|MadSci Network: Genetics|
To experimentally determine if a trait is purebread (dominant or recessive) you must mate a male and female of the same species and determine the phenotypic expression of the offspring. It is best to perform such an experiment on a subject that has multiple births to compare multiple children. If all offspring for one or more generations show the same trait without variation of some significance then you have a purebread dominant or recessive trait. This is the only way I know to tell if a gene is heterozygous or homozygous. See information on Gregor Mendel's experiments. Note: you must know if there is only one gene specific to the trait. If there are more than one gene (i.e. eye color) it would be very difficult to determine.
Try the links in the MadSci Library for more information on Genetics.