Genetics: Key Terms and Glossary

One alternative form of a gene.

The share of all copies of a gene, in a stated population, that are one particular allele.

A polyploid whose chromosome sets came from different species, so that doubling can give each chromosome a partner to pair with.

A bacterial reversion assay of whether a chemical increases mutation, sometimes after a mammalian liver extract has metabolised it. More revertants show mutagenicity under those conditions, not a cancer diagnosis in a person, and a negative result is not proof of safety.

Appearance of a phenotype earlier or more severely in later generations, used here as a genetic sign of an expanding trinucleotide repeat.

A polyploid whose extra chromosome sets all come from one species, so more than two homologues may pair.

A pedigree pattern, for a rare fully penetrant allele on an autosome, of affected people of both sexes in successive generations, including transmission from father to son.

A pedigree pattern of affected siblings born to unaffected carriers, more suggestive when the parents are related.

The compact inactive X chromosome in a mammalian cell, normally one fewer than the cell’s number of X chromosomes.

The older picture in which parental contributions mix for good and cannot be separated again, which would erode heritable differences.

An epistatic failure to make H, so the ABO transferases have no substrate and the red cells type as group O.

In mitochondrial inheritance, a sampling step that lets few genome copies found an egg lineage, so children of one mother can differ sharply; in population genetics, a crash in numbers whose allele sample later growth does not refill.

The heritable effect of an individual’s alleles on a quantitative trait, as distinct from dominance or interaction deviations that are not passed on intact.

The orange-and-black mosaic of a cat heterozygous for an X-linked colour allele, because different cell clones inactivated different X chromosomes. A male with that mosaic is a clue to a second X, not to a separate colour locus.

The frequency of heterozygotes for a recessive allele, about twice the allele frequency when a rare autosomal recessive sits near the null equilibrium.

A map distance of 1% recombinant progeny over a short interval, not a fixed number of base pairs.

The goodness-of-fit comparison, as used in these notes, of observed class counts with the counts predicted by a ratio chosen in advance.

The placement of Mendelian factors on chromosomes, so that segregation follows the separation of homologues.

The unit of function defined when two mutations fail to complement in trans; it matches one polypeptide and can still be split by recombination.

A comparison of two mutations opposite one another, one on each genome, with both mutations on the same genome, to see whether they damage the same function.

Joint detection of both allele products in the heterozygote, rather than a diluted form of one parent.

Observed double crossovers divided by the number expected if exchanges in the two intervals were independent.

Presence of the same trait in both members of a twin pair.

A Mendelian probability revised by conditioning on facts already observed, such as an unaffected child of two carriers. The figure is a probability under stated assumptions, not advice.

A mating of relatives, which raises the chance that a child receives two copies of one ancestral allele.

Uptake of two markers on one piece of naked DNA, which grows rare as the markers lie farther apart.

The input arrangement in which two dominant alleles sit on the same homologue.

The X-linked pattern in which sons receive their X-linked phenotype from the mother and daughters receive theirs from the father.

An exchange between one chromatid of each homologue that yields recombinant chromatids.

Right-handed snail coiling, genetically dominant in the maternal-effect example but displayed according to the mother’s genotype.

An allele whose product spoils the wild-type product, so the heterozygote is worse off than loss of one copy alone would make it.

Two exchanges between outer markers, which put those outer markers back into the parental combination and so hide both exchanges from a two-point count.

Random change in allele frequency from sampling in a finite population, stronger when few individuals breed.

Masking of the phenotypic effect of one gene by the genotype at another, while both genes continue to segregate.

The requirement, if a monozygotic–dizygotic difference in concordance is to be read as genetic, that the two kinds of twins share environments equally.

How far, or in what form, a phenotype shows among those who show it at all.

The frequency pattern in Finland in which some recessive alleles rare elsewhere are unusually common, and some alleles common elsewhere in Europe are comparatively rare. These notes treat that pattern as founder sampling and isolation, not as a judgement of character.

The reproductive success of a genotype relative to other genotypes at the same locus.

An allele whose raised frequency traces to its presence in a small founding group, not to a demonstrated advantage.

The allele sample carried by the few individuals who start a population, which later growth does not replace. Later commonness is not evidence of advantage, and the human illustrations are not stereotypes of a group.

An insertion or deletion whose length is not a multiple of three, so the reading frame downstream is altered.

A Giemsa banding pattern used as a map of positions along metaphase chromosomes.

Transfer of bacterial DNA from any region of the chromosome by a phage particle that packaged host DNA instead of phage DNA.

The alleles an individual carries at the loci under discussion, whether or not they are visible.

Sex determination in which fertilised eggs develop as diploid females and unfertilised eggs as haploid males.

Having a single chromosome set, so that in the bacteria of these notes a new loss of function is expressed at once.

Dominance of a loss-of-function allele because one wild-type copy does not provide enough product.

A null model in which random mating, with selection, mutation, migration, and drift all absent, sets genotype frequencies from allele frequencies alone. It is the expectation under those assumptions, not a claim that a real population meets them.

Having a single copy of a locus, as with an X-linked gene in a male whose Y lacks it, so that copy determines the phenotype.

More than one mitochondrial genotype present in the same individual.

Maintenance of both alleles because heterozygotes leave more offspring than either homozygote, as where severe falciparum malaria favours sickle-cell heterozygotes and not where that advantage is absent.

The relation between two allele copies that both descend from one allele in a recent common ancestor.

