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In this section, you can investigate what genes
are and what they do, and even play a game testing your knowledge
of what causes disease. Just click on where you want to go.
What Are Genes?
Chromosomes contain the recipe for making a living
thing. They are found in almost every cell’s nucleus and are
made from strands of DNA (deoxyribonucleic acid). Segments of DNA
called "genes" are the ingredients. Each gene adds a specific
protein to the recipe. Proteins build, regulate and maintain your
body. For instance, they build bones, enable muscles to move, control
digestion, and keep your heart beating.
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Courtesy
of the National Human Genome Research Institute
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Most of our cells contain 46 chromosomes (here is an early look
at our chromosomes, taken in the 1950s by Dr. Joe Hin Tjio of
the National Institute of Diabetes and Digestive and Kidney
Diseases). Sperm and egg cells contain only 23 chromosomes.
When the sperm and egg cells unite, the resulting fetus inherits
half of its DNA recipe from its mother and half from its father.
Two of these 46 chromosomes determine the sex
of a person. A girl inherits two X-chromosomes, one from her
mother and one from her father. A boy inherits one X-chromosome
from his mother and a small Y-chromosome from his father.
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Chromosomes. Collection of DeWitt Stetten, Jr., Museum of
Medical Research |
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| The Austrian monk Gregor
Mendel (1822-1884) was the first person to describe how traits
are inherited
from generation to generation. He studied how pea plants inherited
traits such as color and smoothness, and discovered that traits
are inherited from parents in certain patterns. Not until the
20th century did other scientists take his ideas further. Mendel is considered to be the father of genetics, although his work was relatively unappreciated until the early 20th century. |
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Gregor
Mendel and Pea Plants.
Courtesy of Medical Arts and Photography Branch |
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A gene can exist in many different forms, calledalleles.
For example, let’s say that there is one gene which determines
the color of your hair. That one gene may have many forms,
or alleles: black hair, brown hair, auburn hair, red hair,
blond hair, etc. You inherit one allele for each gene from
your mother and one from your father.
Each of the two alleles you inherit for a gene
each may be strong ("dominant") or weak ("recessive").
When an allele is dominant, it means that the physical characteristic
("trait") it codes for usually is expressed,
or shown, in the living organism.
You need only one dominant allele to express a dominant trait.
You need two recessivealleles to show a recessive form of
a trait. See the heredity diagram for tongue rolling to see
how dominant and recessive alleles work.
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Tongue Rolling Heredity Diagram
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| For example, mild forms
of red/green color blindness are very common, resulting only
in the inability to tell apart shades of red and green. The
gene for this trait
is located on the X-chromosome. A mother who carries this recessive
trait has normal red/green vision. Any of her sons who inherit
the X-chromosome that carries this trait -- the allele for color
blindedness -- will be mildly red/green color blind. In this
chart used to test for color-blindedness, people with normal
color vision can see the number seven. People with red/green
color blindness cannot see the number seven. |
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Red/Green "Seven" Chart.
Courtesy of the National Eye Institute |
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There are several ways the genetic
code can be altered. Sometimes genes
are deleted or in the wrong place on a chromosome,
or pieces of genes are swapped between chromosomes. As a result,
the gene may not work or may turn on in the wrong part of
the body.
"Point mutations" alter
the genetic code by changing the letters in the codons
-- the three-symbol genetic words that specify which protein
to make. This can change the protein.
Original message: SAM AND TOM ATE THE HAM
| What it does |
Kind of point mutation
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Example of altered protein |
| Frameshift mutation |
Message starts in the wrong place |
AMA NDT OMA TET HEH AMS |
| Stop codon |
Prevents part of the protein from being
made |
SAM AND TOM |
| Missense mutation |
Causes an amino acid substitution |
SAM AND TOM ATE THE DAM |
| mRNA splicing mutation |
Portion of message is left out, leading
to a shortened protein |
SAM THE HAM |
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