Inheritance and population genetics definitions

Meiosis and the sources of genetic variation

Meiosis

Nuclear division in which one round of DNA replication is followed by two divisions, producing four haploid cells that differ genetically from each other and from the parent cell.

Homologous pair

Two chromosomes of the same length carrying the same genes at the same loci, one from each parent, though not necessarily the same alleles.

Diploid

Having chromosomes in homologous pairs, written 2n.

Haploid

Having one chromosome from each homologous pair, written n.

Bivalent

The structure formed when a homologous pair comes together in prophase I: two chromosomes, four chromatids.

Reduction division

A division that halves the chromosome number. Meiosis I is one; meiosis II is not.

Crossing over

The exchange of equivalent sections of DNA between non-sister chromatids of a homologous pair during prophase I.

Chiasma

The visible point at which two non-sister chromatids cross during a bivalent's crossover.

Recombinant

A chromatid, or an offspring, carrying a combination of alleles that neither parental chromosome carried.

Spindle

A framework of microtubules running between the poles of a dividing cell, which attaches to centromeres and moves the chromosomes.

Cytokinesis

The division of the cytoplasm that follows nuclear division: by a cleavage furrow in an animal cell and by a cell plate in a plant cell.

Cell plate

The line of vesicles that fuse across the equator of a dividing plant cell, forming the new cell surface membranes and the new cell wall between the daughter cells.

Non-disjunction

The failure of a homologous pair to separate at anaphase I, or of sister chromatids to separate at anaphase II, giving gametes with one chromosome too many or too few.

Polysomy

The presence of one or more extra copies of a chromosome in a cell, as in trisomy 21.

Monosomy

The presence of only one copy of a chromosome where a diploid cell should have a pair, as in 45,X.

Aneuploidy

A chromosome number that differs from the normal by less than a whole set.

Turner's syndrome

The condition of a female with a single X chromosome and no second sex chromosome, 45,X: a monosomy.

Down's syndrome

The condition of a person with three copies of chromosome 21: a trisomy, and so a polysomy.

Monohybrid inheritance and genetic diagrams

Allele

A version of a gene. The alleles of one gene occupy the same locus on the two chromosomes of a homologous pair.

Locus

The fixed position of a gene on a chromosome or DNA molecule.

Genotype

The alleles an organism carries, for the gene or genes under discussion.

Phenotype

The observable characteristics of an organism, produced by its genotype interacting with its environment.

Homozygous

Carrying two identical alleles of a gene.

Heterozygous

Carrying two different alleles of a gene.

Dominant

An allele whose effect appears in the phenotype even when only one copy is present.

Recessive

An allele whose effect appears only when no dominant allele is present: in a homozygote, or in a male for a gene on the X chromosome.

Carrier

An individual heterozygous for a recessive allele, so not showing the characteristic but able to pass the allele on.

F1 and F2

The first filial generation, the offspring of the original parental cross, and the second, the offspring of F1 individuals crossed with each other.

Test cross

A cross between an organism of dominant phenotype and a homozygous recessive, to find whether the first is homozygous or heterozygous.

Codominance

A relationship between two alleles in which both are expressed in the phenotype of a heterozygote, so it shows both characters fully, as in roan cattle.

Incomplete dominance

A relationship between two alleles in which the heterozygote has a phenotype intermediate between the two homozygotes, as in pink snapdragons.

Sex linkage

The inheritance of a gene carried on one sex chromosome, usually the X, with no equivalent on the other, so a male expresses whichever single allele he carries.

Pedigree diagram

A family tree showing how one characteristic is inherited through several generations, with males as squares, females as circles and affected people shaded.

Affected

Showing the characteristic a pedigree follows; drawn as a shaded symbol.

Autosomal

Carried on an autosome, any chromosome other than the sex chromosomes, so inherited in the same way by sons and daughters.

Dihybrid inheritance, linkage, epistasis and the chi-squared test

Autosomal linkage

The tendency of alleles of genes on the same autosome to be inherited together, unless crossing over separates them.

Recombination frequency

The percentage of offspring of a test cross that are recombinant. It rises with the distance between linked loci and does not exceed 50%.

Epistasis

Interaction between genes at different loci, in which one gene affects or masks the expression of another.

Population genetics and Hardy-Weinberg equilibrium

Population

A group of organisms of the same species living in the same place at the same time and able to interbreed.

Gene pool

All the alleles of all the genes present in a population at a given time.

Allele frequency

The proportion of all the alleles of a gene in a gene pool that are of one particular kind.