| Many of our physical and chemical characteristics are
passed along from parents to children. Examples of these include the
color of our eyes, our hair color, and the chemicals which determine
our blood type. The messages which determine these, and the thousands
of other characteristics which make an individual unique, are called
genes. These genes are packaged on long, string-like structures called
chromosomes. Every cell in the body contains all the chromosomes, and
therefore all of the genes, necessary for life.
Each of our cells contains 23 pairs of chromosomes - and therefore
23 sets of gene pairs. One of each pair of chromosomes is inherited
from our mother while the other is inherited from our father. Since
genes are on these chromosomes, we also inherit one gene (or message)
for a certain characteristic (such as eye color) from our mother and
one gene for the same characteristic from our father.
During egg and sperm production, the total number of 46 parental
chromosomes (23 pairs) is divided in half. One chromosome (23 pairs)
is divided in half. One chromosome of each pair, and only one, is
normally passed on in each egg or sperm. When fertilization of the egg
occurs, the 23 chromosomes contained in the egg combine with the 23
chromosomes in the sperm to restore the total number to 46. In this
way, each parent contributes half of his/her genetic information to
each offspring.
All of the chromosomes except the sex chromosomes are called
autosomes and are numbered from 1-22 according to size. One pair of
chromosomes determines the sex of the individual. The sex chromosomes
are of two types, X and Y chromosomes. Females have two X chromosomes,
and males have an X and a Y. Half of all the sperm produced will
contain an X, and the others carry a Y. The sex of the baby is
determined by which type of sperm fertilizes the egg.
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