Map distance
The standard measure of distance between loci, expressed in centiMorgans (cM). Estimated from recombination fraction via a mapping function (q.v.). For small recombination fractions, map distance equals the percentage of recombination (recombination frequency) between two genes. 1% recombination = 1 cM. Sometimes called a map unit. mapping Determining the location of a locus (gene or genetic marker) on a chromosome. See continuous map; linkage map; physical map.

Mapping function
A mathematical expression relating observed recombination fraction (q.v.) to map distance expressed in centiMorgans.

Map unit
See map distance; crossing-over unit.

Marker
An identifiable DNA sequence that facilitates the study of inheritance of a trait or a gene. Such markers are used in mapping the order of genes along chromosomes and in following the inheritance of particular genes: genes closely linked to the marker will generally be inherited with it. Markers must be readily identifiable in the phenotype, for instance by controlling an easily observable feature (such as eye colour) or by being readily detectable by molecular means, e.g., microsatellite markers (q.v.). See gene tracking.

Maternal effect
An effect attributable to some aspect of performance of the mother of the individual being evaluated.

Maternal inheritance
Inheritance controlled by extrachromosomal (cytoplasmic) factors that are transmitted through the egg.
matric potential A water potential component, always of negative value, resulting from capillary, imbibitional and adsorptive forces. See pressure potential.