A2 OCR Biology: Continuous and Discontinuous Variation

A2 OCR Biology: Continuous and Discontinuous Variation

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  • Created on: 25-05-11 00:07
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Continuous and Discontinuous Variation
Discontinuous Variation
This describes qualitative differences between phenotypes. Qualitative differences fall into
distinguishable categories. There are no intermediate categories ­ e.g. you are either male
or female.
Continuous Variation
This describes quantitative differences between phenotypes. These are the phenotypic
differences where there is a wide range of variation within the population ­ e.g. height and
mass in humans.
Genetic Basis of Discontinuous and Continuous Variation
Both types of variation may be the result of more than one gene. In discontinuous
variation if there is more than one gene involved, the genes interact in an epistatic way. if
only one gene is involved, it's described as monogenic.
In discontinuous (qualitative) variation:
Different alleles at a single gene locus have large effects on the phenotype
Different gene loci have quite different effects on the phenotype
Examples include codominance, dominance and recessive patterns of
inheritance.
In continuous (quantitative) variation:
Traits exhibiting continuous variation are controlled by two or more genes
Each gene provides an additive component to the phenotype
Different alleles at each gene locus have a small effect on the phenotype
A large number of different genes may have a combined effect on the phenotype.
(polygenes, the characteristic they control is polygenic)

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