developmental disorder that results when the
such as polymerase chain reaction and
forebrain of the embryo fails to divide and form
Southern blotting.
the right and left halves of the brain. The disorder
produces a single-lobed brain structure and severe
identical twins: Identical twins are also known
skull and facial abnormalities. The deformities
as monozygotic twins. They result from the
often cause babies to die before birth. In mild
fertilization of a single egg that splits in two.
cases, babies are born with near-normal brain
Identical twins share all of their genes and are
development and facial abnormalities involving
always of the same sex. In contrast, fraternal, or
cleft lip or cleft palate.
dizygotic, twins result from the fertilization of
two separate eggs during the same pregnancy.
homologous recombination: Homologous
They share half their genes, just like any other
recombination is a type of genetic recombination
siblings. Fraternal twins can be of the same or
that occurs during meiosis (the formation of
different sexes.
egg and sperm cells). Paired chromosomes from
the male and female parent align so that similar
in situ hybridization: In situ hybridization is a
DNA sequences from the paired chromosomes
laboratory technique in which a single-stranded
cross over each other. Crossing over results in a
DNA or RNA sequence called a probe is allowed
shuffl ing of genetic material and is an important
to form complementary base pairs with DNA or
cause of the genetic variation seen among offspring.
RNA present in a tissue or chromosome sample.
The probe has a chemical or radioactive label
attached to it so that its binding can be observed.
50
inherited: An inherited trait is one that is
LOD score: LOD stands for “logarithm of the
genetically determined. Inherited traits are passed
odds.” In genetics, the LOD score is a statistical
from parent to offspring according to the rules
estimate of whether two genes, or a gene and a
of Mendelian genetics. Most traits are not strictly
disease gene, are likely to be located near each
determined by genes, but rather are infl uenced by
other on a chromosome and are therefore likely
both genes and environment.
to be inherited together. A LOD score of 3 or
higher is generally understood to mean that
insertion: Insertion is a type of mutation
two genes are located close to each other. In
involving the addition of genetic material.
terms of signifi cance, a LOD score of 3 means
An insertion mutation can be small, involving a
the odds are 1,000 to 1 that the two genes are
single extra DNA base pair, or large, involving a
linked and, therefore, inherited together.
piece of a chromosome.
lymphocyte: A lymphocyte is a type of white
intron: An intron is a portion of a gene that does
blood cell that is part of the immune system.
not code for amino acids. In the cells of plants
There are two main types of lymphocytes: B cells
and animals, most gene sequences are broken
and T cells. The B cells produce antibodies that
up by one or more introns. The parts of the gene
attack invading bacteria, viruses, and toxins.
sequence that are expressed in the protein are
The T cells destroy the body’s own cells that
called exons because they are expressed, while
have themselves been taken over by viruses
the parts of the gene sequence that are not
or become cancerous.
expressed in the protein are called introns
because they come in between the exons.
lyonization: Lyonization is commonly known
as X-inactivation. In mammals, males receive
karyotype: A karyotype is an individual’s
one copy of the X chromosome while females
collection of chromosomes. The term also refers
receive two copies. To prevent female cells from
to a laboratory technique that produces an image
having twice as many gene products from the
of an individual’s chromosomes. The karyotype is
X chromosomes as males, one copy of the X
used to look for abnormal numbers or structures
chromosome in each female cell is inactivated.
of chromosomes.
In placental mammals, the choice of which X
chromosome is inactivated is random, whereas
knockout: A knockout typically refers to an
in marsupials, it is always the paternal copy
organism that has been genetically engineered
that is inactivated.
to lack one or more specifi c genes. Scientists
create knockouts (often in mice) so that they
lysosome: A lysosome is a membrane-bound
can study the impact of the missing genes and
cell organelle that contains digestive enzymes.
learn something about the genes’ function.
Lysosomes are involved with various cell
processes. They break down excess or worn-
linkage: Linkage is the close association of
out cell parts. They may be used to destroy
genes or other DNA sequences on the same
invading viruses and bacteria. If the cell is
chromosome. The closer two genes are to each
damaged beyond repair, lysosomes can help it
other on the chromosome, the greater the
self-destruct in a process called programmed
probability that they will be inherited together.
cell death, or apoptosis.
locus: A locus is the specifi c physical location of
a gene or other DNA sequence on a chromosome,
like a genetic street address. The plural of locus
is “loci.”
