Two kinds of genetic inheritance

Until the recognition of Mendel's laws of inheritance in 1900, only one kind of inheritance was recognised. This was the many-gene, quantitative inheritance of the biometricians, exhibiting every degree of difference between two extremes, usually with a normal distribution. Mendel's laws revealed that a single-gene, qualitative inheritance was also possible, in which a character was either present or absent, and there were no intermediates.

In nature, quantitative inheritance is the rule, and qualitative inheritance occurs only occasionally, usually in specialised systems such as the gene-for-gene relationship (see 4), the vertical subsystem (see 5), and genetic systems of pollen self-incompatibility.

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