Biology Reference
In-Depth Information
FIGURE 1.1. Recessive deafness. The probabilities (in parentheses) of each of the
possible offspring phenotypes and genotypes are shown for parents who are both
carriers.
deafness in the spouses, then all their offspring will probably be heterozy-
gous for two deafness genes, and not homozygous for either of them. Thus,
deaf spouses, who may each have many deaf relatives, can have offspring
with normal hearing.
1.2.3 X-linked Inheritance
Males have both an X and Y chromosome; females have two X chomo-
somes. Thus, a necessary condition for X-linked inheritance is no father-
to-son transmission because a son receives his X chromosome from his
mother. If a disorder has an X-linked dominant pattern of inheritance with
complete penetrance, then all daughters of affected fathers are affected, and
each child of an affected mother has a 50% chance of being affected. In the
case of an X-linked recessive trait, the majority of affected individuals are
male because they have only one X chromosome. Females have two X chro-
mosomes and may be carriers, but are unlikely to be affected. Sons of a
carrier mother each have a 50% chance of inheriting the abnormal allele,
while daughters have a 50% chance of being carriers. Note that, although
females have two X chromosomes in a cell, in general one of the two is ran-
domly inactivated early in embryonic development. Thus, females are
mosaic, with each cell having one or the other X chromosome active.
1.2.4 Mitochondrial Inheritance
Mitochondria are small organelles that are located within the cytoplasm of
a cell; they are independent of the nucleus and have their own DNA
