Modes of Inheritance
- Inheritance 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.
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.
Autosomal Recessive Inheritance
- In autosomal recessive inheritance, two copies of a disease
allele are required for an individual to be susceptible to
expressing the phenotype.
- Typically, the parents of an affected individual are not
affected but are gene carriers.
- As with autosomal dominant inheritance, the proportion of
affected males should be equal to the proportion of affected
females in a given population.
- Examples of diseases with autosomal recessive inheritance
include sickle cell anemia and cystic fibrosis.
X-Linked Dominant Inheritance
- As in autosomal dominant inheritance, only one copy of a
disease allele on the X chromosome is required for an individual to
be susceptible to an X-linked dominant disease.
- Both males and females can be affected, although males may be
more severely affected because they only carry one copy of genes
found on the X chromosome. Some X-linked dominant disorders are
lethal in males.
- When a female is affected, each pregnancy will have a one in
two (50%) chance for the offspring to inherit the disease allele.
When a male is affected, all his daughters will be affected, but
none of his sons will be affected.
X-Linked Recessive Inheritance
- As in autosomal recessive inheritance, two copies of a disease
allele on the X chromosome are required for an individual with two
X chromosomes (a female) to be affected with an X-linked recessive
disease.
- Since males are hemizygous for X-linked genes (they have only
one X chromosome), any male with one copy of an X-linked recessive
disease allele is affected.
- Females are usually carriers because they only have one copy of
the disease allele. Affected males are related through carrier
females.
- For a carrier female, with each pregnancy there is a one in two
(50%) chance her sons will inherit the disease allele and a one in
two (50%) chance her daughters will be carriers.
- Women are affected when they have two copies of the disease
allele. All of their sons will be affected, and all of their
daughters will be unaffected carriers.
- Examples of diseases with X-linked recessive inheritance
include Duchenne muscular dystrophy, hemophilia A and hypohidrotic
or anhidrotic ectodermal dysplasi