Glossary Plant Breeding

Multimeric Structure. A structure composed of several identical or different subunits
held together by weak bonds.
Multiple Allele. A member of a series of several known allelic forms of a gene. The
existence of such a situation is called multiple allelism. The presence of multiple
alleles has been observed for the self-incompatibility system in the Nicotiana spp.
Please notice that the presence of all the allelic forms of a gene in any natural
population is a rare event. As the no. of allelic forms increase, the possible kinds of
gametes (and homozygotes) and total no. of genotypes increase exponentially. For
example, for a single gene pair with four alleles, the kinds of gametes (homozygotes)
and total no. of genotypes will be 4 1 (= 4) and (4 ×5)/2 (= 10), respectively. For two
genes with same no. of alleles at the two loci, the values will be 42 (= 16) and
(16×17)/2 (=136), respectively.
Multiple Correlation Coefficient. A statistic measuring the joint association of all the
independent variables with the dependent one. It thus tells how much of the variation
in the dependent (Y) could be accounted for by reference to these independent ones.
Numerically, it is the square root of the ratio of the regression sum of squares to the
total sum of squares, and is denoted by R.
Multiple Cross. A cross involving many parents. Theoretically its objective is to obtain
recombination from many a parent; the practical utility of multiple cross, however, is
limited in plant breeding for two reasons: (a) the size of the population increases
exponentially with increasing no. of parents to ensure occurrence of the desired
genotype in the segregating generation, and (b) it is often difficult to find many well
adapted parents having different useful genes. (Also called convergent cross).
Multiple-Factor Hypothesis (Nilsson-Ehle 1908). A hypothesis to explain quantitative
variation. Many genes, each with a small but equal effect, segregate to produce a
continuous variation in a quantitative character. Conclusive evidences for this
hypothesis proved beyond doubt that all genetic traits (whether qualitative or
quantitative) follow Mendel’s laws of heredity.
Multiple Resistance. A strategy in resistance breeding that involves placement of two,
three, or more new and still effective resistance genes into a new cultivar to impose
153