Genetic defects: definition, origin, transmission and evolution, and mode of action

Abstract

This article presents an overview of the concepts and principles relative to genetic abnormalities, for which outbreaks are regularly observed in farm animal populations. These genetic defects originate from natural mutations and the frequency of some of them increases under the effect of genetic drift and sometimes of selection. When they are dominant, they are rapidly counter-selected and tend to disappear. But when they are recessive, the affected cases represent only a very small fraction of carrier individuals. Genetic defects are usually monogenic. However, this rule has many exceptions, either because the abnormality is more complex than initially assumed, or because the phenotype presents a variability caused by modulator genes in addition to the major factor. The recessive defects are mainly caused by loss of function mutations, whereas the dominant mutations often result from interactions between genes or between proteins, or from the loss of function of a repressor gene. Chromosomal abnormalities, when they are not lethal, cause syndromes which may vary between the different types of unbalanced caryotypes. Finally, genetic defects sometimes present very peculiar mechanisms,  e.g. when the mutated gene is located on a sex chromosome or when it is imprinted.