Chromosomal abnormalities are a group of conditions that are the
result of a problem with one of the 23 pairs of chromosomes.
Chromosomes are the structures within the body’s cells that
contain genes. Normally, people have 23 pairs of chromosomes.
For each of these pairs, one chromosome comes from the mother
and the other chromosome comes from the father. However, an
abnormality can affect any chromosome, including the sex
chromosomes. Usually, people have two sex chromosomes. Females
have two X chromosomes (XX) and males have one X and one Y
chromosome (XY). A chromosomal abnormality can affect the number
of chromosomes, the structure of certain chromosomes, or the
composition of chromosomes. If the material found in chromosomes
is balanced so that the expected amount is found in each cell,
no abnormalities occur. If that balance is upset, chromosomal
abnormalities occur and can cause a wide range of abnormalities,
usually birth defects or death of the embryo or fetus before
birth. (trisomy 21) is the most c