Single Gene
Disorders and Disability (SGDD) |
What Are Genes?
Genes contain the instructions that tell the cells of people’s bodies how to
grow and work. For example, the instructions in genes control what color a
person’s eyes are and how tall a person will be. Each person gets half of
his or her genes from each parent, and that is why a person tends to look
like his or her parent.
Genes are stored
in a chemical called deoxyribonucleic acid (DNA). DNA is a chemical chain
made up of “bases”. There are four bases: A, C, T, and G. The specific
order, or sequence, of these bases determines the exact information carried
in each gene, like the way that a specific pattern of letters makes up the
words in a sentence.
For more details, click here.
What Are
Mutations?
A mutation is any change in a gene that makes it different from the usual
copy (the copy that most people have). If the change causes a difference in
the way that the gene works, the person with the mutation could have a
particular condition (such as muscular dystrophy) that runs in the family.
Different changes run in families in different ways.
For more details on mutations, click here.
X-Linked
Conditions
X-linked
conditions usually affect only males. A mutation causing a condition can be
passed in the family through female carriers who do not have the condition.
However, each son of a female carrier has a 50% chance of inheriting the
mutation and, therefore, of having the condition.
“X-linked” genes
are genes that are found on the X chromosome. “Recessive” means that a
person can have the condition only if he or she does not have at least one
usual copy of the gene.
A female has two X
chromosomes and, therefore, two copies of each X-linked gene. A woman who
has one usual copy and one copy with a recessive mutation is called a
carrier. She does not have the condition, but can pass the copy with the
recessive mutation on to her children.
A male has only
one copy of the X chromosome, which he has gotten from his mother. If the
copy that he gets is the copy with the recessive mutation, he will have the
condition because he does not have a usual copy of the gene. If he gets his
mother’s X chromosome that has the usual copy of the gene, he will not have
the condition. Therefore, a son of a carrier mother has a 50% chance of
having the condition. A male who has the condition passes his X chromosome
to each of his daughters, so each daughter will be a carrier. A male with
the condition passes his Y chromosome to each of his sons, so none of his
sons will have the condition or be carriers (unless the mother is also a
carrier).
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Date: September 1, 2006
Content source: National Center on Birth Defects and Developmental
Disabilities
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