Glossary of Terms

 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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- A -

abdominal wall defect   click this icon to hear the preceding term pronounced
a genetically inherited condition in which intestines and other abdominal organs protrude through the central abdominal wall.  This must be corrected by surgery at birth.
alleles   click this icon to hear the preceding term pronounced
alternate forms or varieties of a gene.   The alleles for a trait occupy the same locus or position on homologous chromosomes and thus govern the same trait.  However, because they are different, their action may result in different expressions of that trait.
alpha-feto protein (AFP) sampling   click this icon to hear the preceding term pronounced
a routine diagnostic procedure for pregnant women to determine whether or not their fetuses have gross chromosomal anomalies.  This screening procedure is also called maternal serum alpha-feto protein (MSAFP) testing.  The testing is comparatively inexpensive but not as reliable as amniocentesis or chorionic villi sampling.  With AFP sampling, blood is drawn from a pregnant woman in order to determine the amount of alpha-feto protein that has leaked into her system from her unborn child.  Unusually high or low amounts of AFP relative to the stage of pregnancy indicate that there may be specific kinds of genetic defects.  Specifically, it may indicate the likelihood of Down syndrome, neural tube defects, abdominal wall defects, and trisomy 18.  Low AFP levels are associated with Down syndrome, while high levels are indicative of neural tube defects.  The amounts of other diagnostic fetal chemicals (hCG and estriol) also are measured in expanded AFP screening or triple-screening.
Alzheimer syndrome   click this icon to hear the preceding term pronounced
a progressive, irreversible disease characterized by degeneration of brain cells that commonly result in severe loss of memory, the ability to think, and pay attention.  There are also usually major changes in personality.   Alzheimer syndrome most often occurs in late middle and old age.  Genetic factors probably play a role since this syndrome is more common in some families. 
amniocentesis   click this icon to hear the preceding term pronounced
a diagnostic procedure for pregnant women to determine whether or not their fetuses have gross chromosomal anomalies.  It involves sampling the liquid immediately surrounding a fetus within the amnion.  This amniotic fluid is extracted through the mother's abdominal and uterine walls with a hypodermic needle.  The amniotic fluid mostly contains fetal urine but also has millions of fetal skin cells that can be cultured to produce a karyotype.  There is 99+% accuracy in diagnosing Down syndrome and most other gross chromosomal aberrations including neural tube defects such as spina bifida.
drawing of the amniocentesis procedure--amniotic fluid is extracted with a hypodermic needle inserted through the abdomen; the needle is guided with the aid of an ultrasound monitor 
amnion   click this icon to hear the preceding term pronounced
the innermost membrane of the flexible sac enclosing a fetus.  It is filled with amniotic fluid, which helps cushion the fetus and protect it from injury.  See cervix for an illustration.
amniotic fluid   click this icon to hear the preceding term pronounced
the fluid surrounding a fetus inside an amniotic sac.  It mostly consists of fetal urine, but also contains some fetal skin cells.  See cervix for an illustration.
anencephaly   click this icon to hear the preceding term pronounced
the failure to develop a brain at the end of the spinal chord.   This type of neural tube defect always causes the death of a fetus or newborn child.
aneuploidy   click this icon to hear the preceding term pronounced
a chromosomal abnormality in which there is an addition or loss of chromosomes within a set (e.g., 23 + 22 or 23 + 24).  Down syndrome is most often the result of this kind of error.  See monosomy, trisomy, and polyploidy.
autism   click this icon to hear the preceding term pronounced
a neurodevelopmental disorder in children that prevents them from communicating normally and from relating normally with other people.  Symptoms usually appear by age three.
autoimmune disorder   click this icon to hear the preceding term pronounced
any of several disorders in which one's antibodies attack his/her own body tissues.  Lupus erythematosus, rheumatoid arthritis, and even common allergies are caused by such autoimmune reactions.
autosomes   click this icon to hear the preceding term pronounced
any chromosome other than a sex chromosome.  Human somatic cells have 22 pairs of autosomes and 1 pair of sex chromosomes.
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- B -

