4.5: Key Terms

5-alpha reductase deficiency: An autosomal recessive syndrome that manifests when a child having both X and Y sex chromosomes inherits two non-functional (mutated) copies of the SRD5A2 gene, producing a deficiency in a hormone necessary for development in infancy of typical male genitalia. These children often appear at birth to have female genitalia, but they develop a penis and other sexual characteristics when other hormones kick in during puberty.

Adaptive radiation: The situation in which subgroups of a single species rapidly diversify and adapt to fill a variety of ecological niches.

Admixture: A term often used to describe gene flow between human populations. Sometimes also used to describe gene flow between non-human populations.

Africanized honey bees: A strain of honey bees that resulted from the hybridization of African and European honey bee subspecies. These bees were accidentally released into the wild in 1957 in Brazil, and have since spread throughout South and Central America and into the United States. Also known as “killer bees,” they tend to be very aggressive in defense of their hives and have caused many fatal injuries to humans and livestock.

Allele frequency: The ratio, or percentage, of one allele compared to the other alleles for that gene within the study population.

Alleles: Variant forms of genes.

Allopatric speciation: Speciation caused by long-term isolation (physical separation) of subgroups of the population.

Antibiotics: Medicines prescribed to treat bacterial infections.

Artificial selection: Human-directed assortative mating among domestic animals, such as pets and livestock, designed to increase the chances of offspring having certain desirable traits.

Asexual reproduction: Reproduction via mitosis, whereby offspring are clones of the parents.

Autosomal dominant: A phenotype produced by a gene on an autosomal chromosome that is expressed, to the exclusion of the recessive phenotype, in heterozygotes.

Autosomal recessive: A phenotype produced by a gene on an autosomal chromosome that is expressed only in individuals homozygous for the recessive allele.

Balanced translocations: Chromosomal translocations in which the genes are swapped, but no genetic information is lost.

Balancing selection: A pattern of natural selection that occurs when the extremes of a trait are selected against, favoring the intermediate phenotype (a.k.a., stabilizing selection).

Beneficial mutations: Mutations that produce some sort of an advantage to the individual.

Benign: Non-cancerous. Benign tumors may cause problems due to the area in which they are located (e.g., they might put pressure on a nerve or brain area), but they will not release cells that aggressively spread to other areas of the body.

Biometricians: A group of early biological scientists who believed that individual mutations of discrete hereditary units could never account for the continuous spectrum of variation seen in many traits.

Café-au-lait spots (CALS): Flat, brown birthmark-like spots on the skin, commonly associated with Neurofibromatosis Type 1.

Chromosomal translocations: The transfer of DNA between non-homologous chromosomes.

Chromosomes: Molecules that carry collections of genes.

Codons: Three-nucleotide units of DNA that function as three-letter “words,” encoding instructions for the addition of one amino acid to a protein or indicating that the protein is complete.

Cretaceous–Paleogene extinction: A mass disaster caused by an asteroid that struck the earth approximately 66 million years ago and killed 75% of life on Earth, including all terrestrial dinosaurs. (a.k.a., K-Pg Extinction, Cretatious-Tertiary Extinction and K-T Extinction).

Crossover events: Chromosomal alterations that occur when DNA is swapped between homologous chromosomes while they are paired up during meiosis I.

Cutaneous neurofibromas: Neurofibromas that manifest as spherical bumps on or just under the surface of the skin.

Cytosine methylation: A type of point mutation in which a cytosine nucleotide is converted to a thymine. A methyl group is added to a cytosine base, changing it to 5-methyl cytosine, which further converts to thymine after hydrolytic deamination (water-induced removal of an amine group).

Deleterious mutation: A mutation producing negative effects to the individual such as the beginnings of cancers or heritable disorders.

Deletions: Mutations that involve removal of one or more nucleotides from a DNA sequence.

Derivative chromosomes: New chromosomal structures resulting from translocations.

Dictyostelium discoideum: A species of social amoebae that has been widely used for laboratory research. Laboratory strains of Dictyostelium discoideum all carry mutations in the NF1 gene, which is what allows them to survive on liquid media (agar) in Petri dishes.

Directional selection: A pattern of natural selection in which one phenotype is favored over the other, causing the frequencies of the associated advantageous alleles to gradually increase.

Disruptive selection: A pattern of natural selection that occurs when both extremes of a trait are advantageous and intermediate phenotypes are selected against (a.k.a., diversifying selection).

DNA repair mechanisms: Enzymes that patrol and repair DNA in living cells.

DNA transposons: Transposons that are clipped out of the DNA sequence itself and inserted elsewhere in the genome.

Ecological niche: A set of constraints and resources that are available in an environmental setting.

Ellis-van Creveld syndrome: An autosomal recessive disorder characterized by short stature (dwarfism), polydactyly [the development of more than five digits (fingers or toes) on the hands or feet], abnormal tooth development, and heart defects. Estimated to affect approximately one in 60,000 individuals worldwide, among the Old Order Amish of Lancaster County, the rate is estimated to be as high as one in every 200 births.

