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Beyond Biology 101: A glossary

Most scientific fields have a specialized vocabulary. Genetics is no exception. Understanding this vocabulary is an important first step toward understanding genetics.

Allele. One version of a particular gene. Each human cell has two copies of each gene. Those two copies are often different from each other. Each copy is an allele.

Amino acids. The building blocks of proteins. The genetic code works by specifying the order of amino acids in proteins. The proteins act as the functional components of cells.

Autosome. Any chromosome that is not a sex chromosome, for example, in humans any chromosome other than "X" or "Y."

Carrier. An individual who carries an allele of a gene associated with a disease. In many cases, the carrier may not actually have the disease associated with the version of the gene they carry, even though they may pass it on to their offspring.

Chromosome. The physical structure found in the nucleus of cells composed of DNA and proteins that contains a string of genes. Humans have 46 chromosomes: two copies each of 22 autosomes and two sex chromosomes--either two "X" chromosome in the case of females or an "X" and a "Y" in the case of males.

Chromosome abnormality. A chromosome that has become damaged. The damage may be due to a break in the chromosome's linear sequence, a deletion of part of the chromosome, or movement of part of a chromosome to a place where it is not normally found, such as the end of some other chromosome.

Codon. Three sequential nucleotides in DNA. One codon specifies (or codes for) a single amino acid — the building block of proteins. There are as many codons in a gene as there are amino acids in the protein made from that gene.

Cytosine. One of the four building blocks of DNA (a nucleotide). The "C" in the genetic code.

Deoxynucleic acid (DNA). The chemical information molecule used by most biological systems. It's a string of adenosine (A), cytosine (C), guanosine (G) and thymidine (T) nucleotides.

Diploid. Having two copies (alleles) of each gene. Most human cells are diploid. Some cells, such as human eggs and sperm, have only one copy of each gene, so those specialized cells are haploid.

DNA replication. The process of duplicating the DNA to make two identical copies, one of which will be passed along to each daughter cell when the cell divides.

Dominant. A version of a gene that produces an effect (phenotype) even when it occurs as only one of the two copies present in a cell.

Gamete. Sperm and eggs. These cells have the special property of carrying only one copy of each gene instead the two copies that most other cells carry. These cells contribute to the next generation and are also called "germ line" cells.

Gene. A single trait that is passed from generation to generation. A gene corresponds to the biological information that describes a single protein.

Gene expression. Not every gene is expressed in every cell. Expression refers to whether the protein coded by a gene is expressed or not.

Gene mapping. Defining the location of genes to a particular chromosome and a particular location of the gene on a chromosome.

Gene therapy. The introduction of DNA into a cell or animal to cause the expression of a particular protein, often one that can replace a missing or defective protein that may be responsible for a disease.

Genetic code. Each amino acid building block of a protein is specified by the order of nucleotides (A,C,T and G) in the gene for that protein. Three adjacent nucleotides, called a "codon," are required to make one amino acid. There are 64 possible combinations resulting from having one of four nucleotides in each of three possible positions in the codon (4 X 4 X 4 = 64).

Genome. The collection of all the genes in the organism. Also all of the DNA that carries the genes.

Guanosine. One of the four building blocks of DNA (a nucleotide). The "G" in the genetic code.

Haploid. Having one copy of each gene. Gametes usually are haploid while most somatic cells have two copies of each gene (see diploid).

Heterozygous. Describes the situation where cells or organisms carry two different versions (alleles) of a given gene. For example, one of the copies of a gene may be the normal version and one a mutant.

Homozygous. Describes the situation where both copies of a given gene are the same.

Karyotype. A picture of the chromosomes that allows physicians or scientists to determine if there are large abnormalities in the chromosomes.

Linkage. Two genes near each other on the same chromosome are said to be "linked." If one gene is inherited, the probability is high that the other will also be inherited.

Mutation. A change in the sequence of nucleotides of a gene. Some changes will alter the protein so that it no longer works normally, while other changes may have no effect.

Non-coding DNA. DNA that does not code for a protein. Not all DNA codes for proteins. Some DNA between genes is needed to help the genetic machinery express the genes. DNA also contains segments that are repeated many times.

Nucleotide. The chemical name of the building blocks of DNA--A,C,T and G. DNA is a string of nucleotides.

Pedigree. A diagram that shows the pattern of inheritance of a gene in a family.

Phenotype. A heritable genetic trait such as eye color or blood type, or an inherited disease.

Polymorphism. A change in the sequence of DNA associated with a large portion of the population. Polymorphisms may or may not be linked to specific diseases.

Protein. The primary component of cells. A string of linked amino acids whose order is specified by a gene. Proteins build up most of the structures in cells and act as little machines that work together inside of cells to accomplish most of the things cells do.

Recessive. A gene that only produces its effect when both copies present are identical. A recessive gene trait will be passed on to children with only a 1 in 4 chance.

Sex chromosome. One of two chromosomes in humans that defines sex. There are two--the X and the Y chromosome. Males have one X and one Y, while females have two X chromosomes.

Sex-linked. A gene that resides on one of the sex chromosomes.

Somatic cells. Most of the cells in our bodies. Any cell that is not a gamete (a sperm or egg). Mutations that occur in somatic cells aren't passed on to our offspring.

Thymidine. One of the four building blocks of DNA (a nucleotide). The "T' in the genetic code.


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