Glossary of terms

This glossary contains some of the commonly used terms in the field of human genetics. It appears in a new browser window, so you can keep it open and refer to it as you read the material in the rest of the website. If you would like any other terms explained, please click on 'feedback' in the navigation bar of the main window or at the bottom of the page.

You can also visit an extensive online genetics dictionary at The Genomics Lexicon.

adenine

One of the bases that make up the code in a strand of DNA or RNA. abbreviated: A.

allele

when more than one version of a gene exists, each version is called an allele. For example, there are multiple alleles for eye color and blood type.

autosome

The 22 numbered chromosomes are called autosomes, in contrast to the sex chromosomes.

base

Bases are special chemicals that are strung together in a strand of DNA or RNA. The order of the bases determines the information encoded.

base pair

Because DNA is double-stranded, the bases on each strand bond together in pairs. A always bonds with T and G always bonds with C.

cell

The basic building block of the tissues of the body. Each cell in the body contains a copy of the individual's genome. The genes give the cell information on how to function.

chromosome

A long strand of DNA encoding many genes. Proteins help hold together and organize the DNA in a chromosome. Most human cells have 46 chromosomes divided into 23 pairs.

Cystic Fibrosis

A recessive disorder caused by mutations in the CFTR gene. Major symptom is thick mucous in the lungs, leading to infections.

cytosine

One of the bases that make up the code in a strand of DNA or RNA. abbreviated: C.

diploid

having two sets of human chromosomes. Most cells in the human body are diploid.

DNA

A long, coiled, double-stranded chain of base-pairs which encodes genetic information. The bases used in DNA are A (adenine), T (thymine), C (cytosine), and G (guanine).

dominant

A dominant allele controls the phenotype produced, even in the presence of another allele. For example, dark colored eyes are dominant over light colored eyes, so an individual with an allele for brown eyes will have that phenotype, even if he or she also has an allele for blue eyes. (opposite of 'recessive')

enucleated

an adjective meaning 'without a nucleus.' An enucleated egg cell was involved in the process used to clone Dolly the sheep.

enzyme

Enzymes are a special class of proteins. Enzymes control chemical reactions, such as the breakdown of sugar for energy.

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fertilization

The process in which a haploid sperm and a haploid egg join to make a diploid zygote, from which a human will develop.

gamete

an egg or sperm. Human gametes are haploid.

gene

a section of DNA which encodes instructions for a particular function.

genome

all the genetic material (DNA) of an organism referred to collectively. For example, the human genome contains about 80 000 genes on 23 chromosomes.

genotype

describes the specific alleles or combination of alleles in a person's genome.

guanine

One of the bases that make up the code in a strand of DNA or RNA. abbreviated: G.

haploid

having one set of human chromosomes, half the number of most cells.

heterozygous

An individual is heterozygous when the two alleles for a certain locus are different, e.g. A on one copy of the chromosome and C on the other.

homozygous

An individual is homozygous when the two alleles for a certain locus are the same.

Human Genome Project

This international effort is in the process of determining the sequence of the entire human genome.

Huntington Disease

A dominant condition caused by mutations in the Huntington gene. Symptoms begin in adulthood and include uncontrolled movements and cognitive impairment.

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karyotype

Refers to the collection of chromosomes in a cell or in an individual. For example, the human male karyotype has 23 pairs of autosomes, an X chromosome, and a Y chromosome.

locus

A locus is a unique location on the DNA strand; a way of specifying a base or section of the genome. For example, a gene, which is a section of DNA encoding information for a particular function, can be called a locus. A locus can also refer to a section of DNA which does not have a known function.

meiosis

a type of cell division in which the ovaries or testicles produce haploid egg and sperm.

Mendelian

Refers to traits that are inherited in a simple dominant/recessive fashion. Named for Gregor Mendel.

mitosis

the usual process through which a cell divides to make two individual cells, sometimes called daughter cells.

mutation

A change in DNA. Examples of mutations are the insertion or deletion of bases or an alteration of the sequence.

nucleus

The compartment of a cell which contains the genetic material (chromosomes).

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phenotype

The characteristics of a person, such as eye color, height, or disease state. Phenotype is determined through the interaction of an individual's genotype and his or her environment.

Polymorphism

A polymorphism is a place in the human genome where a base can take on multiple forms. For example, a certain base may be A in one person and C in another. The length of some sections of DNA are also polymorphic. For example a particular segment may be 12 bases long in some people and 21 bases in others.

protein

Most genes contain information for the synthesis of proteins. Proteins are a class of molecules which perfrom many functions in the cell, contributing to structure, signalling between cells, and many chemical reactions.

recessive

The phenotype of a recessive allele can be masked by the presence of a dominant allele. The phenotype is only apparent when both alleles are recessive. For example, light colored eyes are recessive to dark colored eyes, so an individual with an allele for blue eyes will only have that phenotype, if he or she has two copies of that allele, i.e. no allele for dark colored eyes. (opposite of 'dominant')

recombination

The process in which strands of DNA are cut open and rejoined. This is done by a complex of proteins called DNA recombinase.

RNA

a long single-stranded chain of bases which encodes a copy of the information in a gene. RNA uses U (uracil) instead of T (thymine).

sequence

The order of bases in a DNA or RNA strand. The sequence encodes specific information.

sex chromosome

The X and Y chromosomes are called sex chromosomes, because they contain the information which makes an individual male or female.

thymine

One of the bases that make up the code in a strand of DNA. abbreviated: T.

transcription

The process in which the information in the DNA of a gene is copied to RNA.

translation

The process in which the information in an RNA sequence is used to make a protein.

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uracil

One of the bases that make up the code in a strand of RNA. abbreviated: U.

vector

A vector is a small piece of DNA used to carry a gene of interest. Besides the gene being studied, a vector may contain elements which help the gene integrate into a genome.

zygote

the cell formed when an egg is fertilized by a sperm, from which a human develops.

Note: Human genetics is an extremely complex topic. This website is meant only as an introduction and overview. If you are concerned about how genetics may affect your health, always consult your physician.