Glossary

-A-

Abnormal

atypical, unusual or uncommon. When used in reference to chromosomes, an abnormal chromosome complement may result in a specific disorder.

Advanced maternal Age

women who are age 35 years old or older at delivery, are at an increased risk for chromosome abnormalities.

Alleles

thereare two copies of each gene. Alleles are alternate forms of the same gene. Different alleles produce variations in inherited characteristics such as eye color or blood type.

Alpha-feto protein (AFP)

a protein produced by the fetus, which is excreted into the amniotic fluid and into the mother's bloodstream through the placenta. The amount of AFP, both in the maternal’s blood and in the amniotic fluid, at particular periods during the pregnancy, may be associated with the presence of neural tube defects or chromosomal problems in the baby.

Amniocentesis

a prenatal diagnosis procedure performed after 15 weeks gestation which involves the removal of a small amount of amniotic fluid. The amniotic fluid contains cells from the baby, which can be examined to look for chromosomal, biochemical or gene alterations in the baby. 
More on amniocentesis...

Amniocyte

cells found in the amniotic fluid which are obtained by amniocentesis

Amnion

a thin, protective membrane surrounding the baby during pregnancy. The amnion is the inner of the two fetal membranes (the chorion is the outer one). It contains the amniotic fluid.

Amniotic fluid

the fluid that surrounds the baby, held inside the amniotic sac. 

Amniotic sac

the membrane-bound compartment that holds the baby and the amniotic fluid in the uterus.

Anaphase lag

an error which can occur during cell division where one chromosome simply fails to get incorporated into the nucleus of a daughter cell. The mechanism for trisomic rescue. 

Aneuploidy

the gain or loss of one chromosome compared to the typical chromosome complement, 46.

Angelman syndrome

a conditioncharacterized by severe mental deficiency, developmental delay and growth deficiency and frequent laughter unconnected to emotions of happiness.
More on Angelman syndrome...

Autosome

any chromosome that is not a sex chromosome. In humans, the autosomes are the numbered chromosomes, numbered from 1 - 22. Chromosome 1 is the largest and chromosome 22 is the smallest.
More on autosomes...

-B-

Birth defects

an abnormality of structure, function, or body metabolism which often results in a physical or mental handicap. The causemay be genetic or environmental.
More on birth defects...

Blastocyst

a very early stage of embryonic development. About 4-5 days after conception the embryo consists of a circle of cells with fluid in the center and a clump of cells at one end with a thinner layer of cells at the other end. 

Blastomere

the first cells resulting from divisions of a fertilized egg cell

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Carrier

a person who has one normal gene and one non-working gene for a recessively inherited disease,or a person with a balanced chromosomal rearrangement. Carriers do not usually develop disease but can pass on the non-working copy or an unbalanced chromosome rearrangement to their children.

Cell division

the process by which cells multiply during the growth of tissues or organs. The type of cell division involved in the growth of the body is called mitosis. The cell division which produces the reproductive cells is called meiosis.

Cell

the basic subunit of any living thing. 
More on cells...

Centromere

the part of a chromosome which separates the two arms. The short arm is called the 'p' arm; the long arm is called the 'q' arm.

Chorion

the outermost protective membrane around the fetus.  The cells of the chorion are sampled during chorionic villus sampling.

Chorionic villi

the tissue that attach the pregnancy to the wall of the uterus.  The cells of the chorionic villi are sampled during chorionic villus sampling.

Chorionic villus sampling (CVS)

a prenatal diagnosis procedure performed at 10 to 12 weeks gestation which involves obtaining a sample of the placenta.The cells in the placenta generally represent the cells of the baby, and can be examined to look for chromosomal, biochemical or gene alterations in the baby.
More on CVS...

Chromatid

exists in a pair as part of two sister chromatids; each of which contains an identical copy of its original chromosome.  Each chromosome duplicates itself making a pair of sister chromatids during the S phase (S stands for DNA Synthesis) of the mitotic cell cycle.  These sister chromatids eventually separate and create two daughter cells identical to the first.

Chromosomal mosaicism

the presenceof two or more cell lines which differ from each other in chromosome number or structure, in an individual that has developed from a single fertilized egg.

Chromosome

thread-like structures found in the nucleus of all cells (except red blood cells), which contain our genetic material (DNA). Chromosomes come in pairs, and a normal human cell contains 46 chromosomes, 22 pairs of autosomes and two sex chromosomes.
More on chromosomes...

Conception

the fusion of the egg and sperm to create an embryo.

Confined embryonic mosaicism

when chromosomally abnormal cells are found in the fetus but not in the placenta .

Confined mosaicism  

when the abnormal cell line is confined to a certain tissue, often the placenta.

Confined placental mosaicism

when chromosomally abnormal cells are found in the placenta but not in the fetus.

