Other Free Encyclopedias » Medicine Encyclopedia » Genetics in Medicine - Part 3

Nomenclature

chromosome genetic clinical genes chromosomes gene chromosome

Like any other field in science, genetics has its own language. However, genetics is also a multidisciplinary field that encompasses expertise, and hence terminology, from diverse areas of science, including molecular biology, statistics, clinical medicine, and, most recently, bioinformatics. Despite all of the new and changing language in the field, two of the most frequently used terms in genetics are still "chromosomes" and "genes."

Humans have twenty-three pairs of chromosomes. One member of each pair is inherited from the person's mother, and the other from the father. Of the pairs, twenty-two are known as autosomes. The remaining pair consists of the sex chromosomes, which determine a person's gender. Females have two X chromosomes, and males have one X chromosome and one Y chromosome.

Chromosomes are located in the nucleus of a cell. During cell division, which is known as mitosis, the chromosomes' long strands coil up tightly, to the point where they can be seen as individual units under the microscope. At this stage, each chromosome is composed of two identical strands, called chromatids (each of which further consists of two strands of nucleotides). Chromatids are attached at a constriction point called the centromere.

Chromosomes can be distinguished by their size and by their "banding pattern." Researchers use a chemical staining process in the laboratory to create the banding pattern, allowing them to see the chromosomes more easily. Each chromosome is divided into two sections, or "arms," with one arm on each side of the centromere. The short arm is called the p arm, and A chromosome, with banding patterns, band numbers, and major physical landmarks. the long arm is the q arm. The bands on each arm are numbered. As new and better staining techniques are developed, the numbering system is also refined, so that band 32, for example, would be subdivided into 32.1, then 32.15, and so on. One particular position on the long arm of chromosome 5 would be referred to as 5q32.15.

Almost every human chromosome contains more than a thousand genes. Therefore, even a small extra piece or missing piece of a chromosome results in hundreds of genes being added or deleted from an individual. When researchers study a person's chromosomes, they try to determine if there are any missing or extra chromosomes or chromosome pieces. The addition or deletion of genes sometimes causes a recognizable genetic disorder. Down syndrome, for example, results when there are three copies of chromosome 21 rather than the normal two.

Genes are very small structures that lie on chromosomes. They are the instructions, or blueprints, for producing proteins, which are the building blocks that our bodies use to grow, develop, and function. Humans have an estimated thirty thousand to forty thousand genes.

As happens with other types of scientific discovery, the person who discovers a gene names it, and a scientist can name a gene anything he or she wants. This has led to some confusion, as different naming schemes are used by different groups. To bring order to the situation, several international working groups are trying to standardize the naming of genes. There are separate working groups that focus on naming genes from humans, mice, fruit flies, plants, and other organisms.

Some scientists choose names based on the clinical disorder that is thought to be associated with changes in the gene. For example, one gene was named CFTR because changes in its sequence are associated with the disease cystic fibrosis. Geneticists studying fruit flies traditionally use single-word names, such as wingless, hunchback, and sevenless, that refer to the effect of a mutation in the gene. "Sevenless" refers to the absence of the R7 protein.

For human genes, abbreviations are commonly used. Abbreviated names are especially useful for genes with long names. WNT2, for example, stands for "wingless-type MMTV integration site family member 2." Although the word "wingless" seems unnecessary (humans, of course, don't have wings!), WNT2 is named after similar genes in fruit flies. Genes are often named after genes they resemble in other organisms.

Sometimes the gene name is actually a variation of the name of the protein it makes. For example, the RELN gene in the human encodes the "reelin" protein. The "reelin" protein was named for the reeling motion exhibited by mice that lack a functional copy of the protein.

Other genes are classified based on what their proteins do. For example, HOX genes (short for homeobox) are genes involved in development. Individual HOX genes are named with additional letters and numbers, such as HOXA1 or HOXD9. The consistent naming system lets scientists know that any gene with the name HOX is likely to play a specific role in development.

There are even playful gene names that have nothing to do with a disorder or protein. An example is the SHH gene, which is involved in the This is a photograph of a high-resolution chromosome analysis commonly referred to as a "karyotype." This analysis shows 46, XX which means that this patient has the normal number of chromosomes and is a female since she has two X chromosomes. development of the brain, spinal cord, and limbs. The SHH gene is named after the cartoon character Sonic the Hedgehog!

Chantelle Wolpert

Bibliography

Internet Resource

HUGO Gene Nomenclature Committee. <http://www.gene.ucl.ac.uk/nomenclature>.

Nondisjunction - The Mechanism Of Nondisjunction, Non-fatal Human Aneuploid Conditions, Fatal Versus Nonfatal Conditions, The Causes Of Nondisjunction And Its Frequency In Humans [next] [back] History Nature of the Gene - From Elements To Genes, The Chromosome Theory, Gene Action And Mutation, The Molecular Gene

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