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 -

albinism  click this icon to hear the preceding term pronounced
the genetically inherited condition in which there is a marked deficiency of pigmentation in skin, hair, and eyes.  An individual with these traits is an "albino."  Since the gene for albinism is recessive, it only shows up in the phenotype of homozygous recessive people.  This is a pleiotropic trait.
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.
amino acids  click this icon to hear the preceding term pronounced
small molecules that are the components of proteins.  There are 20 different kinds of amino acids in living things.  Proteins are composed of different combinations of amino acids assembled in chain-like molecules.  Amino acids are primarily composed of carbon, oxygen, hydrogen, and nitrogen.
Angelman syndrome  click this icon to hear the preceding term pronounced
a rare genetically inherited form of mental retardation due to the deletion or inactivation of specific genes on chromosome 15.  The inheritance of this syndrome is subject to genome imprinting.  Children with Angleman syndrome typically also have small heads, experience seizures, have pronounced speech impairment, are hyperactive, and have balance disorders.
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- B -

blending theory
an incorrect 19th century theory about the inheritance of characteristics.  It proposed that inherited traits blend from generation to generation.  Through his plant cross-breeding experiments, Gregor Mendel proved that this was wrong
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- C -

carrier
an individual who is heterozygous for a trait that only shows up in the phenotype of those who are homozygous recessive.  Carriers often do not show any signs of the trait but can pass it on to their offspring.  This is the case with hemophilia.
cataract  click this icon to hear the preceding term pronounced
an impairment of vision caused by the lenses of the eyes becoming cloudy.  Cataracts are common in elderly people.  They may be inherited or caused by diabetes and environmental factors.
photo of a normal eye and another with a cataract
chronic disease
an illness that lasts for a long period of time or indefinitely.  In contrast, an acute disease is one with a rapid onset and a short but usually severe course.
chromosomes  click this icon to hear the preceding term pronounced
thread-like, gene-carrying bodies in the nucleus of a cell.  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.
codominance  click this icon to hear the preceding term pronounced
the situation in which two different alleles for a trait are expressed unblended in the phenotype of heterozygous individuals.  Neither allele is dominant or recessive, so that both appear in the phenotype or influence it.  Type AB blood is an example.  Such traits are said to be codominant.
cross-pollination
the mating of two genetically different plants of the same species.  Usually, the term is used in reference to the crossing of two pure breeding (homozygous) plants.
cystic fibrosis  click this icon to hear the preceding term pronounced
a genetically inherited disease in children that results in chronic fluid development in the lungs, making breathing difficult.  This disease also prevents normal absorption of fats and other nutrients from food.  Cystic fibrosis occurs as a result of inheriting a recessive allele for it from both parents.  This is ultimately a fatal disease, but with modern medical care, about 2/3 of the people with it survive into early adulthood.  About 30,000 people have cystic fibrosis in the U.S. today.
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- D -

diabetes  click this icon to hear the preceding term pronounced
an inherited metabolic disorder in which there are abnormally high blood sugar levels.  In advanced stages, this often results in blindness from cataracts, nerve damage, gangrene in the feet and legs leading to amputation, heart disease, and kidney failure.  Type 1 diabetes melitis (juvenile onset diabetes) is due to decreased production of insulin by the pancreas.  Type 2 diabetes melitis is due to increased resistance of cells in the body to insulin.  The gene or genes for diabetes are incompletely penetrant.
dominant allele
an allele that masks the presence of a recessive allele in the phenotype.   Dominant alleles for a trait are usually expressed if an individual is homozygous dominant or heterozygous.
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.
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- E -

evolution  click this icon to hear the preceding term pronounced
genetic change in a population of organisms that occurs over time.  The term is also frequently used to refer to the appearance of a new species.
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- F - 

f1 generation  click this icon to hear the preceding term pronounced
the first offspring (or filial) generation.   The next and subsequent generations are referred to as f2, f3, etc.
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 -

