The most familiar gene action is the simple dominant recessive relationship. Examples in livestock include black red in Angus and polled horned in Herefords. Many genetic anomalies, such as dwarfism or hydroceph-alus, are recessive, whereas the normal condition is dominant. This type of gene action is outlined here:

RR black Rr black rr red

PP polled Pp polled pp horned

DD normal Dd normal dd dwarf

The distinguishing characteristic is that the heterozygote has the same phenotype as one of the homozygotes.

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