Human Genetics Glossary
Every field has its own vocabulary that can seem like a foreign language to others. Human genetics is no exception. As you learn about genetics, you are likely to come across words and phrases that are new to you. This is true for patients, family members, health care professionals, researchers, and others.
There are many glossaries of genetic terms available online. The following links may be helpful as starting points.
http://www.genome.gov/glossary.cfm
http://ghr.nlm.nih.gov/ghr/page/Glossary
As you learn about genetics, you are also likely to come across words and phrases describing different types of birth defects. The following link provides definitions for some of the more common birth defects.
This glossary describes some of the more common words and phrases that you are likely to come across. Clicking on the underlined words will take you to the definition for that particular term.
A
allele –One version of a gene. Humans usually have 2 copies of every gene. The copy that came from a person’s father is called the paternal allele, and the copy that came from a person’s mother is the maternal allele. These two versions of the gene may be identical to each other, or they may different.
amino acids - Small molecules that form proteins when linked together in a chain. There are 20 different kinds of amino acids, which join together in different combinations to form different proteins.
anticipation – the tendency for some genetic disorders to show up at an earlier age and with more severe symptoms from generation to generation. For example, a person who does not have symptoms until he or she is 50 years old can have children who have symptoms in their 20’s, and grandchildren who have more severe symptoms in childhood.
autosome – A chromosome that does not determine one’s sex. In humans, these are numbered from 1 to 22. Also see sex chromosome.
B
balanced translocation – A rearrangement between 2 or more chromosomes that does not change the amount of genetic material that is present. This can occur when parts of 2 different chromosomes break off and switch places. This is also called a reciprocal translocation. A person with a balanced translocation may not be affected in any way, but may have an increased chance to have a child with an unbalanced translocation.
band – A section of a chromosome that appears as a light or dark stripe when treated with chemicals in the lab. Bands are given unique numbers and can be used to describe the location of a change on a specific chromosome.
base pair – A set of two complementary molecules that join together between the two strands of DNA. This pairing holds the two strands of DNA together. Base pairs form the “rungs” of the DNA ladder. There are four bases: adenine (A), guanine (G), cytosine (C), and thymine (T). G pairs with A, and C pairs with T. The length of a DNA molecule can be measured in base pairs.
breakpoint – The location on a chromosome where a portion has broken off or is missing. This location can be described using the arm of the chromosome (p or q) and the band number. If chromosome 4 has broken off at band 12 of the short arm, the breakpoint would be called 4p12.
C
carrier - An individual who possesses one altered copy of a recessive gene. Since two altered copies can cause a genetic disorder, carriers are usually not affected. A carrier is at risk for passing on the altered copy to a child. Two parents who are carriers are at risk to have a child that is affected.
cell – The basic unit of all living organisms. In humans, cells are generally too small to see without a microscope. They contain chemicals and structures that carry out all of the body’s functions.
cell division – The process by which cells multiply during the growth of tissues and organs. Cells first make an extra copy of the genetic material. One cell then splits into two new cells, each with its own set of genetic material and other cellular structures. Cell division can occur through mitosis or meiosis.
centromere – Part of the chromosome that appears pinched together. The centromere is somewhere in the middle of the chromosome, and it separates the chromosome into the p-arm and the q-arm.
chromosome – A thread-like structure found in the nucleus of every cell in the body. Chromosomes are made up of long chains of DNA. Many sections of this DNA are discrete units known as genes. Each chromosome is like a string of beads, where every bead represents a gene. Humans usually have 46 chromosomes in every cell.
clone – A genetically identical copy. A clone can be an identical copy of a single gene, a single cell, or an entire plant or animal.
code - The information in DNA that is interpreted by the cells in order to produce proteins. The nucleotides that make up the DNA are described by the letters A (Adenine), T (Thymine), C (Cytosine) and G (Guanine). The genetic code can be written as a series of letters (for example AAA CGT TTC). Each group of 3 letters specifies the specific amino acid that is needed at every step in making a protein.
consanguinity – A partnership or marriage between two individuals from the same family. For example, cousins who marry each other are said to be consanguineous.
D
deletion – The loss of some genetic material from a chromosome. A deletion can be the loss of an entire chromosome, a section of one chromosome, a single gene, or a smaller part of a gene. Some deletions are large enough to see by looking directly at the chromosomes under a microscopic. Smaller deletions may only be found by more detailed techniques.
disomy – Two copies of a chromosome. Humans usually have two copies of every chromosome, so our cells demonstrate disomy. See monosomy and trisomy.