Congenital

present at birth, but does not need to be inherited

Congenital heart def

When a condition is present at birth it is considered congenital.  Heart defects occur during the early part of pregnancy when the heart is developing.  An example of a congenital heart defect is ASD, or atrial septal defect, which means that the wall between the left and right upper chambers of the heart (the atria) does not close completely.
More on congenital heart defects...

Constitutional

From birth

Crossing over

during meiosis homologous pairs of chromosomes may exchange genetic material, part of the maternal chromosome 'crosses over' and exchanges places with the corresponding part of the paternal chromosome. Also known as recombination.

Cystic fibrosis

an autosomal recessive genetic condition, which causes the body to produce excessively thick, sticky mucus that clogs the lungs and pancreas, impairing breathing and digestion.

Cytogenetics

the study of chromosomes and how changes in chromosome structure and number affect the growth, development and health of individuals.

-D-

Daughter cell

the cells resulting from cell division

Deletion

the loss of a segment of the genetic material from a chromosome.  
More on deletions...

Differentiate

Diploid

the fullset of genetic material. In humans, the diploid number is 46. There are two copies of each chromosome, one chromosome from each parent.

Disomic

twocopies of a chromosome.

Disomy

two copies of each chromosome. This is the normal chromosome complement.

DNA

deoxyribonucleicacid; the basic material of heredity. DNA is a large molecule that carries the genetic information that cells need to replicate and to produce proteins. It is made up of chemicals called nucleotide bases, linked together in a spiral-shaped chain called a double helix.
More on DNA...

Down syndrome

trisomy 21. Show me the chromosomes...  

Duplication

when partof chromosome is present in two or more copies.

-E-

Egg

the female reproductive cell, also called the ovum.  The egg carries 23 chromosomes, 22 autosomes and an X chromosome.  The egg fuses with the sperm at conception to produce an embryo.

Embryo

theterm used to describe a developing human from conception to 8 weeks of development

Embryonic progenitor cells

the 1-5 cells of the blastocyst that are destined to develop into the baby 

Extra-embryonic tissue

the tissues that develop from the fertilized egg that do not involve the actual fetus; ie: the placenta and the membranes

-F-

Fertilization

the fusionof the egg and sperm at conception to create an embryo.

Fertilized egg

an egg, or oocyte, becomes fertilized once its genetic complement has fused with that of the sperm which has entered it.  A fertilized egg contains 23 chromosomes from the mother, and 23 chromosomes from the father, that came in the sperm.  Once fertilized, an egg is known as a zygote.

Fetal blood sampling

a prenataldiagnosis procedure used to obtain a blood sample from the baby. The blood sample can be examined to look for chromosomal, biochemical or gene alterations in the baby.

Fetus

term used to describe a developing baby from the nineth week of development to birth.

Fibroblasts

the type of cell that make up skin.

-G-

Gametes

reproductivecells which each contain 23 chromosomes; sperm in males and eggs in females

Gene

a working subunit of DNA that codes for the synthesis of a specific protein. Genes are located on the chromosomes. Each gene has one or more specific effects on the phenotype, and can mutate to various allelic forms. 
More on genes...

Gene markers

landmarks for a specific gene, either detectable traits that are inherited along with the gene, or distinctive segments of DNA.

Generalized mosaicism

if a mistake in cell division occurs very early in development (before differentiation between embryonic and extra-embryonic) that results in some of the cells being trisomic (ie having 47 chromosomes instead of the normal 46), a greater proportion of the cells, and thus most tissues in the body will be affected.

Genetic counselling

an educational and supportive process that helps individuals, couples, or families understand genetic information including recurrence risks and reproductive choices that may be available to them.
More on genetic counselling...

Genotype

the actual genes carried by an individual

Germ cell

a sex cell or reproductive cell; sperm in males and eggs in females

Gestation

the carrying of an embryo or fetus during a pregnancy. Most pregnancies are single gestations, but multiple gestations can occur (twins or triplets).  A 40 week gestational period is considered normal and women are given due dates 40 weeks after they conceived.  A baby born after a gestational period of less than 37 weeks is considered to be premature, and pregnancies are rarely allowed to go beyond 42 weeks gestation (the mother will be induced).

-H-

Haploid

a single set of chromosomes (half of the full set of genetic material). In humans, the haploid number of chromosomes is 23.

Heterodisomy

Heterozygote

having two different alleles for a specific gene.

Homologous chromosomes

a pair of chromosomes, one from each parent, which carry genes for the same traits, in the same order. In a karyotype, the members of a homologous pair look alike, for example, the pair of chromosome 1 look alike.

Homozygote

-I-

Implantation

Imprinting

when agene or part of a chromosome is "turned off" depending on which parent it was inherited from.  During the development of the sperm or egg some genes or parts of chromosomes are "paternally stamped" when inherited from the father and some are "maternally stamped" when inherited from the mother.
More on imprinting...

