See: Cucurbita pepo. Mass selection

Often called population breeding, or recurrent mass selection this is the converse of Mendelian or Pedigree breeding. Mass selection requires a population, as large and as genetically diverse as possible, which is screened for the best individuals that are to become the parents of the next screening generation. It is the method of choice for many-gene characters, and for amateur breeders working with horizontal resistance. The selection criteria can include all aspects of yield, quality, agronomic suitability, and horizontal resistance to all locally important parasites. It is essential to screen for all the desirable characteristics in each breeding cycle. The assessment of each individual must be relative to the neighbouring individuals and the population as a whole. A holistic approach is recommended. Matching

In terms of the gene-for-gene relationship, an allo-infection is described as matching when the vertical parasitism gene(s) of the parasite match the vertical resistance gene(s) of the host (i.e., the biochemical key of the parasite fits the biochemical lock of the host). The vertical resistance then fails to operate and the allo-infection is successful. See also: Non-matching. Maté

See: Ilex paraguariensis. Maximum

The highest possible value of a quantitative variable. Mayetiola destructor

The Hessian fly. A stem borer of wheat. This parasite is interesting in that vertical resistance against it is quantitative. It is thought that the evolutionary function of this resistance is to reduce or even prevent reproduction of the parasite, rather than to reduce the frequency of matching allo-infections. In either event, it appears that the evolutionary function of the gene-forgene relationship is to stabilise the population explosion of an r-strategist parasite. Mean

An alternative term for average. See also: Gaussian curve, Normal distribution, Mode, Skewed distribution. Mechanisms of resistance

The mechanisms of resistance in plants to their parasites are many and varied. As a general rule, they are of little interest to amateur breeders working with horizontal resistance, who should use the holistic approach. It is a great mistake to breed plants for a single, prominent resistance mechanism, such as hairy leaves that resist certain insects. Medicago sativa

Alfalfa, known as lucerne in Britain. This is probably the most important of the fodder legumes and it is used for grazing, hay, and silage. Its origins are ancient and are apparently linked to the domestication of the horse. The plant is pollinated by special bees, although some self-pollination does occur. This is one of the relatively rare examples of recurrent mass selection used by professional breeders to accumulate horizontal resistance during the twentieth century. Meiosis

Reduction division. This is the process in which the two sets of chromosomes in a diploid nucleus separate to form two haploid nuclei that become gametes. See also: Mitosis. Melampsora lini

Flax rust. This is the disease in which H.H. Flor discovered the gene-for-gene relationship.

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