gene flow
the transference of genes from one population to another, usually as a result of migration.  The loss or addition of individuals can easily change the gene pool frequencies of both the recipient and donor populations--that is, they can evolve.
gene pool
all of the genes in all of the individuals in a breeding population.  More precisely, it is the collective genotype of a population.
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 the hereditary traits in plants and animals.
genetic drift
evolution, or change in gene pool frequencies, resulting from random chance.  Genetic drift occurs most rapidly in small populations.  In large populations, random deviations in allele frequencies in one direction are more likely to be cancelled out by random changes in the opposite direction.
genetics  click this icon to hear the preceding term pronounced
the study of gene structure and action and the patterns of inheritance of traits from parent to offspring.  Genetic mechanisms are the underlying foundation for evolutionary change.  Genetics is the branch of science that deals with the inheritance of biological characteristics.
genome  click this icon to hear the preceding term pronounced
the full genetic complement of an individual (or of a species).  In humans, it is estimated that each individual possesses approximately 2.9 billion base units in his or her DNA.  See Human Genome Project.
genome imprinting  click this icon to hear the preceding term pronounced
an inheritance pattern in which a gene will have a different effect depending on the gender of the parent from whom it is inherited.  Genome imprinting is also known as genetic imprinting.
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.
gout  click this icon to hear the preceding term pronounced
a genetically inherited metabolic disorder in which there is an imbalance of uric acid in the blood which causes a build up of urates in joint areas.  Symptoms include painfully inflamed joints, especially of the feet and hands, that can become chronic and result in deformity.  Usually, only one joint is involved, most commonly the base of a big toe.  Gout is a sex-controlled trait, usually being more severe in men.
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- H -

hemophilia  click this icon to hear the preceding term pronounced
an X-linked genetically inherited recessive disease in which one or more of the normal blood clotting factors is not produced.  This results in prolonged bleeding from even minor cuts and injuries.  Swollen joints caused by internal bleeding is a common problem for hemophiliacs.  Hemophilia most often afflicts males.
heterozygous  click this icon to hear the preceding term pronounced
a genotype consisting of two different alleles of a gene for a particular trait (Aa).   Individuals who are heterozygous for a trait are referred to as heterozygotes.  See homozygous.
HLA system  click this icon to hear the preceding term pronounced
the body's genetically inherited system for recognizing and rejecting foreign tissues, such as transplanted organs.  The HLA system has the most genes of any other known human multiple-allele series.  There are at least 30,000,000 possible HLA genotypes.  HLA stands for "human leukocyte antigen" click this icon to hear the preceding term pronounced.
homeotic gene   click this icon to hear the preceding term pronounced
see regulator gene.
homologous chromosomes  click this icon to hear the preceding term pronounced
chromosomes that are paired during the production of of sex cells in 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.
homozygous  click this icon to hear the preceding term pronounced
having the same allele at the same locus on both members of a pair of homologous chromosomes.  Homozygous also refers to a genotype consisting of two identical alleles of a gene for a particular trait.  An individual may be homozygous dominant (AA) or homozygous recessive (aa).  Individuals who are homozygous for a trait are referred to as homozygotes.  See heterozygous.
Human Genome Project
a multinational research effort designed to identify and map the location of all human genes.  The idea of a Human Genome Project began at a 1984 international conference in Utah.  Research to decode the human genome began in earnest in 1986, funded by the U.S. Department of Energy.  The initial stage of discovering all human DNA codons was completed early in 2001 at a cost of 2.7 billion dollars.  The next phase of research will be to identify the proteins for which these genes code.  This decoding of the human proteome will be an even more daunting task than the original Human Genome Project.  See genome.
Huntington's disease
a severe genetically inherited fatal degenerative nerve disorder.  The symptoms usually do not appear until early middle age.  There is a progressive loss of muscle control that inevitably leads to paralysis and death.  This disease is also called Huntington's chorea click this icon to hear the preceding term pronounced
hybrids  click this icon to hear the preceding term pronounced
offspring that are the result of mating between two genetically different kinds of parents--the opposite of purebred.
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- I - 

incomplete penetrance
the situation in which an allele is expressed only if certain factors are present in the environment.  The triggering of genetically inherited diabetes by obesity and possibly severe emotional stress is an example.
intermediate expression
the situation in which a heterozygous genotype results in a phenotype that is intermediate between those resulting from the homozygous genotypes.  The mid-range baritone male voice is an example.
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- M -

