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#phenotypic expression

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Genetic Crosses

Incomplete dominance is characterized by

Incomplete dominance occurs when single dose of functional product from dominant allele insufficient to reach threshold for full dominant phenotype, while recessive allele produces nonfunctional or low activity product. Heterozygote Aa therefore synthesizes intermediate amount of pigment, receptor, or enzyme, leading to phenotype distinct from both homozygotes, such as pink flowers from red RR x white rr. This contrasts with complete dominance where heterozygote resembles dominant homozygote, and with codominance where both allele products fully expressed simultaneously and detectable side by

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Incomplete Dominance and Dosage Effect

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Mendelian Inheritance

The phenotypic expression of a heterozygote in Mendelian inheritance shows

In Mendelian inheritance with complete dominance, heterozygote carries one dominant and one recessive allele but produces sufficient dominant gene product to achieve dominant phenotype threshold, such as enzyme activity or structural protein. Consequently phenotypic expression of heterozygote shows dominant trait, identical to homozygous dominant, e.g., tall Tt same as TT. Recessive trait appears only in tt lacking dominant allele. Blended or intermediate trait characterises incomplete dominance, not Mendelian complete dominance where heterozygote fully expresses dominant phenotype. Cytologica

Ref: Snustad & Simmons, Principles of Genetics, 7th ed., Chapter 3: Heterozygote Expression

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