51
Glossary
Human Genetic Variation
mapping: Mapping is the process of making a
Mendel performed thousands of crosses with
representative diagram cataloging the genes and
garden peas at his monastery during the middle of
other features of a chromosome and showing
the 19th century. Mendel explained his results by
their relative locations. Cytogenetic maps are
describing two laws of inheritance that introduced
made using photomicrographs of chromosomes
the idea of dominant and recessive genes.
stained to reveal structural variations. Genetic
maps use the idea of linkage to estimate the
messenger RNA (mRNA): Messenger RNA
relative locations of genes. Physical maps, made
(mRNA) is a single-stranded RNA molecule that
using recombinant DNA (rDNA) technology,
is complementary to one of the DNA strands
show the actual physical locations of landmarks
of a gene. The mRNA is an RNA version of the
along a chromosome.
gene that leaves the cell nucleus and moves to
the cytoplasm where proteins are made. During
marker: A marker is a DNA sequence with a
protein synthesis, an organelle called a ribosome
known physical location on a chromosome.
moves along the mRNA, reads its base sequence,
Markers can help link an inherited disease
and uses the genetic code to translate each three-
with the responsible genes. DNA segments
base triplet, or codon, into its corresponding
close to each other on a chromosome tend
amino acid.
to be inherited together. Markers are used to
track the inheritance of a nearby gene that has
metagenomics: Metagenomics is the study of a
not yet been identifi ed but whose approximate
collection of genetic material (genomes) from a
location is known. The marker itself may be a
mixed community of organisms. Metagenomics
part of a gene or may have no known function.
usually refers to the study of microbial communities.
meiosis: Meiosis is the formation of egg and
metaphase: Metaphase is a stage during the
sperm cells. In sexually reproducing organisms,
process of cell division (mitosis or meiosis).
body cells are diploid, meaning they contain two
Usually, individual chromosomes cannot
sets of chromosomes (one set from each parent).
be observed in the cell nucleus. However,
To maintain this state, the egg and sperm that
during metaphase of mitosis or meiosis,
unite during fertilization are haploid, meaning
the chromosomes condense and become
they each contain a single set of chromosomes.
distinguishable as they align in the center of the
During meiosis, diploid cells undergo DNA
dividing cell. Metaphase chromosomes are used
replication followed by two rounds of cell
during the karyotyping procedure used to look
division, producing four haploid sex cells.
for chromosomal abnormalities.
Mendel, Johann (Gregor): Gregor Mendel was an
microarray technology: Microarray technology
Austrian monk who in the 19th century worked
is a developing technology used to study the
out the basic laws of inheritance, even before the
expression of many genes at once. It involves
term “gene” had been coined. In his monastery
placing thousands of gene sequences in known
garden, Mendel performed thousands of crosses
locations on a glass slide called a gene chip. A
with garden peas. Mendel explained his results by
sample containing DNA or RNA is placed in
describing two laws of inheritance that introduced
contact with the gene chip. Complementary
the idea of dominant and recessive traits.
base pairing between the sample and the gene
sequences on the chip produces light that is
Mendelian inheritance: Mendelian inheritance
measured. Areas on the chip producing light
refers to patterns of inheritance of organisms that
identify genes expressed in the sample.
reproduce sexually. The Austrian monk Gregor
52
microbiome: A microbiome is all of the genetic
Monosomy can be partial if a portion of the
material found within an individual microbe such
second chromosome copy is present. Monosomy,
as a bacterium, fungal cell, or virus. It also may
or partial monosomy, is the cause of some human
refer to the collection of genetic material found
diseases such as Turner syndrome and Cri du
in a community of microbes that live together.
