An Introduction to Genetics and Genetic Testing
What do you know about your family tree? Have any of your relatives had
health problems that tend to run in families? Which of these problems affected your
parents or grandparents? Which ones affect you or your brothers or sisters now?
Which problems might you pass on to your children? Thanks to advances in genetics,
doctors now have the tools to understand how certain illnesses, or increased risks for
certain illnesses, pass from generation to generation. According to some health
experts, the definition of an inherited or genetic illness should be expanded beyond
the classic inherited disorders (like hemophilia and sickle cell anemia) to include
many types of cancer, Alzheimer's disease, and other illnesses. They look toward a
future where genetic test results are an important part of every healthy person's
medical file.
Genes and Chromosomes
Each person has a unique set of chemical blueprints that determines how his or
her body looks and functions. These blueprints are contained in a complex chemical
called deoxy-ribonucleic acid (DNA), a long, spiral-shaped molecule that is found
inside each body cell. DNA carries the codes for genetic information and is made of
linked subunits called nucleotides. Each nucleotide contains a phosphate molecule, a
sugar molecule (deoxyribose), and one of four coding molecules called bases
(adenine, guanine, cytosine, or thymine). The sequence of these four bases
determines the genetic code.
Specific segments of DNA that contain the instructions for making specific
body proteins are called genes. Right now, scientists believe that human DNA carries
up to 30,000 genes. Some genes direct the formation of structural proteins that
eventually determine physical features such as brown eyes or curly hair. Others
provide the instructions for the body to produce important chemicals called enzymes.
Sometimes, depending on the codes of a specific gene, even a small error within the
DNA structure can mean serious problems for the entire body. Sometimes, an error in
just one gene can result in a life that is shortened or physically difficult. Genes are
found in specific segments along the length of human DNA, neatly packaged within
structures called chromosomes. Every human cell contains 46 chromosomes,
arranged as 23 pairs, with one member of each pair inherited from each parent at the
time of conception. After conception, these 46 chromosomes duplicate again and
again to pass on the same genetic information to each new cell in the developing
child. Human chromosomes are large enough to be seen with a high-powered
microscope, and the 23 pairs can be identified according to differences in their size,
shape, and the way they pick up special laboratory dyes.
Genetic Problems. Abnormal numbers of Chromosomes (Trisomies and
Monosomies)
Genetic problems can happen' for many different reasons. Sometimes a mistake
occurs during cell division, causing an error in chromosome number either before or
shortly after conception. The developing embryo then grows from cells that have
either too many chromosomes or too few. In trisomy, for example, there are three
copies of one particular chromosome instead of the normal two (one from each
parent). Down syndrome, trisomy 18 (Edwards) syndrome, and trisomy 13 (Patau)
syndrome are all examples of this type of genetic problem. In monosomy, another
form of number error, one member of a chromosome pair is missing. There are too
few chromosomes rather than too many.
Deletions, Translocations, and Inversions
Sometimes it's not the number of chromosomes that's the problem Instead, one
or more chromosomes is incomplete or abnormally shaped. In both deletions and
microdeletions, for example, some small part of a chromosome is missing. In a
microdeletion, the missing part of a chromosome is usually so small that it amounts
to a single gene or only a few genes. Some important genetic disorders caused by
deletions and microdeletions include: Wolf-Hirschhom syndrome (affects
chromosomes 4); Cri-du-chat syndrome (chromosome 5); DiGeorge syndrome
(chromosome 22); and Williams syndrome (chromosome 7). In translocations, Bits of
chromosomes shift from one chromosome to another, whereas in inversions, small
parts of the DNA code seem to be snipped out and reinserted flipped over.
Sex Chromosomes
Genetic problems also occur when abnormalities affect the sex chromosomes.
Normally, a child will be a male if he inherits one X chromosome from his mother
and one Y chromosome from his father. A child will be a female if she inherits a
double dose of X (one from each parent) and no Y. Sometimes, however, children are
born with only one sex chromosome (usually a single X) or with an extra X or Y.
Turner syndrome is the name of the disorder affecting girls born with only one X
chromosome, whereas boys with Klinefelter syndrome are born with XXY or XXXY.
Sometimes, too, a genetic problem is X-linked, meaning that it is carried by the V
chromosome. Fragile X syndrome, which causes mental retardation in boys, is one
such disorder. Other diseases that are carried by genes on the X chromosome include
hemophilia and Duchenne muscular dystrophy. Females may be carriers of these
diseases, but because they also inherit a normal X chromosome, the abnormal X may
be canceled out. Males, on the other hand, only have one X and are almost always the
ones who have the disease.
Gene Mutations
Some genetic problems are caused by a single gene that is present but altered in
some way. Such changes in genes are called mutations. When this is the case,
chromosome number and appearance are often entirely normal. To pinpoint the
defective gene, scientists use sophisticated DNA screening techniques. Some
examples of genetic illnesses caused by a single problem gene include: cyclic
fibrosis, sickle cell anemia. Tay-Sachs disease, and achondroplasia (a type of
dwarfism).
