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
macromolecule
A large polymer such as DNA, RNA, protein, lipid or polysaccharide.
major histocompatibility complex (MHC)
A group of highly polymorphic genes whose products appear on the surface of cells imparting the property of self (belonging to that organism). A genetic region found in all mammals whose products are primarily responsible for the rapid rejection of tissue grafts between indiviuals, and function in signalling between lymphocytes and cells expressing antigen.
Manx
Tailless phenotype in cats, caused by an autosomal dominant mutation that is lethal when homozygous.
map unit (m.u.)
The distance between two linked gene pairs where 1 percent of the products of meiosis are recombinant. A unit of distance in a linkage map. The distance equal to 1% recombination between two loci.
mapping
The study of the position of genes on chromosomes.
mapping function
The mathematical relationship between measured map distance and actual recombination frequency. A formula expressing the relationship between distance in a linkage map and recombinant frequency.
Marfan's syndrome
A human disorder of the connective tissue expressed as a range of symptoms, including very long limbs and digits, and heart defects; inherited as an autosomal dominant phenotype. Mutations in a gene called fibrillin.
marker
Any genetic element (locus, allele, DNA sequence or chromosome feature) which can be readily detected by phenotype, cytological or molecular techniques, and used to follow a chromosome or chromosomal segment during genetic analysis. See centromere marker; chromosome marker; DNA marker; genetic marker; inside marker; outside marker.
marker retention
A technique used in yeast to test the degree of linkage between two mitochondrial mutations.
mate-killer
A phenotype of Paramecium induced by intracellular bacteria-like mu particles.
maternal effect
The effect of the maternal parent's genotype on the phenotype of her offspring.
maternal effect lethal
A recessive mutation that is viable in zygotes, but in which homozygous mutant mothers produce inviable offspring.
maternal inheritance
A type of uniparental inheritance in which all progeny have the genotype and phenotype of the female parent.
mating type
In many species of microorganisms individuals can be divided into two types. Mating can take place only between individuals of opposite mating types due to the interaction of cell surface components. The equivalent in lower organisms of the sexes in higher organisms; the mating types typically differ only physiologically and not in physical form.
matroclinous inheritance
Inheritance in which all offspring have the phenotype of the mother.
mean
The arithmetic average; the sum of the data divided by the sample size.
mean fitness of the population W(bar)
The sum of the fitnesses of the genotypes of a population weighted by their proportions; hence a weighted mean fitness.
medium
Any material on (or in) which experimental cultures are grown.
meiocyte
Cell in which meiosis occurs.
meiosis
Two successive nuclear divisions (with corresponding cell divisions) that produce haploid gametes (in animals) or haploid sexual spores (in plants and fungi) having one-half of the genetic material of the original cell. The nuclear and cell division process in diploid eukaryotes that results in four haploid gametes or spores having one member of each original pair of homologous chromosomes only per nucleus.
meiospore
Cell that is one of the products of meiosis in plants.
melting of DNA
Denaturation of duplex DNA by heat or increased pH leading to strand separation.
Mendel's first law
The two members of a gene pair segregate from each other during meiosis; each gamete has an equal probability of obtaining either member of the gene pair.
Mendel's second law
The law of independent assortment; unlinked or distantly linked segregating gene pairs assort independently at meiosis.
Mendelian ratio
A ratio of progeny phenotypes reflecting the operation of Mendel's laws.
merozygote
A bacterial cell having a second copy of a particular chromosomal region in the form of an exogenote. A partially diploid Escherichia coli cell formed from a complete chromosome (the endogenote) plus a fragment (the exogenote).
messenger RNA (mRNA)
An RNA molecule transcribed from the DNA of a gene, and from which a protein is translated by the action of ribosomes. The basic function of the nucleotide sequence of mRNA is to determine the amino acid sequence in proteins.
Met
Methionine (an amino acid).
metabolism
The chemical reactions that occur in a living cell.
metacentric chromosome
A chromosome having its centromere in the middle. See also acrocentric chromosome
metafemale
A fruit fly with an X chromosome:Autosome ratio greater than unity.
metagon
An RNA necessary for the maintenance of mu particles in Paramecium.
metamale
A fruit fly with X chromosome:Autosome ratio below 0.5.
metamere
Segmental repeat unit in higher animals.
metaphase
The stage of mitosis or meiosis in which spindle fibers are attached to kinetochores and the chromosomes are positioned in the equatorial plane of the cell. An intermediate stage of nuclear division when chromosomes align along the equatorial plane of the cell spindle.
metaphase plate
The plane of the equator of the spindle into which chromosomes are positioned during metaphase.
metastasis
The migration of cancerous cells to other parts of the body.
methylation
Modification of a molecule by the addition of a methyl group.
microtubules
Hollow cylinders made of the protein tubulin that form, among other things, the spindle fibers.
midparent value
The mean of the values of a quantitative phenotype for two specific parents.
migration
Movement of individuals between otherwise reproductively isolated populations.
mimicry
A phenomenon in which an individual; gains an advantage by looking like the individuals of a different species.
