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.
http://www.bdid.com
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
de novo –
See sporadic.
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.
X
