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Modern Genetic Analysis Glossary
N
nanometer 109meters.
narrow heritability See hereitability in the narrow sense.
negative assortative mating Preferential mating between phenotypically unlike partners.
negative control Regulation mediated by factors that block or turn off transcription.
neurofibromatosis A human disease with tumors of nerve cells and café au lait spots, both in the skin. The allele generally arises from germinal mutation, but it is inherited as an autosomal dominant.
Neurospora A pink mold, commonly found growing on old food.
neutral mutation (1) A mutation that has no effect on the Darwinian fitness of its carriers. (2) A mutation that has no phenotypic effect.
neutral petite A petite that produces all wild-type progeny when crossed with wild type.
neutrality See selective neutrality.
nicking Nuclease action to sever the sugar-phosphate backbone in one DNA strand at one specific site.
nitrocellulose filter A type of filter used to hold DNA for hybridization.
nitrogen bases Types of molecules that form important parts of nucleic acids, composed of nitrogen-containing ring structures; hydrogen bonds between bases link the two strands of a DNA double helix.
nonautonomous See autonomous phenotype.
nondisjunction The failure of homologs (at meiosis) or sister chromatids (at mitosis) to separate properly to opposite poles.
nonlinear tetrad A tetrad in which the meiotic products are in no particular order.
non-Mendelian ratio An unusual ratio of progeny phenotypes that does not conform to the simple operation of Mendel's laws; for example, mutant:wild ratios of 3:5, 5:3, 6:2, or 2:6 in tetrads indicate that gene conversion has occurred.
nonparental ditype (NPD) A tetrad type containing two different genotypes, both of which are recombinant.
nonsense codon A codon for which no formal tRNA molecule exists; the presence of a nonsense codon causes termination of translation (ending of the polypeptide chain). The three nonsense codons are called amber, ocher, and opal.
nonsense mutation A mutation that alters a gene and produces a nonsense codon.
nonsense suppressor A mutation that produces an altered tRNA that will insert an amino acid in translation in response to a nonsense codon.
nonsynonymous substitution Mutational replacement of an amino acid with one of different chemical properties.
norm of reaction The pattern of phenotypes produced by a given genotype under different environmental conditions.
Northern blot Transfer of electrophoretically separated RNA molecules from a gel onto an absorbent sheet, which is then immersed in a labeled probe that will bind to the RNA of interest.
NPD See nonparental ditype.
nu body See nucleosome.
nuclease An enzyme that can degrade DNA by breaking its phosphodiester bonds.
nucleoid A DNA mass within a chloroplast or mitochondrion.
nucleolar organizer A region (or regions) of the chromosome set physically associated with the nucleolus and containing rRNA genes.
nucleolus An organelle found in the nucleus, containing rRNA and amplified multiple copies of the genes encoding rRNA.
nucleoside A nitrogen base bound to a sugar molecule.
nucleosome A nu body; the basic unit of eukaryotic chromosome structure; a ball of eight histone molecules wrapped about by two coils of DNA.
nucleotide A molecule composed of a nitrogen base, a sugar, and a phosphate group; the basic building block of nucleic acids.
nucleotide pair A pair of nucleotides (one in each strand of DNA) that are joined by hydrogen bonds.
nucleotide-pair substitution The replacement of a specific nucelotide pair by a different pair; often mutagenic.
null allele An allele whose effect is either an absence of normal gene product at the molecular level or an absence of normal function at the phenotypic level.
null mutation A mutation that results in complete absence of function for the gene.
nullisomic A cell or individual with one chromosomal type missing, with a chromosome number such as n 1 or 2n 2.
nurse cells The sister cells of the oocyte in insects. The nurse cells produce the bulk of the cytoplasmic contents of the mature oocyte.

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