DNA (deoxyribonucleic acid) – The chemical that makes up genes and chromosomes. DNA is arranged in a double helix, with two long strands that twist around each other. The two strands are connected by base pairs, which form the “rungs” of the DNA ladder.
dominant – A mutation that causes a disorder when present in only one copy of a gene. The mutated gene is said to be dominant over the other 'correct' copy of the gene. With a dominant mutation, the chance of passing on the mutation (and therefore the disease) to children is 50% in each pregnancy.
duplication – A part of the chromosome that is present in two or more copies.
E
empiric risk –The chance to be affected with a certain condition, based on observation from families and individuals in the general population. Empiric risk can be determined without knowing the specific genetic cause of a condition in a family.
environment - Factors from outside our bodies, which may have an effect on our development or growth. Many physical and mental characteristics are affected by a combination of genes and environment. Environmental factors include diet, cigarette smoke, X-rays, education, social interactions, and many other external influences.
enzyme – A protein involved in chemical reactions in the cell. Enzymes are needed for reactions that break molecules down into smaller molecules, and for reactions that build larger molecules from small ones. Different enzymes have different functions. Many enzymes are involved in the process of breaking down food to generate energy in the body.
exon – A segment of a gene that contains a coding region. The exons provide the instructions for making a protein. Exons are separated by introns.
expressivity - The degree to which an inherited characteristic is seen in a person. If a disorder shows “variable expressivity,” one person may have only a few mild symptoms, while another person may have more severe problems or additional symptoms. This can even happen with members of the same family.
F
familial - A characteristic or disorder which tends to run in families. Characteristics that are genetic are not necessarily familial, since they can be seen in only one member of a family.
G
gamete – Cell that functions in reproduction. In men, gametes are sperm cells, and in women, gametes are egg cells. Gametes have half of the genetic material that is present in the other cells in the body. Human gametes contain 23 chromosomes.
gene – The basic unit of heredity. One gene provides the instructions to make a specific protein. Genes provide the instructions for all of our body’s functions, including growth, development, and physical characteristics.
genetic – Relating to the genes. A feature is said to be genetic if it is influenced by genes or chromosomes. Genetic disorders do not always run in families.
genome – All of the DNA that is contained in an organism or a cell.
genotype – A description of the specific genes or alleles that a person has. A genotype is a description of the DNA in the cells, and cannot be seen in a person’s behavior or physical features. See phenotype.
germline – Reproductive cells that produce gametes, and contain the genetic material that is passed down to offspring. See somatic.
H
heterozygote – An individual that has two different forms of a particular gene.
homozygote – An individual that has two identical forms of a particular gene.
human genome project – The international effort to determine the DNA sequence of all human chromosomes, and to map the location of all of the genes.
I
inherited – Passed on, or transmitted through genes from a parent to a child.
insertion - The addition of a piece of genetic material in a place where it is not normally found. An insertion can be large and result in a chromosomal abnormality. It can also be a smaller addition of one or more base pairs and alter the function of a single gene. This may result in a disorder, because the genetic code may then be read incorrectly.
intron - A segment of a gene that does not contain a coding region. Introns do not contain direct instructions for making a protein. Exons are separated by introns.
inversion – A part of a chromosome that had two breaks, and flipped over and connected again. This results in the genes being in the reverse order along the chromosome. In some cases, this may cause the genetic code to be read incorrectly.
K
karyotype – All of the chromosomes from one cell of an individual. The karyotype describes the number of chromosomes and any structural changes. The photograph of all of the chromosomes from one cell is also called a karyotype.
L
locus – A position on a chromosome where a specific gene is found.
M
marker chromosome - A chromosome, or part of a chromosome, usually small, of unknown origin.
meiosis - The process by which germline cells divide into gametes (egg and sperm cells). A cell that has 46 chromosomes goes through meiosis and results in cells that have 23 chromosomes. In this way, a parent gives half of his or her genetic material to a child, and the other half comes from the other parent. This is different from mitosis, in which the amount of genetic material stays the same.
mitochondria – Structures in the cell that generate energy for the body to use. Mitochondria are called the powerhouses of the cell. The mitochondria contain some DNA that is separate from the DNA that is stored in the nucleus of the cell.
mitosis – The process by which somatic cells divide. In mitosis, an extra copy of the genetic material is made. The cells then divide, and each new cell has the same amount of genetic material as the original cell. This is different from meiosis, in which the amount of genetic material is halved.
monosomy – One copy of a chromosome. If one chromosome from a pair is missing, the cell exhibits monosomy for that chromosome. See disomy and trisomy.
mosaic – The presence of two or more different types of cells. In one person, some cells may have a change in a chromosome or gene, and other cells may not have that change. The severity of the disorder is determined by the number of cells with the change and their location in the body.
multifactorial - A pattern of inheritance which results from the interaction of one or more genes with environmental factors. Multifactorial disorders tend to “cluster” in families, but do not follow a straightforward pattern of inheritance.
mutation – A permanent change in a gene. A change in a gene is like a change in the body’s instructions. Most mutations have no noticeable effect on a person. Some mutations allow the body to work better. Disease-causing mutations cause problems in the body, by changing the way a protein is made.
