for. Example, Prader-Willi syndrome, Angelman syndrome). Codominant inheritance of codominant alleles are considered both. Thus, the phenotype of heterozygotes from homozygotes different. For example, a person who has an allele coding for the blood group A, and one allele coding for the blood group B, both blood groups: the blood group AB. Chromosome inactivation in women who have two (or geschlechtschromosomischen anomalies.> 2) X chromosomes (except eggs), all except one X chromosome inactivated d. H. Most alleles on this chromosome are not expressed. What chromosome is inactivated is random completely determined individually in each cell at the beginning of fetal life, sometimes it is the X of the mother, which is inactivated, sometimes it is the X of the Father. Sometimes the largest share of X chromosome inactivation comes from a parent: the so-called unequal X-inactivation. But does matter occurred in what way inactivation, all descendants of that cell have exactly the same X-inactivation. However, individual alleles on the inactive X chromosome are expressed. Many of these alleles are located on the chromosomal areas associated with the areas of the Y chromosome in relationship (and are thus called pseudoautosomal areas because both men and women receive two copies of these areas). Important Points When a pedigree seems to skip a generation, pull incomplete penetrance, incomplete expression and (less likely) genomic imprinting into consideration. Gene expression can also be modified by gender-limited inheritance, genomic imprinting, co-dominance of alleles, and X-chromosome inactivation.