Quantitative Characteristics

Qualitative, or discontinuous, characteristics possess only a few distinct phenotypes (IFigure22.1a); these characteristics are the types studied by Mendel and have been the focus of our attention thus far. However, many characteristics vary continuously along a scale of measurement with many overlapping phenotypes (I Figure 22.1b). They are referred to as continuous characteristics; they are also called quantitative characteristics because any individual's phenotype must be described with a quantitative measurement. Quantitative characteristics might include height, weight, and blood pressure in humans, growth rate in mice, seed weight in plants, and milk production in cattle.

Quantitative characteristics arise from two phenomena. First, many are polygenic—they are influenced by

(a) Discontinuous characteristic

Ifl A discontinuous (qualitative) characteristic exhibits only a few, easily distinguished phenotypes.

The plants are either tall or dwarf.

Tall

Dwarf

(b) Continuous characteristic

3j A continuous (quantitative) characterisitic exhibits a continuous range of phenotypes.

Phenotype (height)

(b) Continuous characteristic

3j A continuous (quantitative) characterisitic exhibits a continuous range of phenotypes.

Phenotype (height)

Dwarf Tall

Phenotype (height)

Dwarf Tall

Phenotype (height)

22.1 Discontinuous and continuous characteristics differ in the number of phenotypes exhibited.

Ifl A discontinuous (qualitative) characteristic exhibits only a few, easily distinguished phenotypes.

genes at many loci. If many loci take part, many genotypes are possible, each producing a slightly different pheno-type. Second, quantitative characteristics often arise when environmental factors affect the phenotype, because environmental differences result in a single genotype producing a range of phenotypes. Most continuously varying characteristics are both polygenic and influenced by environmental factors, and these characteristics are said to be multifactorial.

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