Evolution: A change in the allele frequencies in a population over time.

Exons: The DNA sequences within a gene that directly encode protein sequences. After being transcribed into messenger RNA, the introns are clipped out, and the exons are pasted together prior to translation.

Fertile offspring: Offspring that can reproduce successfully to have offspring of their own.

Founder effect: A type of genetic drift that occurs when members of a population leave the main or “parent” group and form a new population that no longer interbreeds with the other members of the original group.

Frameshift mutations: Types of indels that involve the insertion or deletion of any number of nucleotides that is not a multiple of three. These “shift the reading frame” and cause all codons beyond the mutation to be misread.

Gametes: The reproductive cells, produced through meiosis (a.k.a., germ cells or sperm or egg cells).

Gene: A sequence of DNA that provides coding information for the construction of proteins.

Gene flow: The movement of alleles from one population to another. This is one of the forces of evolution.

Gene pool: The entire collection of genetic material in a breeding community that can be passed on from one generation to the next.

Genetic drift: Random changes in allele frequencies within a population from one generation to the next. This is one of the forces of evolution.

Genotype: The set of alleles that an individual has for a given gene.

Genotype frequencies: The ratios or percentages of the different homozygous and heterozygous genotypes in the population.

Guevedoces: The term coined locally in the Dominican Republic for the condition scientifically known as 5-alpha reductase deficiency. The literal translation is “penis at twelve.”

Hardy-Weinberg Equilibrium: A mathematical formula (p² + 2pq + q² ) that allows estimation of the number and distribution of dominant and recessive alleles in a population.

Harlequin ladybeetle: A species of ladybeetle, native to East Asia, that was introduced to Europe and the Americas as a form of pest control. After many decades of use, one of the North American strains developed the ability to reproduce in diverse environments, causing it to spread rapidly throughout the Americas, Europe, and Africa. It has hybridized with European strains and is now a major pest in its own right.

Heterozygous genotype: A genotype comprising two different alleles.

Homozygous genotype: A genotype comprising an identical set of alleles.

Hybridization: A term often used to describe gene flow between non-human populations.

Inbreeding: The selection of mates exclusively from within a small, closed population.

Indels: A class of mutations that includes both insertions and deletions.

Inherited mutation: A mutation that has been passed from parent to offspring.

Insertions: Mutations that involve addition of one or more nucleotides into a DNA sequence.

Introns: DNA sequences within a gene that do not directly encode protein sequences. After being transcribed into messenger RNA, the introns are clipped out, and the exons are pasted together prior to translation.

Isolation: Prevention of a population subgroup from breeding with other members of the same species due to a physical barrier or (in humans) a cultural rule.

Lamarckian inheritance: An early model for inheritance that predicted that offspring inherit characteristics acquired during their parents’ lifetimes. This has now been proven incorrect.

Macroevolution: Changes that result in the emergence of new species, the similarities and differences between species, and their phylogenetic relationships with other taxa.

Malaria: A frequently deadly mosquito-borne disease caused by infection of the blood by a Plasmodium parasite.

Malignant: Cancerous. Malignant tumors grow aggressively and their cells may metastasize (travel through the blood or lymph systems) to form new, aggressive tumors in other areas of the body.

Microevolution: Changes in allele frequencies within breeding populations—that is, within a single species.

Missense mutation: A point mutation that produces a change in a single amino acid.

Modern Synthesis: The integration of Darwin’s, Mendel’s, and subsequent research into a unified theory of evolution.

Monosomies: Conditions resulting from a non-disjunction event, in which a cell ends up with only one copy of a chromosome. In humans, a single X chromosome is the only survivable monosomy.

Mutation: A change in the nucleotide sequence of the genetic code. This is one of the forces of evolution.

Mutationists: A group of early biological scientists who believed that variation was caused by mutations in distinct, inherited cells.

Natural selection: An evolutionary process that occurs when certain phenotypes confer an advantage or disadvantage in survival and/or reproductive success. This is one of the forces of evolution.

Negative assortative mating: A pattern that occurs when individuals tend to select mates with qualities different from their own.

Neurofibromas: Nerve sheath tumors that are common symptoms of Neurofibromatosis Type 1.

Neurofibromatosis Type 1: An autosomal dominant genetic disorder affecting one in every 3,000 people. It is caused by mutation of the NF1 gene on Chromosome 17, resulting in a defective neurofibromin protein. The disorder is characterized by neurofibromas, café-au-lait spots, and a host of other potential symptoms.

NF1: An abbreviation for Neurofibromatosis Type 1. When italicized, NF1 refers to the gene on Chromosome 17 that encodes the neurofibromin protein.

Nondisjunction events: Chromosomal abnormalities that occur when the homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II and mitosis) fail to separate after pairing. The result is that both chromosomes or chromatids end up in the same daughter cell, leaving the other daughter cell without any copy of that chromosome.