Inner cell mass

Insertion

the additionof a piece of chromosomal material into a place on a chromosome where it is not normally found.

Inversion

In vitro fertilization (IVF)

the process where an egg is fertilized with a sperm outside of the body and then transplanted into a woman's uterus.
More on IVF...

Isochromosome

anabnormal chromosome with a centromere and two identical arms, e.g. two ' q' arms versus a ' p' arm and a ' q' arm.

Isodisomy

-J-

-K-

Karyotype

a picture of an individual's chromosomes arranged in a standard format. The 23 pairs are organized according to size, location of the centromere and the patterns of the dark and light bands.

Klinefelter syndrome

a condition caused by a two X chromosomes and one Y chromosome (47,XXY); characterized by male development, problems with fertility and some mild learning difficulties.
More on Klinefelter syndrome...

-L-

-M-

Marker chromosome

a part of a chromosome, usually small, of unknown origin.

Maternal

something that originates from the mother

Maternal serum screen

a screening test which provides a calculated risk of fetal abnormalities such as neural tube defects and Down syndrome for a woman in her pregnancy.  The calculation is based on the levels of three analytes in the mother’s blood during pregnancy, alpha-fetoprotein, estriol, human chorionic gonadotropin, and other factors that are unique to her. Also called the triple screen.
More on the maternal serum screen...

Maternal UPD

Meiosis

a specialkind of cell division which produces the reproductive cells, the egg and sperm, whereby the chromosome content is reduced to 23 chromosomes (the haploid number).
More on meiosis...

Meiotic non-disjunction

when a chromosome pair fails to separate correctly during meiosis, resulting in reproductive cells which have missing or extra chromosomes.
A picture of meiotic non-disjunction...

Meiotic origin

originating during the development of the sperm or egg

Miscarriage

loss of a pregnancy prior to 20 weeks. Most miscarriages occur in the first trimester, prior to 12 weeks of pregnancy.
More on miscarriages...

Mitosis

the usually process of cell division involved in growth and development; asingle cell produces two daughter cells which are identical to on another, and to the original parent cell.  

Mitotic non-Disjunction

when a chromosome pair fails to separate correctly during cell division, resulting in daughter cells which have missing or extra chromosomes.
A picture of mitotic non-disjunction...

Monosomy

a condition where a cell has one less chromosome than the normal complement, for a total of 45 chromosomes.

Morula

one of the earliest stages of a fertilized egg, when there are 12 to 15 blastomeres.

Mosaic

an individual who has some cells with an abnormal or unusual genetic or chromosomal makeup while the rest of the cells in the body have the typical genetic or chromosomal constitution.

Mutation

processby which gene experiences a change.

-N-

Non-disjunction

whena chromosome pair fails to separate correctly during cell division, resulting in daughter cells which have missing or extra chromosomes.

Nullisomic

describes a cell in which there are no copies of a specific chromosome

-O-

Ovum

the female reproductive cell, also called the egg.  The ovum carries 23 chromosomes, 22 autosomes and an X chromosome.  The ovum fuses with the sperm at conception to produce an embryo.

Organ

-P-

'p' arm

eachchromosome is divided into two parts, joined by the centromere. The 'p' arm is the shorter of the two segments and is at the top of the chromosome. The longer segment is called the ‘q’ arm.

Paternal

something that originates from the father

Paternal UPD

Phenotype

theobservable characteristics of a person which are determined by their genes in combination with other modifying genes or the environment.

Placenta  

the structure that provides the fetus with nourishment during development. It is attached to the wall of the uterus and connects mother and baby.

Polyploidy

entire extra sets of chromosomes

Post-zygotic

after fertilization

Prader-Willi syndrome

Precursor cell

an original cell

Pregnancy termination

intervention to ensure that a pregnancy does not continue.

Prenatal diagnosis

the procedureof examining fetal cells taken from the amniotic fluid (amniocentesis), placenta (CVS), or umbilical cord (FBS) to detect fetal abnormalities during pregnancy.

-Q-

'q' arm

eachchromosome is divided into two parts, joined by the centromere. The 'q' arm is the longer of the two segments and is at the bottom of the chromosome. The shorter segment is called the ‘p’ arm.

-R-

Recessive

there are two copies of each gene. If a healthy individual has one working copy of a gene and one non-working copy, the mutation is described as being hidden or 'recessive' by the working copy of the gene. An individual with this genetic makeup is a 'carrier' of a recessive gene mutation. An individual with two non-working copies would be affected with the genetic condition.

Recombination

during meiosis homologous pairs of chromosomes may exchange genetic material: part of the maternal chromosome 'crosses over' and exchanges places with the corresponding part of the paternal chromosome. Also known as crossing over.

Recurrence risk

the chances of a genetic problem occuring or reccuring in the family members.

