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Mendelian traits behave according to the model of monogenic or simple gene inheritance in which one gene corresponds to one trait. Discrete traits (as opposed to continuously varying traits such as height) with simple Mendelian inheritance patterns are relatively rare in nature, and many of the clearest examples in humans cause disorders.
Under the Omnigenic Model, trait adaptations that are a result of changes in a single to a few genes may be rare, instead the majority of trait adaptations may be driven by small changes in allele frequency over the whole genome. These small changes across the genome would add up to have profound effects on trait expression.
A null allele is a gene variant that lacks the gene's normal function because it either is not expressed, or the expressed protein is inactive. For example, at the gene locus for the ABO blood type carbohydrate antigens in humans, [13] classical genetics recognizes three alleles, I A, I B, and i, which determine compatibility of blood transfusions.
One example of oligogenic inheritance is a case where one gene is sufficient to cause a trait, however its penetrance or expressivity is influenced by another gene, called a modifier. An example of such a case is the gene TGFB1 which modified a person's risk of developing Alzheimer's disease if they are carrying the disease variant of the gene ...
Genetic disorders are diseases that are caused by a single allele of a gene and are inherited in families. These include Huntington's disease, cystic fibrosis or Duchenne muscular dystrophy. Cystic fibrosis, for example, is caused by mutations in a single gene called CFTR and is inherited as a recessive trait. [16]
The detection of causal genes for diseases impacted by locus heterogeneity is difficult with genetic analysis methods such as linkage analysis and genome sequencing. [9] These methods rely on comparison of affected family members, but when different family members have different disease-causing genes, such genes may not be accurately identified ...
Autosomal traits are associated with a single gene on an autosome (non-sex chromosome)—they are called "dominant" because a single copy—inherited from either parent—is enough to cause this trait to appear. This often means that one of the parents must also have the same trait, unless it has arisen due to an unlikely new mutation.
Directional selection for both traits during the same time period would increase the positive correlation between the traits, while selection on only one trait would decrease the positive correlation between the two traits. Eventually, traits that underwent directional selection simultaneously were linked by a single gene, resulting in pleiotropy.