biopsy   click this icon to hear the preceding term pronounced
the process of removing a tissue sample from a living organism for diagnostic examination (e.g., chorionic villi sampling).
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- C -

cataracts   click this icon to hear the preceding term pronounced
impairments of vision caused by the lenses of the eyes becoming cloudy.   Cataracts are common in elderly people.  They may be inherited or caused by environmental factors.
cervix   click this icon to hear the preceding term pronounced
the neck-like opening at the lower end of the uterus that connects to the vagina.
drawing of the human female reproductive system with a fetus in her uterus--the cervix and other major parts are highlighted
chorion   click this icon to hear the preceding term pronounced
The membrane that develops around an embryo and contributes to the formation of the placenta.  Later, as a fetus develops, the chorion fuses with the amnion.  See chorionic villi sampling for an illustration.
chorionic villi sampling   click this icon to hear the preceding term pronounced   (or biopsy click this icon to hear the preceding term pronounced)
a diagnostic procedure for pregnant women to determine whether or not their fetuses have gross chromosomal anomalies.  It involves collecting a small sample of chorion cells for karyotyping.  The biopsy usually is done by inserting a small flexible plastic tube through the vagina and the cervix of the uterus to draw out a sample of chorion tissue.   Alternately, the cells may be extracted with a hypodermic needle through the uterine wall, as in the case of amniocentesis.   With CVS, there is approximately 98% accuracy in diagnosing Down syndrome and many other conditions due to gross chromosomal abnormalities.  However, the accuracy in predicting neural tube defects is lower than with amniocentesis.  Some sources refer to CVS as "chorionic villus sampling".  "Villus" is the singular of "villi".   
drawing of the chorionic villi biopsy process--a flexible plastic catheter tube is inserted through the vagina into the uterus guided by an ultrasound monitor and a small amount of the chorion is sucked off   
chromatids   click this icon to hear the preceding term pronounced
one of two or more strands of a chromosome that are joined at a centromere.
chromosomes   click this icon to hear the preceding term pronounced
thread-like, gene-carrying bodies in the cell nucleus.   Chromosomes are composed primarily of DNA and protein.  They are visible only under magnification during certain stages of cell division.  Humans have 46 chromosomes in each somatic cell and 23 in each sex cell.
crossing-over
see recombination.
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- D -

diabetes    (diabetes mellitus click this icon to hear the preceding term pronounced)
an inherited metabolic disorder in which the pancreas does not produce enough of the hormone insulin.  Symptoms include excessive urination, persistent thirst and hunger, as well as persistent fatigue and weight loss.  Without treatment, this disease can be debilitating and even fatal.
diploid click this icon to hear the preceding term pronounced number
referring to the full component of chromosomes normally found in somatic cells.   In humans, the number is 46.  See haploid number.
DNA  click this icon to hear the preceding term pronounced   (deoxyribonucleic acid click this icon to hear the preceding term pronounced)
a large organic molecule that stores the genetic code for the synthesis of proteins.  DNA is composed of sugars, phosphates and bases arranged in a double helix shaped molecular structure.  Segments of DNA in chromosomes correspond to specific genes.
Down syndrome  click this icon to hear the preceding term pronounced
a genetically inherited form of mental retardation usually resulting from the inheritance of an extra autosome 21.   Down syndrome individuals also typically are short and stocky in build with short appendages.  They usually have broad round faces, saddle-shaped nose profiles, and thick tongues that are often stuck out of their mouths.  The incidence of Down syndrome children goes up rapidly with the age of the mother, particularly after 40.
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- E -

embryo  click this icon to hear the preceding term pronounced
the earliest stage of development of an unborn child.  In humans, the embryonic stage lasts up till about the 8th week of pregnancy.   Following, is the fetal stage.
 
