CONTENTS Glossary of Abbreviations B-1
Glossary of Genetic Terms B-2
Glossary of Malformations B-7
AFP alpha fetoprotein
AFAFP amniotic fluid alpha fetoprotein
AS Angelman syndrome
ASO allele specific oligonucleotide
CAH congenital adrenal hyperplasia
CF cystic fibrosis
CVS chorionic villus sampling
DNA deoxyribonucleic acid
DS Down syndrome
EA early amniocentesis
EDD expected date of delivery
FISH fluorescence in situ hybridization
hCG human chorionic gonadotropin
HD Huntington disease
LMP last menstrual period
MCA multiple congenital anomaly
MoM multiple of median
MSAFP maternal serum alpha fetoprotein
NF neurofibromatosis
NTD neural tube defect
OTIS Organization of Teratogen Information Services
PAH phenylalanine hydroxylase
PCB polychlorinated biphenyl
PCR polymerase chain reaction
PKU phenylketonuria
PWS Prader-Willi syndrome
Rb retinoblastoma
RNA ribonucleic acid
T4 thyroxine or thyroid hormone
TIS Teratogen Information Service
TS Tay-Sachs disease
TSH thyroid stimulating hormone
UPD uniparental disomy
US ultrasound
VNTR variable number of tandem repeats
WBC white blood cell
ALLELE Refers to the different forms of a gene at one locus. Example:
A, B and O are alleles for a blood group gene. A person with blood
type AB has the A allele on one chromosome and the B allele at
the same locus on the other homologous chromosome.
AMNIOCENTESIS A clinical procedure in which a few milliliters
of amniotic fluid is withdrawn from the uterus. This fluid and
the fetal cells that are present in the fluid may be tested to
detect various genetic conditions.
ANEUPLOID The total number of chromosomes in a cell if not an
exact multiple of the haploid number of 23. The most common aneuploid
numbers are 45 (Turner syndrome) and 47 (Down syndrome).
AUTOSOME The chromosomes males and females have in common; chromosomes
1 through 22.
BIRTH DEFECT An abnormality of structure, function, or body metabolism
which often results in a physical or mental handicap. It may be
inherited (genetic) or environmental.
CARRIER A person who has one normal gene and a gene coding for
a recessively inherited disease, or a person with a balanced chromosomal
rearrangement; in either case a carrier has a normal phenotype.
CHORIONIC VILLUS A procedure done between 10 and 12 weeks of gestation
in
SAMPLING (CVS) which a few milligrams of fetal placenta are aspirated
vaginally or abdominally. The aspirated tissue is then tested
for various genetic diseases.
CODOMINANT Alleles are codominant if each is expressed independent
of the presence of the other. Example: AB blood type.
CONGENITAL Present at birth; may or may not have a genetic cause.
CONSANGUINITY Refers to a mating between persons who share a common
ancestor.
CONTIGUOUS GENE Conditions that occur secondary to microdeletions
or
SYNDROME microduplications involving several neighboring genes.
DIPLOID The number of chromosomes normally present in a somatic
cell. In humans there are 46 chromosomes in a somatic cell. This
is twice the number of chromosomes present in a haploid (23 chromosomes)
gamete.
DOMINANT A gene (allele) which is expressed clinically in the
heterozygous state. In a dominant disorder only one mutant allele
need be present as it covers up, or masks, the normal allele.
EXPRESSIVITY The extent to which a gene is clinically evident
(expressed) in an individual. Variable expressivity refers to
the variation in severity of symptoms produced by the same gene
in different individuals.
FAMILIAL Any trait that occurs more often in the relatives of
an affected person than in the general population; traits that
tend to "run in families" (e.g., diabetes, neural tube
defects).
GAMETE An ovum or sperm.
GENE A small segment of DNA that codes for the synthesis of a
specific protein. Genes are located on the chromosomes. Examples:
ABO blood group gene, Rh blood group gene.
GENETIC A communication process intended to help affected
COUNSELING individuals and/or their families to comprehend the
diagnosis, prognosis, recurrence risks and reproductive
choices and to make the best possible adjustment to the
condition.
GENOME All of the genes present on a set of chromosomes.
GENOTYPE The specific pair of alleles present at a single locus.
HAPLOID The normal number of chromosomes present in an egg or
sperm. In humans, the haploid number of chromosomes is 23.