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.
Mendelian genetics  click this icon to hear the preceding term pronounced
inheritance patterns which can be explained by simple rules of dominance and recessiveness of genes.
modifying gene
a gene that can alter the expression of another gene in the phenotype of an individual.
monozygotic twins  click this icon to hear the preceding term pronounced
identical twins.  Twins that come from the same zygote are essentially the same genetically.  Differences between monozygotic twins later in life are virtually always the result of environmental influences rather than genetic inheritance.  Fraternal twins may look similar but are not genetically identical.
multiple-allele series  click this icon to hear the preceding term pronounced
a situation in which a gene has more than two alleles.  The ABO blood type system is an example.  Multiple-allele series only partly follow simple Mendelian genetics.
multiple sclerosis  click this icon to hear the preceding term pronounced    (MS)
a genetically inherited progressive disease of the central nervous system.  MS occurs as a consequence of one's own immune system attacking the insulating sheath that normally protects neurons.  Symptoms range from numbness and tingling to paralysis.  There is a loss of motor and cognitive functions.  The gene(s) responsible for MS are incompletely penetrant in that the onset of the disease is apparently triggered by a virus and possibly other environmental factors.  There is a correlation between the amount of sunlight that children are exposed to and the likelihood that they will develop MS later in life.  People who spend much of their first 16 years in tropical and subtropical regions of the world are much less likely to develop this disease than those who live in far northern and far southern regions of our planet.  It is believed that the lack of abundant sun exposure early in life is somehow responsible for the later onset of MS.
muscular dystrophy  click this icon to hear the preceding term pronounced    (MD)
any of a group of inherited progressive muscle disorders caused by a defect in one or more genes that control muscle function.   MD is characterized by a gradual, irreversible wasting of skeletal muscle.  It is a sex-linked trait most often passed on to sons by their mothers.  The most common form, Duchene MD, begins to weaken the legs of boys by age 3 and inevitably gets worse with each passing year.  There is no cure for this disorder, and it usually results in death before the age of 30.
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.
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- N -

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- O -

ovum click this icon to hear the preceding term pronounced    (plural ova click this icon to hear the preceding term pronounced)
a female sex cell or gamete.
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- P -

pangenesis  click this icon to hear the preceding term pronounced
Charles Darwin's incorrect theory about the inheritance of traits.  He proposed that hereditary particles in the body are affected by the things an individual does during his or her lifetime.  These modified particles were thought to migrate via blood to the reproductive cells and subsequently could be inherited by the next generation.  This was a variation of Lamarck's incorrect idea of the "inheritance of acquired characteristics."
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.  
pleiotropy  click this icon to hear the preceding term pronounced
the situation in which a single gene is responsible for a variety of traits.  The collective group of symptoms known as sickle-cell trait is an example.
polygenic trait  click this icon to hear the preceding term pronounced
an inherited trait that is determined by genes at two or more loci.  Simple Mendelian rules of dominance do not apply to the complex interaction of these genes.  As a result, phenotypes may appear as apparent blends or intermediate expressions.  Human skin and hair color are polygenic traits.  Many polygenic traits are also influenced by environmental factors.
principle of independent assortment
Gregor Mendel's second principle of genetic inheritance.   It states that different pairs of genes are passed to offspring independently so that new combinations of genes, present in neither parent, are possible.  In other words, the distribution of one pair of alleles does not influence the distribution of another pair.  The genes controlling different traits are inherited independently of one another.
principle of segregation
Gregor Mendel's first principle of genetic inheritance.  It states that, for any particular trait, the pair of genes of each parent separate (during the formation of sex cells) and only one gene from each parent passes on to an offspring.  In other words, genes occur in pairs (because chromosomes occur in pairs).  During gamete production, the members of each gene pair separate, so that each gamete contains one member of each pair.  During fertilization, the full number of chromosomes is restored, and members of gene pairs are reunited.
probability
the likelihood that a specific event will occur.   Probability is usually expressed as the ratio of the number of actual occurrences to the number of possible occurrences.
proteins  click this icon to hear the preceding term pronounced
any of a large number of complex organic molecules that are composed of one or more chains of amino acids.  Proteins can serve a wide variety of functions through their ability to bind to other molecules.  Proteins may be enzymes, hormones, antibodies, structural components, or gas-transporting molecules.  
proteome  click this icon to hear the preceding term pronounced
the full complement of proteins produced by an individual (or a species).  It is estimated that each human produces approximately 90,000 types of proteins.  See Human Genome Project.
psoriasis   click this icon to hear the preceding term pronounced
an inherited disease characterized by recurring thick, reddish patches of inflamed skin.  It affects men and women equally.  This trait is subject to genome imprinting.
Punnett square  click this icon to hear the preceding term pronounced
a simple graphical method of showing all of the potential combinations of offspring genotypes that can occur and their probability given the parent genotypes.  See example below.  Punnett squares are commonly used by genetics counselors to predict the odds of a couple passing on particular inherited traits.