photo of a normal Chinese chiild of about 4 years of age--the epicanthic folds of her eyelids can be seen
epicanthic fold  click this icon to hear the preceding term pronounced
a fold of skin over the inner corner of each eyelid.  This is characteristic of normal East Asians (see photo on right).  It is also typical of people with Down syndrome from any population in the world.  The presence of epicanthic folds does not imply Down's.
epilepsy  click this icon to hear the preceding term pronounced
a usually recurrent disorder of the brain characterized by abnormal electrical activity which causes mental and physical dysfunction.  In serious cases, there are convulsions and unconsciousness when seizures occur.
estrogen  click this icon to hear the preceding term pronounced
a class of feminizing hormones.   Both men and women produce them, but females normally produce much more.
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- F -

false positive
a test result that indicates a medical problem when there actually is not one.
fertilization
conception; the process of sexual reproduction by which the chromosomes from a sperm cell enter the nucleus of an ovum and combine with its chromosomes.
fetus  click this icon to hear the preceding term pronounced
an unborn child during the later stages of pregnancy.  In humans, an embryo usually becomes a fetus after the 8th week of gestation.  Unlike embryos, fetuses have clearly recognizable physical characteristics of the species and gender.
fragile-X syndrome   click this icon to hear the preceding term pronounced
a relatively common genetically inherited abnormality of the X chromosome which results in mental retardation.  Since it is an X-linked trait, males more often have it expressed in their phenotypes.   Most fragile-X males have large testes, big ears, narrow faces, and sensory integration dysfunctions that result in learning disabilities.  It is likely to occur 1 in 1000 births.  Approximately 1 in 700 females are carriers of the gene for this trait.
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- G -

gametes  click this icon to hear the preceding term pronounced
reproductive cells--sperm or unfertilized ovum cells produced in the testes and ovaries of animals.  Gametes are produced by meiosis.  They normally have half the number of chromosomes found in somatic cells.
genes  click this icon to hear the preceding term pronounced
units of inheritance usually occurring at specific locations, or loci, on a chromosome.  Physically, a gene is a sequence of DNA bases that specify the order of amino acids in an entire protein or, in some cases, a portion of a protein.  A gene may be made up of hundreds of thousands of DNA bases.  Genes are responsible for hereditary characteristics in plants and animals.
genotype  click this icon to hear the preceding term pronounced
the genetic makeup of an individual.  Genotype can refer to an organism's entire genetic makeup or the alleles at a particular locus.  See phenotype.
gestation  click this icon to hear the preceding term pronounced
the period between conception and birth during which an unborn young is within its mother's uterus.  Pregnancy is another word for gestation.
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- H -

haploid click this icon to hear the preceding term pronounced number
referring to the number of chromosomes in gametes.  Human sperm and ova normally contain only 23 chromosomes, which is the result of the halving of chromosome pairs in meiosis.  See diploid number.
hernias   click this icon to hear the preceding term pronounced
protrusions of an organ through a tear in the wall of the surrounding structure (e.g., the protrusion of part of the intestine through the abdominal muscle and surrounding fascia tissue).
homologous click this icon to hear the preceding term pronounced chromosomes
chromosomes that are paired during meiosis.  Such chromosomes are alike with regard to size and also position of the centromere.  They also have the same genes, but not necessarily the same alleles, at the same locus or location.
hormones  click this icon to hear the preceding term pronounced
substances (usually proteins) that are produced by specialized cells and that travel to other parts of the body, where they influence chemical reactions and regulate various cellular functions.  Hormones include the secretions of the endocrine glands that affect metabolism and behavior.  Testosterone, estrogen, and insulin are examples of such hormones.
hypodermic click this icon to hear the preceding term pronounced needle
a hollow needle, connected to a syringe, used to either inject fluid materials into the body tissues or to extract blood or other fluids from the body.   "Hypodermic" literally means "beneath the skin."
hypothyroidism  click this icon to hear the preceding term pronounced
a disorder caused by the thyroid gland not producing enough hormones.  Symptoms usually include a decreased metabolic rate which results in physical and mental depression, sluggishness, and puffiness.
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- I -

