Inheritance Patterns

The presence of a dominant allele, or two recessive alleles, is not always a guarantee that the trait will be displayed in the phenotype, a phenomenon called incomplete penetrance. Variable expressivity also occurs, with some individuals more affected than others of the same genotype. In most cases the reasons for these differences are unknown, but they are assumed to be due at least in part to other differences between individuals. For instance, if there are differences between individuals in the other genes with which the product of incompletely penetrant or variably expressed allele interacts, this may account for some of these differences in expression.

One phenomenon also associated with changes in both timing and severity of expression is anticipation. Anticipation refers to the successive decrease in the age of symptom onset over several generations, so that a condition might first manifest at age 60 in a grandfather, at age 40 in a father, and at age 20 in a son. This increasingly earlier onset is often accompanied by Mitochondrial inheritance passes only through the mother. increasingly severe symptoms as well. Some cases of anticipation are known to be due to changes in the allele itself over time. Spinocerebellar ataxia, for example, is an autosomal dominant disease that causes balance disorders. The normal allele has a section of its DNA that includes approximately 20 repeats of a nucleotide triplet, CAG. The disease allele has 40 or more CAG repeats. This number can increase between generations, leading to earlier onset and more severe disease over several generations. Other so-called triplet repeat diseases show the same pattern of anticipation.