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 -
-
adaptive
radiation
- the relatively rapid
expansion and diversification of an evolving group of organisms as they
adapt to new ecological
niches. Adaptive radiation is the process by which one species
evolves into
two or more species. This occurs as a result of different populations becoming reproductively isolated
from each other, usually by adapting to different environments. The
branching pattern of evolution resulting from adaptive radiation
is known as cladogenesis
.
-
AIDS
(Acquired Immunodeficiency Deficiency Syndrome)
- a usually fatal, slow acting disease caused by the HIV retrovirus. Important disease-fighting white blood cells are destroyed resulting in a weakened immune system. Death usually comes as a result of cancer or other diseases that are normally fought off by healthy immune systems. HIV is spread from person to person via bodily fluids such as blood and semen. The common methods of transmission are via sexual intercourse or sharing hypodermic needles.
- albinism
- 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. Albinos have sunlight sensitive eyes and skin. They are also more likely to develop skin and eye cancers.
- alleles
- 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
- organic molecules that are building block 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.
-
Amish
- members of a conservative Protestant sect related to the Mennonites. The Amish migrated to Pennsylvania from Switzerland in the late 18th century. The Old Order Amish are a relatively closed group that shuns most modern conveniences in their farming lifestyle. They use horse drawn carriages, dress very simply, and reject those who marry non-Amish.
-
anagenesis
- The pattern of non-branching evolution that results from successive speciation in a single evolutionary line.
-
autosomes
- any chromosomes other than a sex chromosome.
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- B -
- balanced
polymorphism
- the maintenance of two or more alleles for a trait in a population at a more or less constant frequency ratio due to the selective advantage of heterozygotes. See polymorphism.
- biocultural
evolution
- the pattern of human evolution in which the effects of natural selection are altered by cultural inventions. Culture can alter the direction of evolution by creating non-biological adaptations to environmental stresses (e.g., wearing insulating clothes on very cold days). This potentially reduces the need to evolve genetic responses to the stresses. This has meant that we have been able to remain essentially tropical animals biologically and live in colder regions of our planet. Biocultural evolution can also involve a mutual, interactive evolution of human biology and culture. An example of this has been the selection favoring sickle-cell trait in Africa. Human agricultural practices altered the environment, which resulted in factors that were advantageous to both the malarial microorganisms and the mosquitoes that transmit them between people. This, in turn, selected for the sickling allele.
- bottleneck effect
- a dramatic reduction in genetic diversity of a population or species resulting from an ecological crisis that wipes out most of its members. The limited genetic diversity of the few survivors is the pool from which all future generations are based. This is one of the small population size effects.
-
bubonic plague
- a highly contagious disease caused by the bacterium Yersinia pestis. It is normally spread by fleas that have taken blood from an infected human or other animal. Symptoms include a high fever and extremely swollen lymph nodes in the groin, armpits, and/or throat. An agonizing death usually occurs within a few days.
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- C -
- chromosomes
- 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.
-
cladogenesis
- the branching pattern of evolution resulting from adaptive radiation.
- codon
- a sequence of three nucleotide bases in a DNA or RNA molecule that code for a specific type of amino acid that will be used in the synthesis of a protein.
- color blindness
- the inability to see certain colors as they normally appear to others. The most common form of this vision deficiency is X-linked genetically inherited red-green color blindness or deficiency.
- consanguineous
mating
- the mating of closely related individuals, such as brothers, sisters, or cousins. Another name for this mating pattern is "inbreeding." This is an extreme form of positive assortative mating.
- crossing-over
- the exchange of genetic material (DNA) between homologous chromosomes at the beginning of meiosis. This results in sperm and ova with greater genetic diversity due to a recombination of genes. Specifically, a portion of the end of a chromosome is broken and reattached on another chromosome.
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- D -
- diabetes
- 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.
- diploid
number
- referring to the full component of chromosomes normally found in somatic cells. In humans, the number is 46. See haploid number.
- directional selection
- Selection for or against one extreme of a trait. In the case of polygenic traits that are expressed as a continuum of phenotypes, such as human stature, it would be selection for people who are either very tall or very short. The result would be a progressive increase in the form of the trait that is being selected for and a reduction in the form that is being selected against. In the case of a trait controlled by only two alleles, it would be selection against one of the alleles. When that allele is recessive, it is usually selection against the recessive homozygote and for the dominant homozygote and the heterozygote. An example is selection against people who have a fatal genetically inherited disease that only shows up in homozygous recessive people. The result would be a progressive reduction of the recessive allele in the gene pool of the population and, subsequently, a reduction in the number of people who have the disease. Continuous directional selection can result in evolution. See disruptive selection and stabilizing selection.
