The genes for many traits are passed down in families from parents to
children. For example, if your parents have black hair, then it is likely you
and your brothers and sisters will have black hair. Once in awhile,
though, this doesn't happen and parents with black hair will have a baby
with blond hair.
How this happens is explained by the principle of "segregation." It is one
of the basic principles of genetics and works for plants and animals alike.
Because you come from two parents, each parent will provide one half of
your genes for any trait. That is called an "allele." So you have two alleles
for each gene, one from each parent.
When both alleles tell your body the same thing, then your body
"expresses" that gene. But what if you have two alleles that say different
things? For instance, your earlobes, not something you usually think
about, may be attached or free to flop. This is determined by one gene
that comes in two forms: one for attached earlobes and one for free. If
you got one allele for attached from your mom and one for free from your
dad, how does your body know what kind of an earlobe to make? One of
the earlobe alleles dominates over the other. No matter if the genes
came from your mother or father, certain genes will dominate.
In the case of earlobes, "free to flop" is the dominant allele. So for you to
have attached earlobes, the recessive trait, both your parents must have
given you alleles for attached earlobes. Which means that they must
have gotten them from their parents, who got them from their parents.
Because attached earlobes is a recessive trait, it can sneak thru without
being seen, from generation to generation, unlike a dominant allele. By
watching how a trait is expressed in several generations and throughout
an extended family, you can figure out some of the genes that each
person must have.
The game will help you understand a tricky concept - how two parents
who each have black hair (a dominant trait) can have a child with red
hair (a recessive trait).
Another way a child can have an unexpected trait is when a mutation
occurs. Mutations are changes in the DNA. They sometimes happen for
no reason we can find, and sometimes they happen because we (or
our mothers while they're pregnant with us) are exposed to something
in the environment (like cigarette smoke or certain chemicals) that
makes the DNA more likely to change, or mutate. Most mutations are
neutral - they don't hurt us or help us, and we may not even know
they've happened. Some mutations are harmful, though, and can
make us more prone to certain
diseases or other problems.
Our cells have repair kits to fix most
mutations and keep them from
affecting us, but sometimes the repair
kits don't work as well as they should.
You can help keep your DNA repair kit
working by taking good care of
yourself: eat a balanced, healthy diet
with lots of fresh fruits and
vegetables; avoid cigarettes and other
things that we know are harmful;
wear sunscreen when you're outside;
exercise regularly; maintain a healthy
weight for your age and height; get
enough sleep at night; keep a good
balance between school, work and
fun; and see your health care
provider for regular check-ups.
http://genetics.gsk.com/kids/heredity01.htm