All About Genetics

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 deoxyribonucleic acid (DNA), a long, spiral-shaped molecule that's 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.

The specific segments of DNA that contain the instructions for making specific body proteins are called genes. Right now, scientists believe that human DNA carries from 25,000 to 35,000 genes. Some genes direct the formation of proteins that eventually determine physical features such as brown eyes or curly hair. Others provide instructions for the body to produce important chemicals called enzymes (which help control the chemical reactions in the body).

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's 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 reasons. Sometimes, a mistake occurs during cell division, causing an error in the 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 examples of this type of genetic problem.

Trisomy 18 syndrome affects 1 out of every 3,000 newborns. Children with this syndrome have a low birth weight and a small head, mouth, and jaw. Their hands typically form closed fists with abnormal finger positioning. They may also have malformations involving the hips and feet, heart and kidney problems, and mental retardation. Only about 5% of these children live longer than 1 year.

Trisomy 13 syndrome affects 1 out of every 5,000 newborns. This syndrome causes cleft lip, flexed fingers with extra digits, hemangiomas (blood vessel malformations) of the face and neck, and many different structural abnormalities of the skull and face. It can also cause malformations of the ribs, heart, abdominal organs, and sex organs. Long-term survival is unlikely but possible.

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, but that chromosomes are 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.

Important genetic disorders caused by deletions and microdeletions include Wolf-Hirschhorn syndrome (affects chromosome 4), Cri-du-chat syndrome (chromosome 5), DiGeorge syndrome (chromosome 22), and Williams syndrome (chromosome 7).

In translocations (which affect 1 out of every 500 newborns), bits of chromosomes shift from one chromosome to another. With inversions (which affect about 1 out of every 100 newborns), small parts of the DNA code seem to be snipped out and reinserted flipped over. Translocations may be either inherited from a parent or arise spontaneously in a child's own chromosomes.

Both translocations and inversions typically cause no malformations or developmental problems in the kids who have them. However, adults with either translocations or inversions who wish to become parents may have an increased risk of miscarriage or chromosome abnormalities in their own children.

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's carried by the X 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 chromosome and are almost always the ones who have the disease.

Gene Mutations

Some genetic problems are caused by a single gene that's present but altered in some way. Such changes in genes are called mutations. When this is the case, the number and appearance of the chromosomes 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: phenylketonuria (PKU), cystic fibrosis, sickle cell anemia, Tay-Sachs disease, and achondroplasia (a type of dwarfism).

Although experts originally believed that no more than 3% of all human diseases were caused by errors in a single gene, new research suggests that this may be an underestimate. Within the last few years, scientists have discovered genetic links to many different diseases that weren't originally thought of as genetic, including several different types of cancer.

Oncogenes (Cancer-Causing Genes)

Researchers have identified 20 to 30 cancer-susceptibility genes that greatly increase a person's odds of getting some form of malignancy. For example, a gene has been identified on chromosome number 9 that may be linked to a common skin cancer called basal cell carcinoma. This gene, labeled PTC or patched, may someday be important in screening for this type of cancer. Another gene, called HNPCC, is carried by 1 out of every 300 Americans and may greatly increase an individual's chance of getting colon cancer. And the doubly dangerous gene called BRCA-1 seems to give women an 85% chance of developing breast cancer, as well as a 50% chance of ovarian tumors.

Other Genetically Linked Diseases

Altered genes may play a role in the development of many other devastating illnesses. Parkinson's disease, for example, may be linked to a gene on chromosome number 4, and multiple sclerosis may be linked to alterations in a gene on chromosome number 6. Alzheimer's disease, linked to a gene on chromosome 19, can already be diagnosed (in some cases) by screening for that altered gene, although such screening is viewed by many as controversial.

Although heart disease and diabetes appear to be related to simultaneous changes in many different genes, the first of these may already have been identified. According to the American Heart Association, this gene may be an artery-clogging gene that almost doubles the risk of fatty deposits blocking the coronary arteries. Having the gene may also triple someone's chances of getting adult-onset diabetes.

It's important to note that much of the newest information from genetic research has not yet been translated into useful screening tests. However, experts predict that this will soon change, and they estimate that the number of available genetic tests will increase dramatically in the years to come.