N
nucleotides – Small molecules that are the building blocks of DNA and RNA. One nucleotide is made up of one base, one sugar, and one phosphorous molecule.
nucleus- The structure in a cell that contains the genetic material in the form of chromosomes.
P
p-arm – The short arm of a chromosome. The letter ‘p’ comes from the French word petit, meaning small. Pictures of chromosomes are usually arranged with the p-arm above the centromere. See q-arm.
pedigree – A diagram of a family tree that often includes health information of family members.
penetrance – The percentage of individuals who have a specific gene that also have symptoms. “Complete penetrance” means that 100% of people who have the gene will have symptoms. “Incomplete” or “reduced penetrance” means that some people who have the gene will not have any symptoms.
phenotype - The observable traits or characteristics of a person, for example hair color, weight, or the presence or absence of a disease. Phenotypic traits are determined by genes and/or the environment.
polymorphism – A common variation in genetic material that occurs naturally in the population and does not affect an individual.
predisposition – Susceptibility to a particular condition due to a person’s genes. Symptoms may or may not be present, depending on the environmental factors.
protein – A large molecule made up of amino acids. Proteins perform many different functions in the body. The type of protein in foods that we eat is only one type of the proteins found in our bodies.
Q
q-arm - The long arm of a chromosome. The letter ‘q’ is used because it comes after the letter ‘p’ in the alphabet. (See p-arm.) Pictures of chromosomes are usually arranged with the q-arm below the centromere.
R
recessive - A mutation that causes a disorder only when present in two copies of a gene. When one copy of the gene is altered, a person is not affected. (See carrier.) The mutated gene is said to be hidden or recessive to the correct copy of the gene.
reciprocal translocation – A rearrangement between 2 or more chromosomes that does not change the amount of genetic material that is present. This can occur when parts of 2 different chromosomes break off and switch places. This is also called a balanced translocation. A person with a reciprocal translocation may not be affected in any way, but may have an increased chance to have a child with an unbalanced translocation.
recurrence risk - The risk that a disorder will occur again in a family. This can be based on the empiric risk, or the presence of a known change in a gene or chromosome in individual family members.
RNA (ribonucleic acid) – A chemical that is similar to DNA. There are several different types of RNA that are involved in carrying the genetic message from DNA and making proteins.
S
sequence –The exact order of bases or base pairs in a segment of DNA. Sequence is also a verb, which refers to a laboratory technique used to determine the exact order of base pairs.
sex chromosome – One of the chromosomes that determine one’s sex. In humans, these are called X and Y. Males have one X chromosome and one Y chromosome. Females have two X chromosomes. Also see autosome.
sex-linked - A disorder or characteristic that is determined by genes carried on the X chromosome. A sex-linked disorder affects males and females differently, since males have one X chromosome and females have two X chromosomes. This is also called X-linked.
somatic – All the cells of the body, except for the germline cells.
sporadic - A mutation that appears for the first time in a family. The mutation takes place in either the egg or the sperm, or at conception. Also called "de novo."
stem cell – A cell that has not yet differentiated, or become a specific type of cell. Stem cells have the capacity to become many different kinds of cells (like a muscle cell, a skin cell, a blood cell, etc.).
subtelomere – The region of the chromosome just before the telomere. Subtelomeres are unique to each chromosome, and can be identified in the laboratory.
T
telomere - The terminal or end segment of each chromosome arm.
translocation – When a piece of one chromosome has broken off and attached to a different chromosome. Translocations can be balanced or unbalanced.
trisomy – Three copies of a chromosome. If a cell has one extra chromosome, then the cell exhibits trisomy for that chromosome. See monosomy and disomy.
U
unbalanced translocation – A rearrangement between 2 or more chromosomes that results in a change in the amount of genetic material that is present. An unbalanced translocation can result in extra genetic material from a chromosome, missing genetic material from a chromosome, or both missing and extra genetic material.
uniparental disomy (UPD) - When both members of a chromosome pair are contributed by one parent rather than one from each parent. Uniparental disomy may be maternal or paternal.