Autosomal recessive inheritance

Most mutations inactivate genes and act recessively. Autosomal
recessive disorders occur in individuals who are homozygous
for a particular recessive gene mutation, inherited from healthy
parents who carry the mutant gene in the heterozygous state.
The risk of recurrence for future offspring of such parents is
25%. Unlike autosomal dominant disorders there is usually no
preceding family history. Although the defective gene is passed
from generation to generation, the disorder appears only
within a single sibship, that is, within one group of brothers
and sisters. The offspring of an affected person must inherit
one copy of the mutant gene from them, but are unlikely to
inherit a similar mutant gene from the other parent unless the
gene is particularly prevalent in the population, or the parents
are consanguineous. In most cases, therefore, the offspring of
an affected person are not affected.