Mendellian and Human
A dominant trait is always expressed phenotypically.
→ In the case of co-dominance, both traits will be expressed phenotypically.
→ In the case of incomplete dominance, a mixing of the traits will occur - the dominant is "diluted".
A recessive trait must be in the homozygous state in order to be expressed phenotypically.
In the heterozygote condition, when a dominant allele is inherited with a recessive allele, only the dominant allele will be expressed.
(Campbell/Reese. Biology:6th Ed. Copyright 2002 Pearson Ed. Inc.)
Offspring can be predicted using a Punnett Square (a probability tool). Online tutorial can be found at:
If P=Purple (and is dominant) and p=yellow (and is recessive)
All genotypes would be Pp. All phenotypes would be purple.
Genotype Ratio is 2PP: 2Pp. All phenotypes would be purple.
Genotypic Ratio is 1PP:2Pp:1pp. Phenotypic Ratio is 3:1 purple to yellow.
When two heterozygous organisms are crossed for a single trait, the cross is called a monohybrid cross.
Genotypic Ratio is 1Pp:1pp. Phenotypic Ratio is 1:1 purple to yellow.
graphics from www.geocities.com
Dihybrid Punnett Square