Chat syndrome.
microsatellite: Microsatellite sequences are
mouse model: A mouse model is a laboratory
repetitive DNA sequences usually several base
mouse used to study some aspect of human
pairs in length. Microsatellite sequences are
physiology or disease. A variety of different model
composed of noncoding DNA and are not parts
organisms are used in this regard, but mice are
of genes. They are used as genetic markers to
especially useful because they share mammalian
follow the inheritance of genes in families.
features with humans and suffer from many of the
same diseases. A large number of mouse models
missense mutation: A missense mutation is
have been created by selective breeding and genetic
when the change of a single base pair causes
engineering to target specifi c human diseases.
the substitution of a different amino acid in the
resulting protein. This amino acid substitution
mutagen: A mutagen is a chemical or physical
may have no effect, or it may render the protein
phenomenon, such as ionizing radiation, that
nonfunctional.
promotes errors in DNA replication. Exposure to
a mutagen can produce DNA mutations that cause
mitochondria: Mitochondria are membrane-
or contribute to diseases such as cancer.
bound cell organelles (mitochondrion, singular)
that generate most of the chemical energy
mutation: A mutation is a change in a DNA
needed to power the cell’s biochemical reactions.
sequence. Mutations can result from DNA copying
Chemical energy produced by the mitochondria
mistakes made during cell division, exposure to
is stored in a small molecule called adenosine
ionizing radiation, exposure to chemicals called
triphosphate (ATP). Mitochondria contain
mutagens, or infection by viruses. Germ line
their own small chromosome(s). Generally,
mutations occur in eggs and sperm and can be
mitochondria, and therefore mitochondrial
passed on to offspring, while somatic mutations
DNA, are inherited only from the mother.
occur in body cells and are not passed on.
mitochondrial DNA: Mitochondrial DNA is
nanotechnology: Nanotechnology is the science of
the small circular chromosome found inside
manipulating matter on the atomic and molecular
mitochondria. Mitochondria are the organelles in
scales to solve problems. Nanotechnology is a
cells where energy is produced. The mitochondria,
developing applied science that has the potential
and thus mitochondrial DNA, are passed from
to make signifi cant contributions to many
mother to offspring.
fi elds, including engineering, computer science,
and medicine.
mitosis: Mitosis is a cellular process that
replicates chromosomes and produces two
newborn screening: Newborn screening is
identical nuclei in preparation for cell division.
testing performed on newborn babies to detect
Generally, mitosis is immediately followed by
a wide variety of disorders. Typically, testing is
the equal division of the cell nuclei and other
performed on a blood sample obtained from a
cell contents into two daughter cells.
heel prick when the baby is two or three days
old. In the United States, newborn screening is
monosomy: Monosomy is the state of having
mandatory for several different genetic disorders,
a single copy of a chromosome pair instead
though the exact set of required tests differs
of the usual two copies found in diploid cells.
from state to state.
53
Glossary
Human Genetic Variation
noncoding DNA: Noncoding DNA sequences
nucleic acid: Nucleic acids are an important class
do not code for amino acids. Most noncoding
of macromolecules found in all cells and viruses.
DNA lies between genes on the chromosome
The functions of nucleic acids have to do with the
and has no known function. Other noncoding
storage and expression of genetic information.
DNA, called introns, is found within genes.
Deoxyribonucleic acid (DNA) encodes the
Some noncoding DNA plays a role in the
information the cell needs to make proteins. A
regulation of gene expression.
related type of nucleic acid, called ribonucleic
acid (RNA), comes in different molecular forms
nondirectiveness: Nondirectiveness refers to
that participate in protein synthesis.
the nature of the genetic counseling process.
According to the principle of nondirectiveness,
nucleolus: The nucleolus is a region found
the genetic counselor has the responsibility to
within the cell nucleus concerned with producing
provide the client with accurate information
and assembling the cell’s ribosomes. Following
about a test or outcome but should remain
assembly, ribosomes are transported to the cell
neutral and not try to infl uence the decisions
cytoplasm, where they serve as the sites for
made by the client.
protein synthesis.
nonsense mutation: A nonsense mutation is
nucleopore: A nucleopore is one of a series of
the substitution of a single base pair that leads to
small holes found in the nuclear membrane.
the appearance of a stop codon where previously
The nucleopore serves as a channel for
there was a codon specifying an amino acid. The
transporting nucleic acids and proteins into
presence of this premature stop codon results
and out of the cell nucleus.
in the production of a shortened, and likely
nonfunctional, protein.
nucleosome: A nucleosome is the basic repeating
unit of eukaryotic chromatin. In a human cell,
northern blot: Northern blot is a laboratory
about six feet of DNA must be packaged into a
technique used to detect a specifi c RNA sequence
nucleus with a diameter less than a human hair.
