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By nature, everyone is born with an inheritance: 23
chromosomes from each parent that arrange themselves
into 23 pairs in each cell. Chromosomes are made up
of genes, the body’s blueprint. A gene can change
(mutate), and the altered gene can be passed on within
a family. These altered genes, alone or in a pair (one
from each parent) cause the body to function differently,
often in an imperceptible way, but sometimes with serious
results. Also, many genes have common variations called
polymorphisms, which can increase the risk of certain
diseases.
When the inheritance pattern is dominant, only one
flawed gene in a pair is necessary for a condition to
occur. A person who has an autosomal dominant disorder
like neurofibromatosis 1, which causes tumors to grow
along the nerves, may have a parent with the condition.
The chances are 50-50 that a parent will pass along
an autosomal dominant disorder to a child.
Autosomal
Dominant Disorder |
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| In autosomal dominant disorders,
if one affected parent has a disease-causing gene
that dominates its normal counterpart, each child
in the family has a 50% chance of inheriting the
disease-causing gene and the disorder. Image and
caption provided by Science
+ Literacy for Health, a project of the AAAS
Directorate for Education and Human Resources.
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An autosomal recessive pattern is different: both parents
“carry” one altered gene. There’s
a 50 percent chance that a child will also be a carrier;
a 25 percent chance that a child will have received
one altered gene from the mother and one altered gene
from the father, and thus have the disorder; and a 25
percent chance that a child will neither be a carrier
nor have the disorder. Sickle cell anemia, a blood disorder,
is an example of an autosomal recessive condition. The
mother and father of a child with sickle cell anemia
themselves each have one altered sickle cell gene, but
because they each also have an unaffected gene, they
do not have the condition.
Autosomal
Recessive Disorder |
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| In diseases associated with altered
recessive genes, both parents -- although disease
free themselves -- carry one normal gene and one
altered gene. Each child has one chance in four
of inheriting two altered genes and developing the
disorder, one chance in four of inheriting two normal
genes, and two chances in four of being a carrier
like both of the parents. Image and caption provided
by Science + Literacy for Health, a project of the
AAAS Directorate for Education and Human Resources.
Image and caption provided by Science
+ Literacy for Health, a project of the AAAS
Directorate for Education and Human Resources.
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X-linked recessive disorders are inherited through
the X chromosome, which is passed on by the mother.
X-linked diseases almost always occur in males. Males
have only one X chromosome, so a single recessive gene
on that X chromosome will cause the disease. The Y chromosome,
present in all males, does not protect the male from
the recessive disorder. Females inheriting one affected
x chromosome become carriers. It is extremely rare for
a female to inherit an x-linked recessive disorder.
In that case, the mother must be a carrier of the gene
and the father must be affected with the disease; the
female would then have a 25% chance of inheritance.
X-linked recessive disorders can be quite serious, such
as in the case of hemophilia and Duchenne muscular dystrophy.
X-Linked
Recessive |
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| In this example, an unaffected
woman carries one copy of a gene mutation for an
X-linked recessive disorder. She has an affected
son, an unaffected daughter who carries one copy
of the mutation, and two unaffected children who
do not have the mutation. Image and caption provided
by Science + Literacy for Health, a project of the
AAAS Directorate for Education and Human Resources.
Image and caption provided by Science
+ Literacy for Health, a project of the AAAS
Directorate for Education and Human Resources.
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Inheritance
Patterns
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