The probability that the two alleles at a locus in an individual are identical by descent, which adds homozygotes without changing allele frequency.

A relationship in which the heterozygote is intermediate, so phenotype distinguishes the same classes as genotype.

Distribution of alleles of different genes into gametes independently, when the genes are unlinked or effectively so.

The departure of double crossovers from independence, scored as one minus the coefficient of coincidence.

The chromosome set arranged by size, centromere position, and bands.

An allele that removes a genotypic class after it has been formed, so the ratio among survivors is not the ratio at conception.

Recovery together of alleles that entered on the same chromosome, which is a failure of independent assortment.

* The snail in which shell coiling is controlled by a nuclear gene in the mother, not by an organelle she passes on.

The account that one X in each XX mammalian cell is inactivated at random early on, and that mitotic descendants keep that choice.

A heterozygote for an X-linked allele who shows the phenotype because enough relevant cells inactivated the wild-type X.

A phenotype set by products the mother places in the egg, so the young report her genotype and their own genotype shows in the next generation.

Transmission through cytoplasm from the egg, so an organelle trait can reach children of both sexes from the mother and, in ordinary pedigrees, not from the father.

A partial diploid in which a second copy is present for only some genes, supplied in bacteria by an F′ or a specialised transducing phage.

Movement of alleles from one population into another, which shifts the receiving frequency towards the source.

* The four o’clock in which branch colour follows the seed parent, the plastid case that is separate from nuclear inheritance of its flower colour.

A codon change that specifies a different amino acid.

A pedigree in which an affected mother can transmit to children of both sexes, an affected father does not, and heteroplasmy can leave some children unaffected or less severely affected.

The presence of more than two alleles of one gene in a population, although any one diploid still carries only two.

The allele frequency at which new copies added by mutation are offset by copies removed by selection.

The ratio of additive genetic variance to phenotypic variance in one population and one range of environments. It is not the fraction of one person’s trait caused by genes, and it does not cap a shift of the mean in a new environment.

The wheat-colour analysis in which several additive loci still segregate, while kernel colour is graded by the number of contributing alleles.

Failure of chromosomes to disjoin, in meiosis I, meiosis II, or mitosis.

A codon change that creates a stop codon.

Of an inversion that leaves the centromere outside the reversed segment, so a crossover in the pairing loop yields a dicentric bridge and an acentric fragment.

The proportion of individuals of a stated genotype who show the phenotype at all.

Of an inversion that includes the centromere, so crossover products stay monocentric but each carries a duplication and a deletion.

An environmentally produced phenotype that mimics a genetic one in someone who lacks that genotype.

The character that is observed, which need not reveal every allele that is transmitted.

Several effects of one gene, rather than an interaction between two genes.

Influenced by many loci, each usually of small effect, plus environment, so the trait forms a curve instead of a few discrete classes.

The person through whom a family is brought to attention on a pedigree.

The rule that the probability of independent events occurring together is the product of their separate probabilities.

Belonging to the shared tips of X and Y, which recombine, so a father can pass those alleles to a daughter.

The dominant-looking pedigree produced when someone homozygous for a recessive allele has children with a carrier, so about half the children are affected.

A meiotic association of four chromosomes, either four homologues of an autopolyploid or the cross-shaped figure of a reciprocal translocation.

The input arrangement in which two dominant alleles sit on opposite homologues.

The shift from the original population mean to the offspring mean, predicted for one generation, in the same environment, as narrow-sense heritability times the selection differential.

A fusion of the long arms of two acrocentric chromosomes; a balanced carrier keeps that long-arm material in fewer chromosomes and can still make unbalanced gametes.

Separation of the two alleles of a diploid so that each gamete receives one of them.

The difference between the mean of the parents who are chosen to breed and the mean of the population they were chosen from.

Reduced fertility of a translocation heterozygote because unbalanced segregation yields gametes that fail.

Expressed in both sexes, usually from an autosome, but with different dominance or intensity in each sex.

Expressed in only one sex, usually from an autosome, so the sex that does not show the trait can still transmit it.

Left-handed snail coiling, recessive to dextral in the maternal-effect example and likewise expressed a generation after the genotype that sets it.

Phage-mediated transfer restricted to bacterial genes beside a prophage attachment site, after an incorrect excision.

A change at an intron boundary that alters how the transcript is cut and joined.

The rule that the probability of one or another of several mutually exclusive events is the sum of their probabilities.

A mating to a homozygous recessive, used to show whether a dominant phenotype hides a heterozygote.

A cross of a triple heterozygote to a triple recessive that exposes every gamete class, so the middle locus is the one altered in the rare double-crossover classes.

The mutant fraction in a tissue above which a mitochondrial defect becomes apparent. These notes do not treat that fraction as one figure for every mutation and tissue.

The earliest time an Hfr marker appears among recipients, taken as its map position because the bridge between cells usually breaks.

A base substitution within the purines or within the pyrimidines.

A base substitution that swaps a purine for a pyrimidine, or the reverse.

Three copies of human chromosome 21, so the effect is extra dose of the genes on that chromosome rather than a change confined to one of them.

Sorting of mixed organelle genomes at mitosis, without pairing, so that cell lineages can come to differ.

A pedigree pattern with more affected males than females, no father-to-son transmission, and passage through carrier females.

The *Drosophila* sex signal, read from the dose of X chromosomes relative to autosome sets, not from a gene on the Y.