minimal medium
A culture medium for microorganisms that contains the minimal necessities for growth of the wild-type. A medium containing only inorganic salts, a carbon source, and water.
mismatch repair
A form of excision repair initiated at the sites of mismatched bases in DNA.
missense mutation
mutations that at change a codon for one amino acid into a codon for a different amino acid.
mitochondrial cytopathy
Human disorder caused by point mutation or deletion in mitochondrial DNA; inherited maternally.
mitochondrion
A eukaryotic organelle that is the site of ATP synthesis and of the citric acid cycle. The eukaryotic cellular organelle in which the Krebs cycle and electron transport reactions take place.
mitosis
The nuclear division producing two daughter nuclei identical to the original nucleus. A type of nuclear division that produces two daughter nuclei identical to the parent nucleus normally just prior to cell division. See endomitosis, cytokinesis.
mitotic crossover
A crossover resulting from the pairing of homologous chromosomes in a diploid during mitosis.
mixed codon family
Group of four codons sharing their first two bases but coding for more than one amino acid.
mobile genetic element
See transposable genetic element.
mode
The single class in a statistical distribution having the greatest frequency.
modifier gene
A gene that affects the phenotypic expression of another gene.
molecular chaperone
A protein that aids in the folding of a second protein. The chaperone prevents proteins from taking conformations that would be inactive.
molecular evolutionary clock
A measurement of evolutionary time in nucleotide substitutions per year.
molecular genetics
The study of the molecular processes underlying gene structure and function.
molecular imprinting
The phenomenon in which there is differential expression of a gene depending on whether it was maternally or paternally inherited. Paternal imprinting means that an allele inherited from the father is not expressed in offspring. Maternal imprinting means that an allele inherited from the mother is not expressed in offspring.
monocistronic mRNA
An mRNA that codes for only one protein.
monoclonal antibody
The unique immunoglobulin molecule (antibody) produced by a clone of cells derived from a fusion of a B lymphocyte with a myeloma cell.
monoecious plant
A plant species in which male and female organs are found on the same plant but in different flowers (for example maize).
monohybrid cross
A cross between two individuals identically heterozygous at one gene pair for example, Aa x Aa.
monohybrid
Offspring of parents that differ in only one genetic characteristic. Usually implies heterozygosity at a single locus under study.
monoploid
A cell having only one chromosome set (usually as an aberration) or an organism composed of such cells.
monosomic
A diploid cell missing a single chromosome. A cell or individual that is basically diploid but that has only one copy of one particular chromosome type and thus has chromosome number 2n - 1.
morphological species concept
Organisms are classified in the same species if they appear identical by morphological (anatomical) criteria.
mosaic
A chimera; a tissue containing two or more genetically distinct cell types, or an individual composed of such tissues. Individual made up of two or more genetically distinct cell lines.
mtDNA
Mitochondrial DNA.
mu particle
Bacteria-like particle found in the cytoplasm of Paramecium that have the mate-killer phenotype.
mu phage
A kind (species) of phage with properties similar to those of insertion sequences, being able to insert, transpose, and cause chromosome rearrangements and mutations.
Mullerian mimicry
A form of mimicry in which noxious species evolve to resemble each other.
multihybrid
An organism heterozygous at numerous loci.
multimeric structure
A structure composed of several identical or different subunits held together by weak bonds.
multinomial expansion
The terms generated when a multinomial is raised to a power.
multiple allelism
The existence of several known alleles of a gene.
multiple-factor hypothesis
A hypothesis to explain quantitative variation by assuming the interaction of a large number of genes (polygenes) each with a small additive effect on the character.
multiple-hit hypothesis
The proposal that a single cell must receive a series of mutational events in order to become malignant or cancerous.
multiplicity of infection (moi)
The average number of phage particles that infect a single bacterial cell in a specific experiment.
mutability
The ability to change.
mutagen
An agent that is capable of increasing the mutation rate .
mutant
An organism or cell carrying a mutation. An alternative phenotype to the wild-type; the phenotype produced by a non-wildtype allele.
mutant allele
An allele differing from the allele found in the standard or wild type organism.
mutant screening
The process of screening for mutations showing abnormalities in a certain structure or in a particular function, as a preparation for genetic analysis of that strucutre or function.
mutant site
The damaged or altered area within a mutated gene.
mutation
(1) The process producing a gene or a chromosome differing from the wild-type.
(2) The gene or chromosome that results from such a process.
mutation breeding
Use of mutagens to develop variants that can increase agricultural yield.
mutation event
The actual occurrence of a mutation in time and space.
mutation frequency
The frequency of mutations in a population.
mutation rate
The number of mutation events per gene per unit of time (for example, per cell generation). The proportion of mutations per cell division in bacteria or single-celled organisms or the proportion of mutations per gamete in higher organisms.
mutational dissection
The study of the components of a biological system through a study of mutations affecting that system.
mutational load
Genetic load caused by mutation.
mutator mutation
mutation of DNA polymerase that increases the overall mutation rate.
muton
A term coined by Benzer for the smallest mutable site within a cistron. The smallest part of a gene that can be involved in a mutation event; now known to be a single nucleotide pair.

myeloma
A cancer of the bone marrow.