in a blood or tissue sample. The sample RNA
A single nucleosome consists of about 150 base
molecules are separated by size using gel
pairs of DNA sequence wrapped around a core of
electrophoresis. The RNA fragments are transferred
histone proteins. The nucleosomes are arranged
out of the gel to the surface of a membrane. The
like beads on a string. They are repeatedly folded
membrane is exposed to a DNA probe labeled
in on themselves to form a chromosome.
with a radioactive or chemical tag. If the probe
binds to the membrane, then the complementary
nucleotide: A nucleotide is the basic building
RNA sequence is present in the sample.
block of nucleic acids. RNA and DNA are
polymers made of long chains of nucleotides.
nuclear membrane: A nuclear membrane is a
A nucleotide consists of a sugar molecule
double membrane that encloses the cell nucleus.
(either ribose in RNA or deoxyribose in DNA)
It serves to separate the chromosomes from the
attached to a phosphate group and a nitrogen-
rest of the cell. The nuclear membrane includes
containing base. The bases used in DNA are
an array of small holes, or pores, that permit the
adenine (A), cytosine (C), guanine (G), and
passage of certain materials, such as nucleic acids
thymine (T). In RNA, the base uracil (U)
and proteins, between the nucleus and cytoplasm.
takes the place of thymine.
54
nucleus: A nucleus is a membrane-bound
peptide: A peptide is one or more amino acids
organelle that contains the cell’s chromosomes.
linked by chemical bonds. The term also refers
Pores in the nuclear membrane allow molecules
to the type of chemical bond that joins the amino
to pass into and out of the nucleus.
acids together. A series of linked amino acids is
a polypeptide. The cell’s proteins are made from
oncogene: An oncogene is a mutated gene that
one or more polypeptides.
contributes to the development of a cancer. In
their normal, unmutated state, oncogenes are
personalized medicine: Personalized medicine
called proto-oncogenes, and they play roles in the
is an emerging practice of medicine that uses
regulation of cell division. Some oncogenes work
an individual’s genetic profi le to guide decisions
like putting your foot down on the accelerator of
about the prevention, diagnosis, and treatment
a car, pushing a cell to divide. Other oncogenes
of disease. Knowledge of a patient’s genetic profi le
work like removing your foot from the brake while
can help doctors select the proper medication or
parked on a hill, also causing the cell to divide.
therapy and administer it using the proper dose or
regimen. Personalized medicine is being advanced
open reading frame: An open reading frame is a
through data from the Human Genome Project.
portion of a DNA molecule that, when translated
into amino acids, contains no stop codons. The
pharmacogenomics: Pharmacogenomics is a
genetic code reads DNA sequences in groups of
branch of pharmacology concerned with using
three base pairs, which means that a double-
DNA and amino acid sequence data to inform
stranded DNA molecule can read in any of six
drug development and testing. An important
possible reading frames—three in the forward
application of pharmacogenomics is correlating
direction and three in the reverse. A long open
individual genetic variation with drug responses.
reading frame is likely part of a gene.
phenotype: A phenotype is an individual’s
organ: In biology, an organ (from the Latin
observable traits, such as height, eye color, and
“organum,” meaning an instrument or tool) is
blood type. The genetic contribution to the
a collection of tissues that structurally form a
phenotype is called the genotype. Some traits are
functional unit specialized to perform a particular
largely determined by the genotype, while others
function. Your heart, kidneys, and lungs are
are largely determined by environmental factors.
examples of organs.
phosphate backbone: A phosphate backbone is
organelle: An organelle is a subcellular structure
the portion of the DNA double helix that provides
that has one or more specifi c jobs to perform in
structural support to the molecule. DNA consists
the cell, much like an organ does in the body.
of two strands that wind around each other like a
Among the more important cell organelles are
twisted ladder. Each strand has a backbone made
the nuclei, which store genetic information;
of alternating sugar (deoxyribose) and phosphate
mitochondria, which produce chemical energy;
groups. Attached to each sugar is one of four
and ribosomes, which assemble proteins.
bases—adenine (A), cytosine (C), guanine (G), or
thymine (T). The two strands are held together by
pedigree: A pedigree is a genetic representation
bonds between the bases, with adenine forming
of a family tree that diagrams the inheritance of
a base pair with thymine and cytosine forming a
a trait or disease though several generations. The
base pair with guanine.
pedigree shows the relationships between family
members and indicates which individuals express
or silently carry the trait in question.
55