HETEROGENEITY The phenomenon by which a certain phenotype (or clinical feature) can be produced by different genetic mechanisms.
HETEROZYGOTE An individual who has two different alleles at a
given locus on a pair of homologous chromosomes.
HOMOLOGOUS A pair of chromosomes, one from each parent, carrying
CHROMOSOMES genes for the same traits, in the same order. In a
karyotype,
the members of a homologous pair look alike (e.g., a pair of
1s, 2s, etc.).
HOMOZYGOTE An individual who has two identical alleles at a given
locus on a pair of homologous chromosomes.
IMPRINTING Refers to the modification of a gene as it is transmitted
through the father or the mother. One mechanism may be the methylation
(inactivation) of a gene.
INDEX CASE The affected individual who brings the family to the
attention of the geneticist, also known as the proband or propositus.
KARYOTYPE The chromosome complement of an individual arranged
in a standard order from large to small.
LINKAGE Refers to genes (or DNA markers) that are located in close
proximity to one another on the same chromosome. Closely linked
genes tend to be inherited together as a unit.
LOCUS Position or location of a gene on a chromosome.
LYONIZATION The random inactivation of loci on one of a pair of
X chromosomes. In each cell, only one X chromosome is active.
This process of inactivation occurs early in fetal development
and explains the variable expressivity of X-linked traits in females.
MEIOSIS A special type of cell division that occurs when mature
eggs and sperm are formed. Through the process of meiosis the
number of chromosomes present in a cell is decreased by half.
In humans the diploid number of chromosomes is 46. This is decreased
to 23 during meiosis.
MITOSIS The division process that occurs in somatic cells resulting
in the formation of two cells, each with the same number of chromosomes
that were present in the original parent cell.
MONOSOMY One chromosome of a pair is missing.
MOSAICISM The presence of two or more cell lines (cell populations)
which differ from each other in genotype or chromosome number.
MULTIFACTORIAL A trait that is determined by the interaction between
a number of genes and environmental factors.
MUTATION A permanent, heritable change in a gene or chromosome
structure.
NONDISJUNCTION Failure of paired chromosomes to separate (into
each daughter cell) during cell division.
PEDIGREE A graphic representation of a person's family created
using a standardized set of symbols.
PENETRANCE The frequency with which individuals carrying a given
gene will show the clinical manifestations associated with the
gene.
PHENOTYPE The clinical features or the observable characteristics
of an individual determined by a pair of genes at a given locus
(or genotype). The phenotype can vary following interaction with
modifying genes or the environment.
PLEIOTROPIC A single gene or gene pair which produces multiple
effects, usually because of clinical abnormalities in various
organ systems.
RECESSIVE A gene (allele) which is only expressed clinically in
the homozygous state. In a recessive disorder, both genes at a
given locus must be abnormal to manifest the disorder.
RESTRICTION Naturally occurring enzymes which cut DNA at specific
sites.
ENDONUCLEASES Each enzyme recognizes a specific sequence of base
pairs
and will only cut the DNA at these sites.
RESTRICTION Variations in DNA nucleotide sequences which occur
FRAGMENT LENGTH throughout the human genome and have no apparent
POLYMORPHISMS phenotypic effects since they usually do not involve
the
(RFLPs) coding sequence of genes. Each is inherited codominantly.
RFLPs are used as markers to track the inheritance of
abnormal genes in families in linkage analysis.
SEX CHROMOSOMES The X and Y chromosomes which are responsible
for sex determination.
SIBLINGS or SIBS A person's brothers and sisters.
SOMATIC CELLS All of the cells in the body except the eggs and
sperm.
SYNDROME A collection of physical findings that occur together
frequently enough to be recognized as a distinct clinical entity.
TERATOGEN An agent capable of causing congenital malformation(s),
(e.g., environmental agents, recreational drugs, medications,
infectious diseases, etc.).
TRANSLOCATION A rearrangement occurring when a piece of one chromosome
is broken off and joined to another chromosome. An individual
with a balanced translocation has the normal amount of
chromosomal material. A person with an unbalanced translocation
will have a loss or gain of chromosomal material.
TRIPLET REPEAT A unit of three DNA bases (e.g., CAG, CTG, CGG)
that is present in multiple copies, scattered throughout the genome.
A number of neurogenetic conditions have been ascribed to expansion
of triplet repeat size.
