A genoype is the genetic representation on chromosomes of factors (genes) that code for a trait or characteristic. The Rh blood group antigens are coded for by a pair of genes --the dominant "D" gene and the recessive "d" gene. Consequently, a human may inherit one of three Rh genotypes --D/D, D/d or d/d . The D/D, D/d, and d/d are homozygous dominant, heterozygous, and homozygus recessive, respectively.
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A phenotype is the physical or chemical expression of a gene. The phenotyes of the foregoing genotypes are: D/D --Rh+ blood D/d -- also Rh+ blood (because the D gene is dominant). d/d---Rh- negative blood; the recessive gene is expressed only in the double recessive genotype.