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- J -

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- K - 

karyotype  click this icon to hear the preceding term pronounced
pictures of chromosomes cut out from a microphotograph of a cell and rearranged into homologous pairs according to size and other physical characteristics.  The standardized arrangement of karyotypes allows researchers to discover if an individual is a male or female and if he/she has any gross chromosomal abnormalities.
Klinefelter syndrome  click this icon to hear the preceding term pronounced
a genetically inherited sex chromosome abnormality only affecting males.  Genotypically, they are XXY or more rarely  XXXY, or XXXXY.  They have asexual to feminine body contours as well as breast enlargement and relatively little body hair.  They are sterile or nearly so and their penis, testes, and prostate gland are small.  Like metafemales, most Klinefelter syndrome men are an inch or so above average height.  They usually have slight learning difficulties, especially with language.  However, most are sufficiently ordinary in appearance and mental ability to live in society without notice.  The frequency of Klinefelter syndrome has been reported to be between 1 in 500 and 1 in 1,000 male births.   Males with Down syndrome sometimes also inherit Klinefelter syndrome.
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- L -

leukemia   click this icon to hear the preceding term pronounced
a class of cancers of the blood cells and blood-forming organs resulting in exceptionally high leukocyte (white blood cell) production.  Other organs can become involved as well.  Leukemia can be caused by some viruses, chemicals, and radiation.  However, there also may be a genetic connection.  For instance, individuals with Down syndrome have a much higher chance of developing leukemia.
linked genes
genes that are on the same chromosome and subsequently are inherited together as a package unless crossing-over separates them.
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- M -

medical syndrome
see syndrome.
meiosis  click this icon to hear the preceding term pronounced
cell division in specialized tissues of ovaries and testes which results in the production of sperm or ova.  Meiosis involves two divisions and results in four daughter cells, each containing only half the original number of chromosomes--23 in the case of humans.  These cells can develop into gametes.  See mitosis.  
metafemales   click this icon to hear the preceding term pronounced
a genetically inherited sex chromosome abnormality only affecting females.  Metafemales inherit three X chromosomes--their genotype is XXX.  As adults, these individuals are usually an inch or so taller than average with unusually long legs and slender torsos.  They have normal development of sexual characteristics and are fertile.  They may have slight learning difficulties and are usually in the low range of normal intelligence.  They tend to be emotionally immature for their size during childhood.  None of these traits prevent them from being socially accepted as ordinary women.  This type of chromosomal abnormality is apparently rare and little is known about it.  However, the frequency is approximately 1 in 1,000 female infants and it may be more common when the mother is older.  Metafemales are also called "triple-X females."
miscarriage
the expulsion of an embryo or fetus from the uterus before it is sufficiently developed to survive.  A miscarriage, or abortion, may be spontaneous or induced by some action.
mitosis  click this icon to hear the preceding term pronounced
the simple cell division process that occurs in somatic cells.  One cell divides into two offspring cells that are identical to each other in their chromosome complement.  Mitosis produces cells with diploid numbers of chromosomes--46 in the case of humans.   See meiosis.
Mongoloidism    click this icon to hear the preceding term pronounced
a now rejected alternate term for Down Syndrome.
monosomy   click this icon to hear the preceding term pronounced
the form of aneuploidy in which there is only one chromosome instead of the usual two homologous chromosomes of each type.  Conceptions with autosomal monosomies die before birth.
mosaicism   click this icon to hear the preceding term pronounced   (mosaic click this icon to hear the preceding term pronounced)
a genetic pattern in which an individual has two distinct cell groupings--one with normal cells and another with a genetic problem.  For instance, some people with Down syndrome produce both normal somatic cells and cells with an extra chromosome 21.   Usually, this mosaic pattern results in a lessening of symptoms.
mutagen  click this icon to hear the preceding term pronounced
an agent that can cause a mutation to occur.  Various kinds of chemicals, viruses, and radiation have been identified as mutagens.
mutation  click this icon to hear the preceding term pronounced
an alteration of genetic material such that a new variation is produced.  For instance, a trait that has only one allele (A) can mutate to a new form (a).   This is the only mechanism of evolution that can produce new alleles of a gene.  Technically, mutation refers to changes in DNA bases as well as changes in chromosome number and /or structure.  See mutagen.
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- N -

