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| Single Gene
Disorders and Disability (SGDD) |
The DNA is packaged into small units
called chromosomes. Each cell in a person’s body contains a set of
chromosomes, and thus a set of genetic instructions. Every person should
have 23 pairs of chromosomes. One chromosome from each pair is from the
person’s mother and one chromosome of each pair is from the father.
The first 22 pairs of chromosomes are
the same in everybody. The 23rd pair are called the sex chromosomes, and
they help determine if a person is male or female. A female has two X
chromosomes, and a male has one X and one Y chromosome. A mother will give
one of her two X chromosomes to each of her children. A father will give
either his X or his Y chromosome. If the child gets his father’s Y
chromosome, he will be a boy. If the child gets her father’s X chromosome,
she will be a girl.
The following figure shows how
children get their chromosomes, and therefore their genes, from their
parents. In this figure, three pairs of chromosomes are shown: pair #1
(green), pair #2 (yellow), and the sex chromosomes (pink and blue). The
father’s chromosomes are shown in solid color, and the mother’s are striped.
Children randomly get one of each pair of chromosomes from their mother
(striped) and one of each pair from their father (solid). Each daughter gets
an X from her mother (striped) and an X from her father (solid). Each son
gets an X from his mother (striped) and a Y from his father (solid).
[Return to Top]
Date: September 1, 2006
Content source: National Center on Birth Defects and Developmental
Disabilities
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