Book iga

M

M phase The mitotic phase of the cell cycle.

Mb See megabase.

macromolecule A large polymer such as DNA, a protein, or a polysaccharide.

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.

mapping function A formula expressing the relation between distance in a linkage map and recombinant frequency.

Marfan syndrome A human disorder of the connective tissue expressed as a range of symptoms, including very long limbs and digits as well as heart defects; inherited as an autosomal dominant phenotype.

marker See genetic markers.

marker retention A technique used in yeast to test the degree of linkage between two mitochondrial mutations.

maternal effect The environmental influence of the mother's tissues on the phenotype of the offspring.

maternal effect lethal A mutation that is viable in zygotes, but mothers having the mutation produce inviable offspring.

maternal inheritance A type of uniparental inheritance in which all progeny have the genotype and phenotype of the parent acting as the female.

maternally expressed gene A gene that contributes to the phenotype of an offspring on the basis of its expression in the mother.

mating types 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 nucleus-determined phenotype of the mother.

mean The arithmetic average.

medium Any material on (or in) which experimental cultures are grown.

Megabase (Mb) One million nucleotide pairs.

meiocyte Cell in which meiosis takes place.

meiosis Two successive nuclear divisions (with corresponding cell divisions) that produce gametes (in animals) or sexual spores (in plants and fungi) having one-half of the genetic material of the original cell.

meiospore Cell that is one of the products of meiosis in plants.

melting Denaturation of DNA.

Mendelian ratio A ratio of progeny phenotypes in accord with the application of Mendel's laws.

Mendel's first law The two members of a gene pair segregate from each other in 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.

merozygote A partly diploid E. coli cell formed from a complete chromosome (the endogenote) plus a fragment (the exogenote).

messenger RNA See mRNA.

Met Methionine (an amino acid).

metabolism The chemical reactions that take place in a living cell.

metacentric chromosome A chromosome having its centromere in the middle.

metamere A segmental repeat unit in higher animals.

metaphase An intermediate stage of nuclear division when chromosomes align along the equatorial plane of the cell.

methylation Modification of a molecule by the addition of a methyl group.

microfilaments The smallest-diameter cable system of the cytoskeleton. Microfilament cables are composed of actin polymers.

microsatellite DNA A type of repetitive DNA based on very short repeats such as dinucleotides.

microtubule organizing center The part of the microtubule cytoskeleton in which all of the minus ends of the microtubules are clustered. Ordinarily, this cluster is near the center of the cell.

microtubules The largest-diameter cable system of the cytoskeleton. Microtubules are composed of polymerized tubulin subunits forming a hollow tube.

midparent value The mean of the values of a quantitative phenotype for two specific parents.

minimal medium A medium containing only inorganic salts, a carbon source, and water.

minisatellite DNA A type of repetitive DNA sequence based on short repeat sequences with a unique common core; used for DNA fingerprinting.

misexpression The occurrence of gene expression in a tissue in which the gene is normally not expressed. Such ectopic expression can be caused by the juxtaposition of novel enhancer elements to a gene.

missense mutation A mutation that alters a codon so that it encodes a different amino acid.

mitochondrial cytopathies Human disorders caused by point mutations or deletions in mitochondrial DNA; inherited maternally.

mitochondrion A eukaryotic organelle that is the site of ATP synthesis and of the citric acid cycle.

mitosis A type of nuclear division (occurring at cell division) that produces two daughter nuclei identical with the parent nucleus.

mitotic crossover A crossover resulting from the pairing of homologs in a mitotic diploid.

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 clock A constant rate of change in DNA or protein sequences as a consequence of random mutation and genetic drift of unselected molecular variants.

molecular genetics The study of the molecular processes underlying gene structure and function.

monocistronic mRNA An mRNA that encodes one protein.

monoecious plant A plant species in which male and female organs are found on the same plant but in different flowers (for example, corn).

monohybrid cross A cross between two individuals identically heterozygous at one gene pair

for example, A/a × A/a.

monoploid A cell having only one chromosome set (usually as an aberration) or an organism composed of such cells.

monosomic Refers to 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 [minus sign]

1.

morphogen A molecule that can induce the acquisition of different cell fates according to the level of morphogen to which a cell is exposed.

mosaic A chimera; a tissue containing two or more genetically distinct cell types or an individual composed of such tissues.

motor protein A protein that is able to move unidirectionally along a specific type of cytoskeletal cable. Kinesins and dyneins are microtubule-based and myosins are microfilament-based motor proteins. By attaching to other subcellular components, motor proteins are capable of directed movement of these components within the cell.

mRNA (messenger RNA) An RNA molecule transcribed from the DNA of a gene and from which a protein is translated by the action of ribosomes.

mtDNA Mitochondrial DNA.

m.u. See map unit.

mu phage A kind ("species") of phage with properties similar to those of insertion sequences, being able to insert, transpose, inactivate, and cause rearrangements.

multimeric structure A structure composed of several identical or different subunits held together by weak bonds.

multiple allelism The existence of several known alleles of a gene.

multiple cloning site See polylinker.

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 to become malignant or cancerous.

multiplicity of infection The average number of phage particles that infect a single bacterial cell in a specific experiment.

mutagen An agent that is capable of increasing the mutation rate.

mutant An organism or cell carrying a mutation.

mutant allele An allele differing from the allele found in the standard, or wild type.

mutant hunt The process of collecting different mutants showing abnormalities in a certain structure or in a certain function, as a preparation for mutational dissection of that function.

mutant site The damaged or altered area within a mutated gene.

mutation (1) The process that produces a gene or a chromosome set differing from the wild type. (2) The gene or chromosome set 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 mutants in a population.

mutation rate The number of mutation events per gene per unit of time (for example, per cell generation).

mutational dissection The study of the components of a biological function through a study of mutations affecting that function.

muton The smallest part of a gene that can take part in a mutation event; now known to be a nucleotide pair.

myeloma A cancer of the bone marrow.