Non-random mating: A scenario in which mate choice within a population follows a non-random pattern (a.k.a., Assortative Mating).

Nonsense mutation: A point mutation that converts a codon that encodes an amino acid into a stop codon.

Non-synonymous mutation: A point mutation that causes a change in the resulting protein.

Old Order Amish: A culturally isolated population in Lancaster County, Pennsylvania, that has approximately 50,000 current members, all of whom can trace their ancestry back to a group of approximately eighty individuals. This group has high rates of certain genetics disorders including Ellis-van Creveld syndrome.

Origins of life: How the first living organism came into being.

Pangenesis: An early model for inheritance that combines the Lamarckian idea of inheriting acquired characteristics with the idea that particles from different parts of the body make their way to the sex cells. This has now been proven to be incorrect.

Peacock: The male sex of the peafowl, famous for its large, colorful tail, which it dramatically displays to attract mates. (The female of the species is known as a peahen.)

Peppered moth: A species of moth found in England that has light and dark phenotypes. During the Industrial Revolution, when soot blackened the trees, the frequency of the previously rare dark phenotype dramatically increased, as lighter-colored moths were easier for birds to spot against the sooty trees. After environmental regulations eliminated the soot, the lighter-colored phenotype gradually became most common again.

Phenotype: The observable traits that are produced by a genotype.

Phylogenetic tree of life: A family tree of all living organisms, based on genetic relationships.

Phylogenies: Genetically determined family lineages.

Plasmodium: A genus of mosquito-borne parasite. Several Plasmodium species cause malaria when introduced to the human bloodstream via a mosquito bite.

Plexiform neurofibromas: Neurofibromas that involve whole branches of nerves, often giving the appearance that the surface of the skin is “melting.”

Point mutation: A single-letter (single-nucleotide) change in the genetic code, resulting in the substitution of one nucleic acid base for a different one.

Polymorphisms: Multiple forms of a trait; alternative phenotypes within a given species.

Population: A group of individuals who are genetically similar enough and geographically near enough to one another that they can breed and produce new generations of individuals.

Population bottleneck: A type of genetic drift that occurs when the number of individuals in a population drops dramatically due to some random event.

Positive assortative mating: A pattern that results from a tendency for individuals to mate with others who share similar phenotypes.

Retrotransposons: Transposons that are transcribed from DNA into RNA, and then are “reverse transcribed,” to insert the copied sequence into a new location in the DNA.

Sexual reproduction: Reproduction via meiosis and combination of gametes. Offspring inherit genetic material from both parents.

Sexual selection: An aspect of natural selection in which the selective pressure specifically affects reproductive success (the ability to successfully breed and raise offspring).

Sickle cell anemia: An autosomal recessive genetic disorder that affects millions of people worldwide. It is most common in Africa, countries around the Mediterranean Sea, and eastward as far as India. Homozygotes for the recessive allele develop the disorder, which produce misshapen red blood cells that cause iron deficiency, painful episodes of oxygen-deprivation in localized tissues, and a host of other symptoms. In heterozygotes, though, the sickle cell allele confers a greater resistance to malaria.

Somatic cells: The cells of our organs and other body tissues (all cells except gametes) that replicate by mitosis.

Speciation: The process by which a single population divides into two or more separate species.

Species: Organisms whose individuals are capable of breeding because they are biologically and behaviorally compatible to produce viable, fertile offspring.

Splice site mutation: A mutation that changes the genetic code so that the correct area to be modified for mRNA splicing is not recognized by the appropriate enzymes.

Spontaneous mutation: A mutation that occurs due to random chance or unintentional exposure to mutagens. In families, a spontaneous mutation is the first case, as opposed to mutations that are inherited from parents.

Subspecies: A distinct subtype of a species. Most often, this is a geographically isolated population with unique phenotypes; however, it remains biologically and behaviorally capable of interbreeding with other populations of the same species.

Sympatric speciation: When a population splits into two or more separate species while remaining located together without a physical (or cultural) barrier.

Synonymous mutation: A point mutation that does not change the resulting protein.

Transposable elements: Fragments of DNA that can “jump” around in the genome.

Transposon: Another term for “transposable element.”

Trisomies: Conditions in which three copies of the same chromosome end up in a cell, resulting from a non-disjunction event. Down syndrome, Edwards syndrome, and Patau syndrome are trisomies.

Unbalanced translocations: Chromosomal translocations in which there is an unequal exchange of genetic material resulting in duplication or loss of genes.

Universal ancestor: The first living organism, from which all living things are descended.

UV crosslinking: A type of mutation in which adjacent thymine bases bind to one another in the presence of UV light.

Viable Offspring: Offspring that are healthy enough to survive to adulthood.

Xeroderma pigmentosum: An autosomal recessive disease in which DNA repair mechanisms do not function correctly, resulting in a host of problems, especially related to sun exposure, including severe sunburns, dry skin, heavy freckling, and other pigment changes.