Replication

Reproductive cells

also called germ cells, the egg or sperm cells, each mature reproductive cell carries a single set of 23 chromosomes.

Ring chromosome

this occurs as a result of the fusion of the two ends of the same chromosome; there is loss of genetic material at the ends of the chromosome prior to the fusion.

-S-

Sex cells

a germ cell or reproductive cell; sperm in males and eggs in females

Sex chromosome

the chromosomes responsible for sex determination. Females have two X chromosomes; males have one X and one Y.
More on sex chromosomes...

Sex determining chromosome

sex is determined at fertilization by the sex chromosome carried by the sperm (either an X chromosome or a Y chromosome) that fertilizes the egg.

Sister chromatids

two exact copies of a chromosome attached together by the centromeres.

Somatic cells

all the cells in the body except the reproductive cells.

Somatic mutation

a change occurring in any cell that is not destined to become a reproductive cell. 

Somatic origin

a somatic cell

Sperm

the male reproductive cell. The sperm carries 23 chromosomes, 22 autosomes and an X chromosome or a Y chromosome.  The sperm fuses with the ovum at conception to produce an embryo.

Syndrome

a recognizable pattern or group of characteristics or symptoms.

-T-

Tetraploidy

four copies of every chromosome in a cell, for a total of 92 chromosomes, instead of the typical number, 46 chromosomes.

Tetrasomy

four copies of a particular chromosome present in a cell, for a total of 48 chromosomes in the cell instead of the typical number, 46 chromosomes.

Tissue

a group of cells that have a similar structure and function.  Many different tissue types can make up an organ.

Totipotent

a cell that is totipotent has the potential to become any differentiated, or specialized, cell in the body, including extra-embryonic tissuses (such as a placenta). Totipotent cells are found in the zygote, and begin to specialize a few days after fertilization and after several cycles of cell division.

Translocation

a chromosome rearrangement where a piece of one chromosome breaks off and joins 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. 
More on translocations...

Triploidy

three copies of every chromosome in a cells, for a total of 69 chromosomes, instead of the typical number, 46 chromosomes.

Trisomic rescue

Trisomy

three copies of a particular chromosome present in a cell, for a total of 47 chromosomes in the cell instead of the typical number, 46.

Trisomy 13 
(Patau syndrome)

three copies of chromosome 13, for a total of 47 chromosomes instead of the typical number, 46 chromosomes.
More on trisomy 13...

Trisomy 18 
(Edward syndrome)

three copies of chromosome 18, for a total of 47 chromosomes instead of the typical number, 46 chromosomes.
More on trisomy 18...

Trisomy 21 
(Down syndrome)

three copies of chromosome 21, for a total of 47 chromosomes instead of the typical number, 46 chromosomes.
More on Down syndrome...

Trophoblast cells

after fertilization, the zygote begins to divide mitotically (each 2 daughter cells identical to the parent cell).  The cells begin to differentiate into the inner cell mass (defined above) and trophoblast cells, creating a blastocyst. Trophoblast cells are found all around the outside of the blastocyst, they are the cells that are responsible for implantation into the uterine wall and will eventually differentiate further into all the extra-embryonic tissues, such as placenta, amnion and chorion.

Turner syndrome

only one copy of the X chromosome, for a total of 45 chromosomes instead of the typical number, 46 chromosomes.
More on Turner syndrome...

-U-

Ultrasound

the use of sound waves to obtain an image of body tissues and structures. Prenatal ultrasound provides an image of the fetus in order to assess growth and development.
More on ultrasound...

Uniparental disomy

when both members of a chromosome pair are inherited from one parent rather than one from each parent. Uniparental disomy may be maternal or paternal.
More on uniparental disomy...

Uterus

the female organ in which the fetus develops into a baby; also known as the womb.

-V-

Villus stroma

one of two cell types in the inner layer of the chorionic villi of the placenta (the layer closer to the amniotic sac and developing fetus). Trophoblast cells make up the outer later of the chorionic villi.

-W-

-X-

X-inactivation

early in the development of a normal female, one of the X-chromosomes in each cell is randomly inactivated. All descendent cells of that cell will have the same X-chromosome inactivated. Thus, all females are mosaic for some cells which express the paternal X-chromosome and others with express the maternal X-chromosome.

XYY syndrome

a genetic condition in males with extra Y chromosome (in 1 in 1000 male births). These individuals are tall in stature (over 6') and may experience problems with fertility, developmental delay, learning problems.

X chromosome

a sex chromosome; typical females carry two X chromosomes.

-Y-

Y chromosome

a sex chromosome; typical males carry one Y and one X chromosome.

-Z-

Zygote

a fertilized egg. Through cell division the zygote develops into a multicellular embryo and then into a fetus.

This page was last updated on04/06/2007 11:10:32 AM