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Genes that encode for continuous traits are found widely across the genome, with a gene that has a significant effect on trait expression occurring every 10,000-100,000 base pairs. The distance between these significant effects within the genome implies that these significant genes are not tied to similar regulatory pathways.
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.
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.
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]
Mendel consciously chose pairs of genetic traits, represented by two alleles for his inheritance experiments. In nature, such genes often exist in several different forms and are therefore said to have multiple alleles. An individual usually has only two copies of each gene, but many different alleles are often found within a population.
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 ...
Pseudoplesiomorphy – is a trait that cannot be identified as neither a plesiomorphy nor an apomorphy that is a reversal. [18] Reversal – is a loss of derived trait present in ancestor and the reestablishment of a plesiomorphic trait. Convergence – independent evolution of a similar trait in two or more taxa. Hemiplasy [19] [20]
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.