Marker - An identifiable physical location on a chromosome (e.g., restriction enzyme cutting site, gene) whose inheritance can be monitored. Markers can be expressed regions of DNA (genes) or some segment of DNA with no known coding function but whose pattern of inheritance can be determined.

Megabase (Mb) - Unit of length for DNA fragments equal to 1 million nucleotides and roughly equal to 1 cM.

Meiosis - The process of two consecutive cell divisions in the diploid progenitors of sex cells. Meiosis results in four rather than two daughter cells , each with a haploid (single) set of chromosomes.

Mendel's first law - Also known as Mendel's law of segregation. The law of segregation states that two alleles for each trait separate during gamete formation, then unite at random, one from each parent, at fertilisation.

Mendel's second law - Also known as the law of independent assortment. This law states that during gamete formation, different pairs of alleles segregate independenly of each other.

Messenger RNA (mRNA) - RNA that serves as a template for protein synthesis.

Metaphase - A stage in mitosis or meiosis during which the chromosomes are alligned along the equatorial plane of the cell.

Metaphase Plate -  A section in the equatorial plane of the metaphase spindle having the chromosomes oriented upon it.

Methionine - A crystalline sulfur-containing essential amino acid. It is the translation initiation codon.

Microtubule - Any of the minute tubules in eukaryotic cytoplasm that are composed of the protein tubulin and form an important component of the cytoskeleton, mitotic spindle, cilia, and flagella.

Mitosis - The process of nuclear division in cells that are genetically identical to one another and to the parent cell.
 
 Mutation - Any heritable change in DNA sequence.