illustration of how to set up a Punnett square

purebred   click this icon to hear the preceding term pronounced
offspring that are the result of mating between genetically similar kinds of parents--the opposite of hybrid.  Purebred is the same as true breeding.
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- Q -

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- R -

recessive allele  click this icon to hear the preceding term pronounced
an allele that is masked in the phenotype by the presence of a dominant allele.  Recessive alleles are expressed in the phenotype when the genotype is homozygous recessive (aa).
regulator gene
a gene that can initiate or block the functions of other genes.  Regulator genes control the timing of production of a variety of chemicals in humans and other organisms.  Shortly after conception, regulator genes work as master switches orchestrating the timely development of our body parts.  They are also responsible for changes that occur in our bodies as we grow older.  Regulator genes are also called homeotic click this icon to hear the preceding term pronounced genes.
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- S -

sex cell
a gamete, either a sperm or an ovum.  Sex cells are produced by the meiosis process.  See somatic cell.
sex-controlled gene
a gene that can be inherited by both genders but is usually expressed differently in males and females.  See sex-limited gene.
sex-limited gene
a gene that can be inherited by both genders but is usually expressed in only males or females.  See sex-controlled gene.
sickle-cell trait
a genetically inherited recessive condition in which red blood cells are distorted resulting in severe anemia and related symptoms that are often fatal in childhood.  Sickle-cell trait is the result of a pleiotropic gene.  Sickle-cell trait is also known as sickle-cell anemia.
somatic cell  click this icon to hear the preceding term pronounced
any cell in the body except those directly involved with reproduction.  Most cells in multicellular plants and animals are somatic cells.  They reproduce by mitosis.  See sex cell.
sperm  click this icon to hear the preceding term pronounced
a male sex cell or gamete.
stuttering alleles
defective alleles that have segments which are doubled in their transmission from generation to generation.  In the case of genetically inherited diseases, the result is increasingly severe symptoms each generation.  The myotonic form of muscular dystrophy is an example.  Stuttering alleles are also known as unstable alleles.
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- T -

Tay-Sachs Disease  click this icon to hear the preceding term pronounced
a genetically inherited condition caused by the inability to produce the enzyme hexosaminidase A.  This results in progressively increased fluid pressure on the brain and the subsequent degeneration of the brain and nervous system beginning about 6 months of age and inevitably resulting in death usually by age 2-3.  The gene responsible for Tay-Sachs Disease is recessive.  It has been most common among the descendents of Eastern European Jews (Ashkenazi Jews).
true breeding
see purebred.
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- U -

unit inheritance
Gregor Mendel's idea that the characteristics of parents are passed on to descendants unchanged as units.  In other words, the hereditary material of any organism is made up of discrete units (now called genes).
unstable alleles
see stuttering alleles.
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- V -

virus  click this icon to hear the preceding term pronounced
a category of extremely small microscopic parasites of plants, animals, and bacteria. Viruses are not cells but rather RNA or DNA molecules surrounded by a protein coating.  Since viruses cannot reproduce without a host cell, they are not strictly speaking living organisms.
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- X -

X-linked
referring to a gene that is carried by an X sex chromosome.
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- Z -

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 at conception.  A zygote contains the full complement of chromosomes (in humans 46) and has the potential of developing into an entire organism.
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Copyright 1997-2012 by Dennis O'Neil. All rights reserved.
illustration credits