neural tube defects   click this icon to hear the preceding term pronounced
defective developments of the brain or spinal cord.  By the mid embryonic stage (5 weeks in humans), a neural tube extends from the top of the head to the lower end of the spinal column.  Later, this tube becomes the brain and spinal chord.  Spina bifida and anencephaly are examples of neural tube defects.
nondisjunction errors   click this icon to hear the preceding term pronounced
a mechanical error that can occur during meiosis and mitosis.   Specifically, members of a pair of homologous chromosomes move to the same pole rather than opposite poles.  In meiosis, this can result in one gamete receiving two of the same type of chromosome and another receiving none of this type.  Nondisjunction is a cause of gross chromosomal abnormalities.
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- O -

oöcyte  click this icon to hear the preceding term pronounced
a female sex cell in humans and other animals.  Immature, primary oöcytes are produced before birth.  These become secondary oöcytes usually one by one after puberty when hormones trigger ovulation.
oögenesis   click this icon to hear the preceding term pronounced
the meiosis process of female sex cells that produces ova.  It takes place mostly in the ovaries of humans and other animals.
osteoporosis   click this icon to hear the preceding term pronounced
a bone disorder in which there is a progressive reduction in bone density resulting in brittleness and increased porosity.    People with osteoporosis are at a high risk for bone fractures.  This disorder is most commonly found among post-menopausal women.  However, it also occurs at a high frequency among men who have low testosterone levels  (e.g., men with Klinefelter syndrome).
over expression
the abnormal expression of a trait in the phenotype of an individual resulting from the presence of extra chromosomes or copies of the genes responsible for the trait.  For example, Down syndrome traits are likely the consequence of inheriting three chromosome 21's instead of the usual two.  Apparently, the genes on the extra chromosome are also expressed. 
ovulation   click this icon to hear the preceding term pronounced
the release of a secondary o�cyte from an ovary to begin its path down a fallopian tube (or oviduct) to the uterus.
ovum  click this icon to hear the preceding term pronounced    (plural click this icon to hear the preceding term pronounced ova)
a female sex cell or gamete that has completed all of the phases of the meiosis process.
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- P -

penis   click this icon to hear the preceding term pronounced
the external male sex organ through which urine and semen pass.   The penis mostly consists of the urethra and three cylindrical bodies of erectile tissues.  See prostate gland for illustration.
phenotype  click this icon to hear the preceding term pronounced
the observable or detectable characteristics of an individual organism; the detectable expression of a genotype.
placenta   click this icon to hear the preceding term pronounced
an organ in the uterus of a mammal that develops from the chorion of an embryo during gestation.  The placenta is connected to an unborn child by the umbilical cord.  The placenta provides oxygen, nutrients, and antibodies for a fetus.  Following birth the placenta and umbilical cord are naturally expelled from the uterus as the "after birth."  See cervix for illustration.
polyploidy   click this icon to hear the preceding term pronounced
a chromosomal abnormality in which there are extra complete multiples of chromosome sets (e.g., 23 + 23 + 23).   Polyploids are rare among animals but common in plants.  A mosaic polyploidy (with only some ployploid cells) is the common animal form.  See aneuploidy.
postmenopausal   click this icon to hear the preceding term pronounced
after the period in a woman's life when her ovulations and menstruations have reduced in frequency and finally stopped altogether--i.e., after menopause.  Menopause usually takes place between the ages of 45 and 55.
primary o�cyte
  drawing of the human male reprodcutive system with the prostate gland and other major parts highlighted
see o�cyte.
prostate gland  click this icon to hear the preceding term pronounced
a gland surrounding the urethra at the base of the bladder.   It secretes most of the seminal fluid that is expelled with sperm cells during ejaculation.
puberty   click this icon to hear the preceding term pronounced
the stage of maturation of an individual when secondary sexual characteristics begin to develop and sexual reproduction first becomes possible.   These changes are genetically controlled and triggered by hormones.
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- Q -

