3.1 Modes of Inheritance
Inheritance patterns describe how a disease is transmitted in families.
These patterns help to predict the recurrence risk for relatives.
In general, inheritance patterns for single gene disorders are classified based on whether they are autosomal or X-linked and whether they have a dominant or recessive pattern of inheritance. These disorders are called Mendelian disorders, after the geneticist Gregor Mendel.
.2 Autosomal Dominant Inheritance
In autosomal dominant inheritance, only one copy of a disease allele is necessary for an individual to be susceptible to expressing the phenotype.
With each pregnancy, there is a one in two (50%) chance the offspring will inherit the disease allele.
Unless a new mutation has occurred, all affected individuals will have at least one parent who carries the disease allele.
Autosomal dominant inheritance is often called vertical inheritance because of the transmission from parent to offspring.
Across a population, the proportion of affected males should be equal to the proportion of affected females.
Male-to-male transmission can be observed.
Examples of diseases with autosomal dominant inheritance include myotonic muscular dystrophy and Huntington disease.