- disruptive selection
- Selection for both extremes of a trait and against the middle. In the case of polygenic traits that are expressed as a continuum of phenotypes, such as human stature, it would be selection for both very tall and very short people and against those who are average in height. The result would be a progressive increase in both of the extreme forms of this trait and a reduction in the middle range. In the case of a trait controlled by only two alleles, it would be selection for both recesssive and dominant homozygotes and against heterozygotes. The result would be a progressive reduction in the number of people who are heterozygous for the trait in the population. See directional selection and stabilizing selection.
- DNA
(deoxyribonucleic acid
)
- a large organic molecule that stores the genetic code for the synthesis of proteins. Each chromosome consists mostly of a DNA molecule. DNA is composed of sugars, phosphates and bases arranged in a double helix shaped molecular structure. Segments of DNA correspond to specific genes.
- dominant allele
- an allele that masks the presence of a recessive allele.
-
double helix
- the twisted ladder shape that is characteristic of DNA molecules.
-
Down syndrome
- 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 -
specific micro-habitats in nature to which populations or organisms adapt. They are usually seen in terms of being food getting opportunities in the environment.
-
Ellis-van Creveld syndrome
- a genetically inherited disorder characterized by dwarfism, extra fingers, and malformations of the arms, wrists, and heart. The majority of the known cases in the world of this rare syndrome have been found among the Amish and 7% of them carry the responsible recessive autosomal allele.
-
epilepsy
- 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.
-
erythroblastosis fetalis
- a blood disease of fetuses and newborn infants caused by the mother's anti-Rh+ antibodies agglutinating or bursting the red cells of her infant's Rh+ blood. Symptoms include life threatening anemia, jaundice, fever, swollen tissues from edema, and an enlarged liver and spleen. Serious cases are treated by blood replacement. Erythroblastosis fetalis is also referred to as "hemolytic anemia" and "hydrops fetalis."
- evolution
- 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. More specifically, it is change in the frequencies of alleles in a population's gene pool from one generation to the next.
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- F -
- fertilization
- the process of sexual reproduction by which the chromosomes from a sperm cell enter the nucleus of an ovum and combine with its chromosomes to create a zygote.
- first cousin
- someone who is related as a result of being a child of one's uncle or aunt.
- founder principle
- a small population size effect in which the genes of a few people (the population founders) are inherited over time by a large number of descendents. This is also known as the "founder effect" and the "Sewall Wright effect." See genetic drift.
- frequency
- the number of times that something happens. For example, the number of people out of 100 who get divorced would be the frequency of divorce.
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- G -
- gametes
- 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
- 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 a protein or, in some cases, a small RNA molecule referred to as a microRNA. A gene may be made up of hundreds to thousands of DNA bases. Genes are responsible for hereditary characteristics.
- gene flow
- gene pool
- the sum of all of the alleles of genes in all of the individuals in a population.
- 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.
- genetic load
- a measure of all of the harmful recessive alleles in a population or family line. A high genetic load would be one in which there is a high frequency of deleterious recessive alleles.
- genetic equilibrium
- the situation in which a population is not evolving from generation to generation--that is, the population's gene pool frequencies remain unchanged.
-
genome
- the full genetic complement of an individual (or a species). In humans, it is estimated that each individual possesses approximately 2.9 billion nucleotides in all of the DNA that makes up his or her genome.
- genotype
-
glucose-6-phosphate dehydrogenase deficiency
(G6PD)
- a genetically inherited X-linked error in metabolism caused by an inadequate amount of the enzyme glucose-6-phosphate dehydrogenase in red blood cells. When exposed to certain environmental influences, such as fava beans and some drugs (sulphonamide antibiotics, some antimalarials including primaquine, and several other sulphur-containing drugs) , the red cells of individuals with this deficiency burst, resulting in severe anemia. As in the case of sickle-cell trait and thalassemia, this anemia may provide some immunity to falciparum malaria. G6PD deficiency is also known as "favism." It is the most common type of human enzyme deficiency--about 400 million people have this condition around the world. It is a common problem especially in populations around the Mediterranean Basin, the Middle East, and Southeast Asia.
-
gradualism
- see phyletic gradualism.
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- H -
- haploid
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.
- Hardy-Weinberg equilibrium
equation
- the equation (p²+2pq+q² = 1) used by population geneticists to determine probable genotype frequencies of a population for specific traits. By comparing these frequencies for subsequent generations, it is possible to track the direction and rate of evolution.
- hemophilia
- 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 are a common problem for hemophiliacs. Hemophilia most often afflicts males.
-
heterosis
- increased health of individuals who are the result of negative assortative mating, or crossbreeding. This is due to the fact that when mates are from very different genetic lines they are more likely to have lower frequencies of homozygous recessive conditions. Subsequently, they are liable to be more viable. This phenomenon has also been referred to as hybrid vigor.
- heterozygous
- 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.
-
hip dysplasia
- abnormal development of one or both hip joints. The joint socket may be partly or completely missing.
-
HIV
(Human
Immunodeficiency Virus)
- the virus that is responsible for causing AIDS.