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- R -

recombination
the exchange of genetic material (DNA) between homologous chromosomes at the beginning of meiosis. This results in sperm and ova with greater genetic diversity.  Specifically, a portion of a chromosome is broken and reattached on another chromosome.  Recombination is also referred to as crossing-over.
rickets disease   click this icon to hear the preceding term pronounced
a disease of the skeletal system in which the bones are softened and often bent as a result of vitamin D deficiency in the diet that hinders the normal development of bones and teeth.  Rickets most frequently affects malnourished children.
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- S -

schizophrenia  click this icon to hear the preceding term pronounced
a severe mental disorder characterized by disorganized, faulty perceptions of reality that create difficulty in relating to other people.  Schizophrenics often experience intense hallucinations, delusions, and/or psychotic defenses.
secondary o�cyte
see o�cyte.
sex cell
a gamete, either a sperm or an ovum.  Sex cells are produced by the meiosis process.  See somatic cell.
sex chromosome
the X and Y chromosomes which are responsible for determining whether an individual is a male or a female.  Normal males inherit an X from their mother and a Y from their father.  Normal females get an X chromosome from both parents.  See SRY gene.
sex-linked
referring to a gene that is part of a sex chromosome.  Since all of the genes on a chromosome are inherited as a package, they are essentially linked together.

sickle-cell trait  click this icon to hear the preceding term pronounced

a genetically inherited recessive condition in which some or all of the red blood cells are abnormally distorted to a sickle shape.  Symptoms include pain and severe anemia as well as heart, lung, and kidney problems.  People who are heterozygous for this trait rarely have these debilitating and ultimately fatal problems but do have a high degree of immunity to malaria.  Sickle cell trait is at its highest frequency among Central African populations and among people whose ancestors came from that region.  Sickle-cell trait is often referred to as sickle-cell anemia.
somatic cell  click this icon to hear the preceding term pronounced
basically, all the cells in the body except those directly involved with reproduction.  Most cells in multicellular plants and animals are somatic cells.  They reproduce by mitosis and have a diploid number of chromosomes.  See sex cell.
sperm  click this icon to hear the preceding term pronounced
a male sex cell or gamete.
spermatogenesis   click this icon to hear the preceding term pronounced
the meiosis process of male sex cells that produces sperm.  It takes place in the testes of humans and other animals.
spina bifida   click this icon to hear the preceding term pronounced
a neural tube birth defect in which there are malformations of vertebrae and the protrusion of the spinal cord.  This can result in paralysis of the legs, loss of bowel and bladder control, and "water" on the brain (hydrocephaly).   Spina bifida requires surgical correction.
SRY gene  click this icon to hear the preceding term pronounced
sex-determining region Y gene.   A gene on the Y chromosome which, if present and functioning, causes the undifferentiated sex organs of an animal embryo to become testes.  These then produce the hormone testosterone which, in turn, causes the development of masculine physical traits.
syndromes   click this icon to hear the preceding term pronounced
a number of conditions that occur together and characterize a particular disease or condition (e.g., Klinefelter syndrome).
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- T -