TRISOMY The presence of three homologous chromosomes rather than
the normal two (e.g., trisomy 21).
UNIPARENTAL DISOMY A pair of homologous chromosomes inherited
from one parent with no contribution from the other parent.
X-LINKED A gene located on the X chromosome (formerly known as
sex-linked).
ZYGOTE A fertilized egg.
ANOPHTHALMIA A developmental defect characterized by absence of
the eyes (rare).
ARACHNODACTYLY A condition characterized by abnormally long and
slender fingers and toes.
BRACHYCEPHALY A "short" head due to a short anteroposterior
diameter.
BRACHYDACTYLY Short fingers, all digits or only one or two.
BRUSHFIELD SPOTS A speckled ring about 2/3 of the distance from
the pupil to the periphery of the iris with a relative lack of
patterning beyond the ring. Found in about 80% of babies with
Down syndrome.
CAMPTODACTYLY Permanent and irreducible flexion of one or more
fingers.
COLOBOMA The absence of a portion of ocular tissue resulting in
a notch or cleft-like defect in the structure, usually resulting
from the failure to close part of a fetal fissure (e.g., coloboma
of iris, coloboma of retina).
CRANIOSYNOSTOSIS Premature closure of the sutures of the skull.
EPICANTHAL FOLD A vertical fold of skin on either side of the
nose, covering the inner canthus (corner of the eye).
HERNIA Weakness in the body wall (e.g., umbilical hernia, inguinal
hernia).
HIRSUTISM Excess body hair.
HYDROCEPHALUS A condition marked by dilatation of the ventricles
of the brain.
HYPERTELORISM Increased distance between the eyes.
HYPOPLASTIC Poorly formed or small (e.g., hypoplastic toenails,
fingernails, digits).
HYPOSPADIAS Urethral opening along the ventral surface (rather than the tip) of the penis.
HYPOTELORISM Decreased distance between the eyes.
MACROCEPHALY Large head size.
MACROSTOMIA Increased width of the mouth, resulting from failure
of union of the maxillary and mandibular processes, with extension
of the oral orifice toward the ear.
MICROCEPHALY Small head size, usually associated with mental retardation.
MICROGNATHIA Small jaw.
MICROMELIA Abnormally small or short limb.
MICROPHTHALMIA Abnormally small eyes (one or both).
MICROSTOMIA Small mouth.
NEURAL TUBE A congenital abnormality of the spinal cord or brain
resulting
DEFECT (NTD) from the abnormal closure of the neural groove early
in
embryogenesis (e.g., spina bifida, anencephaly).
OLIGODACTYLY Missing fingers.
PALPEBRAL FISSURES Pertaining to the eye slits.
PHILTRUM The vertical groove in the median portion of the upper
lip.
PHOCOMELIA Absence of the proximal portion of a limb or limbs;
the hands or feet being attached to the trunk by a small, irregularly
shaped bone.
PIGMENTARY SKIN Cafe-au-lait spots (brown spots in neurofibromatosis),
ash
ANOMALIES leaf spots (depigmented leaf shaped spots in tuberous
sclerosis), axillary freckles (freckles under the arms in
neurofibromatosis).
POLYDACTYLY The presence of extra digits (fingers and toes) on
the hands and feet; postaxial (on the ulnar side), preaxial (on
the radial side).
PREAURICULAR PITS An indentation situated in front of the auricle
of the ear.
PREAURICULAR TAGS An appendage situated in front of the auricle of the ear.
PROGNATHISM Marked protrusion of the jaw.
RETROFLEXED Bent backward.
THUMBS
ROCKER-BOTTOM The plantar surface of the foot is shaped like the
rocker of a
FEET rocking chair.
SIMIAN LINE Single crease on the palm, common in Down syndrome.
SKELETAL Lordosis (swayback), kyphosis (hunchback), scoliosis
ANOMALIES - SPINE (s-shaped curve).
SYNDACTYLY Persistence of the webbing between adjacent digits
(digits that are more or less completely attached) usually between
the 3rd and 4th fingers and the 2nd and 3rd toes.
SYNOPHRYS The eyebrows meet or fuse in the midline.
TELECANTHUS Increased distance between the inner corners of the
eyes.
TRIPHALANGEAL The presence of three phalanges in the thumb normally
THUMB composed of only two.
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