What are "Gene Mutations"?
is a change in a gene that prevents it from working properly.
mutation is a change that occurs in the egg or the sperm, or both,
and is passed from one parent or both parents to the child.
bone marrow failure syndromes
(IBMFS) result from mutations, or genetic changes, in one of the
many genes which
have been found to cause these diseases (see
individual syndrome descriptions). These mutations occur in
several different patterns within families. These patterns define
the specific kind of genetic inheritance by which each disease
is transmitted. Examples include:
inheritance: Females have two X chromosomes
(XX), and males have an X and a Y chromosome (XY). Combinations
of these two chromosomes determine if a person is to be either
male or female. These two chromosomes (the X and the Y chromosomes)
are therefore called the "sex
chromosomes." If a mutation occurs on one of the motherís
X chromosomes, and if she passes that abnormal X chromosome to
her son, who has only one X chromosome, he will have the disease.
If he receives the other, unaffected, normal X chromosome from
his mother instead, he will not have the disease. Since daughters
have two X chromosomes, the normal X chromosome can compensate
for the abnormal X chromosome should she inherit it, and so daughters
do not have the disease; they are "carriers" of the
inheritance: Each person has a total of 23 pairs of chromosomes.
One chromosome from each pair is inherited from a personís mother,
and the other chromosome in the pair is inherited
from the personís father. 22 pairs are called "autosomes",
and the other pair is called the sex
chromosomes (XX or XY). In order for a person to develop an
autosomal recessive disease, they must inherit two abnormal
copies of the disease-related gene,
one from each parent. Each parent of a patient with an autosomal
recessive disease has one copy of the abnormal gene and one copy
of the normal gene. As a result, the parents typically have no
disease, because the one normal gene is sufficient to prevent
disease from developing.
dominant inheritance: Disease occurs in the presence of
only one mutant chromosome,
whose effect is stronger than the effect of the paired normal
chromosome. Thus, anyone with one mutant gene
is affected. Children of a person with one mutant and one normal
gene have a 1 in 2 chance of receiving the mutant gene, and thus
having the disease. However, the strength of the effect of the
mutant gene is not the same in every person, and thus the appearance
of the disease may be of variable severity or even inapparent
in someone who carries the mutant gene.
are small structures inside each cell which contain DNA
(molecular material). This DNA is separate from the usual DNA
found in the nucleus
of each cell.
Mitochondria produce much of the energy that cells need to function
in a normal manner. Mitochondria are passed from mother to child
in the motherís egg. Sperm do not contain mitochondria. Thus,
only mothers can pass diseases caused by mutations in mitochondrial
DNA to their offspring.