testes   click this icon to hear the preceding term pronounced   (or testicles click this icon to hear the preceding term pronounced)
the two male sex organs within the scrotum.  Testes produce the hormone testosterone and sperm cells.   See prostate gland for illustration.
testosterone  click this icon to hear the preceding term pronounced
a class of masculinizing hormones.   Both men and women produce them, but males normally produce much more.   Testosterone is mainly produced in the testes of males.   Smaller amounts are produced by the cortex of the adrenal glands in both males and females.
thyroid  click this icon to hear the preceding term pronounced
the large gland in the neck next to the trachea that secretes the hormone thyroxine, which regulates body growth and metabolism.
translocation
a structural modification of a chromosome in which a portion of a chromatid is broken off and attached to a different location on that chromosome or on another chromosome.  This occurs as a result of crossing-over.
triple-X female
see metafemale.
trisomy   click this icon to hear the preceding term pronounced
the form of aneuploidy in which there are 3 chromosomes instead of just 2 for a specific homologous pair.  An example is the common form of Down syndrome in which there are 3 chromosome 21's.  Most fetuses with autosomal trisomies die before birth.
trisomy 18  click this icon to hear the preceding term pronounced
a rare chromosomal abnormality  in which there is a trisomy of chromosome 18.  This inherited condition is also called Edward's syndrome.  It is characterized by severe mental retardation, a small head with malformed ears set low, and a face with a pinched appearance.  Other likely defects include a cleft palate, webbed hands, missing thumbs, clubfeet,  as well as heart and genitourinary defects.  Individuals with trisomy 18 usually die within a few months after birth.
trisomy 21  click this icon to hear the preceding term pronounced
see Down syndrome.
Turner syndrome   click this icon to hear the preceding term pronounced
a genetically inherited sex chromosome abnormality only affecting females.  Women with Turner's only have one X chromosome--their genotype is X0.   These individuals are short in stature, averaging 4'7" and have distinctive webbed necks (i.e., extra folds of skin).  They have exceptionally small breasts and generally lack secondary sexual characteristics.  Their ovaries do not develop normally and they do not ovulate.  They are in a sense postmenopausal from early childhood and are sterile.  In some individuals, there is slight mental retardation.   Turner syndrome is rare--current estimates of its frequency range from 1 in 3000 to 1 in 5,000 females.
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- U -

ultrasound monitoring
the use of high frequency sound to form an image or picture of internal organs for immediate medical diagnostic purposes.  The Ultrasonography device transmits sound waves directly into the body and then accurately records the amount of time for echoes to return to a receiver, much like radar.  The sound waves used are at frequencies above the threshold of human hearing--e.g., 20,000 vibrations per second.
uterus   click this icon to hear the preceding term pronounced
a pear-shaped, muscular organ at the top of the vagina.  A fertilized ovum normally attaches onto the inner wall of the uterus following conception.  Unborn babies are nurtured in the uterus by means of the placenta.   See cervix for an illustration.

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- V -

vagina   click this icon to hear the preceding term pronounced
the muscular canal between the vulva and the uterus in female mammals.  See cervix for an illustration.
variable penetrance
the expression of a genetically inherited trait in different forms or degrees in different people.  For example, the alleles for the Down syndrome genes may cause an effect in the phenotype of some individuals but not others.  As a result, there is considerable variability within the Down syndrome population in regards to susceptibility to the characteristic unpleasant medical problems. 
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X-linked  click this icon to hear the preceding term pronounced
referring to a gene that is carried by an X chromosome.  See sex-linked gene.
XXX syndrome  click this icon to hear the preceding term pronounced
see metafemale.
XYY syndrome  click this icon to hear the preceding term pronounced
a genetically inherited sex chromosome abnormality only affecting males.  Men with XYY syndrome inherit an extra Y chromosome--their genotype is XYY.  They are usually quite tall (i.e., above 6 feet) and generally appear and act normal.  During adolescence, they frequently have severe facial acne.  They are usually fertile and lead ordinary lives as adults.  Many, if not most, are unaware that they have a chromosomal abnormality.  The frequency of XYY syndrome is not certain due to statistical differences between different studies of this condition.  It may be as common as 1 in 900 male births to as rare as 1 in 1,500 or even 1 in 2,000. 
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Y-linked  click this icon to hear the preceding term pronounced
referring to a gene that is carried by a Y chromosome.  See sex-linked gene.
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zygote  click this icon to hear the preceding term pronounced
a "fertilized" ovum. More precisely, this is a cell that is formed when a sperm and an ovum combine their chromosomes.  A zygote is the beginning of a multicellular plant or animal, such as a human.
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Copyright © 1997-2012 by Dennis O'Neil. All rights reserved.
illustration credits