- homologous
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.
- homozygous
- 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.
- 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.
- hybrid vigor
- see heterosis.
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- I -
- inbreeding
- inbreeding depression
- The reduction in viability and subsequent loss of reproductive potential of purebred varieties. This is a consequence of a pattern of consanguineous mating that goes on for many generations in a family line or a small inbred population.
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- J
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- K -
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- L -
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- M -
- macroevolution
- major evolutionary changes in a population's gene pool, occurring over many generations, resulting in the evolution of new species. See microevolution.
-
malaria
- a group of similar life threatening diseases found mostly in tropical and subtropical regions of the world. Malaria is caused by any of four different microorganisms called plasmodia (Plasmodium falciparum, vivax, ovale, and malariae). These single-celled organisms are transmitted from person to person by anopheles mosquitoes as they extract blood. Symptoms of malaria include chills, high fever, and sometimes fatal irregularities of the brain, liver, kidney, and/or blood. There are at least 100,000,000 new cases of malaria reported annually around the world. Approximately 1,500,000 people die from it each year (mostly children--3,000 children die of malaria every day).
-
meiosis
-
microcephaly
- an extremely rare genetically inherited condition in which babies are born with extremely small brains and heads. This fatal condition has been found only among the children of 23 Old Order Amish families in Lancaster County Pennsylvania.
-
microevolution
- small changes in a population's gene pool occurring over a few generations. The accumulation of microevolutionary changes can result in macroevolution.
- mitosis
- 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.
- mutagen
- mutation
- 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.
the cell division process in specialized tissues of female ovaries and male testes which results in the production of sex cells, or gametes. Meiosis involves two divisions and produces four sperm cells in males and one ovum in females from a starting cell. Each sperm and ovum contains only half the original number of chromosomes--23 in the case of humans. Subsequently, meiosis is also called "reduction division." Spermatogenesis is the term used for meiosis in males and oögenesis refers to the same process in females. See mitosis.
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- N -
- natural selection
- an evolutionary mechanism that occurs when some individuals of a population are better able to adapt to their environment and, subsequently, produce more offspring. Nature, in effect, selects which members of a population are fit to survive long enough to reproduce. Differential productive success between individuals is the key. Those who produce more offspring have a greater influence on the gene frequencies of the next generation. This mechanism of evolutionary change was first articulated by Charles Darwin.
-
negative
assortative mating
- a form of non-random mating in which individuals who are not genetically alike for particular traits mate and those who are alike do not. The result is a progressive increase in the frequency of heterozygotes and a decrease in the homozygotes for the discriminated traits. See positive assortative mating and random mating.
-
non-random
mating
- mate selection based on one or more traits that are discriminated for or against. This is a mechanism of evolution. See positive assortative, negative assortative, and random mating.
- nucleic
acids
- the largest type of molecule in living organisms. It is composed of a chain of nucleotides that code for the synthesis of specific proteins. DNA and RNA are types of nucleic acid.
- nucleotide
- the basic building block of nucleic acid. It consists of any one of four specific purine or pyrimidine bases attached to a ribose or deoxyribose sugar and phosphate group.
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- O -
- ovum
(plural
ova
)
- a female sex cell or gamete.
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- P -
- paleontology
- the field of geology that studies the fossil record of ancient life forms.
-
paradox
- a statement that is seemingly contradictory or opposed to common sense and yet is true.
-
phenotype
- the observable or detectable characteristics of an individual organism. The detectable expression of a genotype.
-
phyletic
gradualism
- a theoretical model of evolution in which species change slowly at a more or less constant rate through time into other species. See punctuated equilibrium.
- point mutation
-
polygenic
trait
- 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.
- polymorphism
- a genetic trait controlled by more than one allele, each of which has a frequency of 1% or greater in the population gene pool. See balanced polymorphism.
- population
- a more or less distinct group of individuals within a species who are reproductively isolated from other groups. In other words, they restrict their mate selection to members of their own population. This is usually due to geographic and/or social barriers to mating with outsiders. Members of a completely isolated small population tend to have similar genetic characteristics due to generations of inbreeding.
- population genetics
- the study of biological inheritance patterns and changing gene pool frequencies in populations largely through the determination of allele frequencies. Population geneticists also identify processes resulting in evolution. See synthetic theory of evolution.
-
positive
assortative mating
- a form of non-random mating in which individuals who are alike for particular traits mate and those who are not alike do not. The result is a progressive increase in the frequency of homozygotes and a decrease in the heterozygotes for the discriminated traits. See negative assortative mating, consanguineous mating, and random mating.
- proteins
- 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
-
punctuated
equilibrium
- a theoretical model of evolution in which species remain unchanged for long periods of time and then at times rapidly change as a result of major alterations in the environment and, subsequently, in natural selection. See phyletic gradualism.
-
-
Punnett square
-
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.
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