Law of Dominance — Core Principles

Core Principles

The Law of Dominance, proposed by Gregor Mendel, is a fundamental principle of heredity. It states that when two pure-breeding parents with contrasting traits are crossed, only one trait, the dominant one, will appear in the first filial (F1) generation.

The other trait, the recessive one, remains hidden. This occurs because traits are controlled by discrete units called factors (now known as genes), which exist in pairs. When an individual inherits two different forms (alleles) of a gene, the dominant allele masks the expression of the recessive allele.

For example, in pea plants, tallness (T) is dominant over dwarfness (t). A cross between pure tall (TT) and pure dwarf (tt) plants yields all tall (Tt) F1 offspring. If these F1 plants are self-pollinated, the F2 generation will show a 3:1 phenotypic ratio of dominant to recessive traits, demonstrating the reappearance of the masked recessive trait.

This law is crucial for understanding basic inheritance patterns but has exceptions like incomplete dominance and co-dominance.

Important Differences

vs Incomplete Dominance and Co-dominance

Aspect	This Topic	Incomplete Dominance and Co-dominance
Definition	Law of Dominance (Complete Dominance)	Incomplete Dominance
Definition	One allele completely masks the expression of the other in a heterozygote.	Neither allele is completely dominant; the heterozygote exhibits an intermediate phenotype.
Heterozygous Phenotype	Identical to the homozygous dominant phenotype.	A blend or intermediate between the two homozygous phenotypes.
F1 Generation (from pure parents)	Uniformly expresses the dominant trait.	Uniformly expresses an intermediate trait.
F2 Phenotypic Ratio (from F1 x F1)	3:1 (Dominant:Recessive)	1:2:1 (Parental 1:Intermediate:Parental 2)
F2 Genotypic Ratio (from F1 x F1)	1:2:1 (Homozygous Dominant:Heterozygous:Homozygous Recessive)	1:2:1 (Homozygous 1:Heterozygous:Homozygous 2)
Example	Pea plant height (Tall/Dwarf), Seed color (Yellow/Green)	Snapdragon flower color (Red x White -> Pink), Andulasian fowl feather color (Black x White -> Blue)
Definition	Law of Dominance (Complete Dominance)	Co-dominance
Definition	One allele completely masks the expression of the other in a heterozygote.	Both alleles express themselves fully and equally in the heterozygote, without blending.
Heterozygous Phenotype	Identical to the homozygous dominant phenotype.	Shows characteristics of both homozygous phenotypes simultaneously.
F1 Generation (from pure parents)	Uniformly expresses the dominant trait.	Uniformly expresses both parental traits simultaneously.
F2 Phenotypic Ratio (from F1 x F1)	3:1 (Dominant:Recessive)	1:2:1 (Parental 1:Both expressed:Parental 2)
F2 Genotypic Ratio (from F1 x F1)	1:2:1 (Homozygous Dominant:Heterozygous:Homozygous Recessive)	1:2:1 (Homozygous 1:Heterozygous:Homozygous 2)
Example	Pea plant height (Tall/Dwarf), Seed color (Yellow/Green)	ABO blood groups ($I^A I^B$ results in AB blood type), Roan cattle (Red x White -> Roan)

While the Law of Dominance describes situations where one allele completely masks another, leading to a 3:1 phenotypic ratio in the F2 generation of a monohybrid cross, incomplete dominance and co-dominance represent deviations from this simple pattern. In incomplete dominance, the heterozygous phenotype is an intermediate blend, resulting in a 1:2:1 phenotypic ratio in F2, mirroring the genotypic ratio. Co-dominance, on the other hand, involves the simultaneous and full expression of both alleles in the heterozygote, also leading to a 1:2:1 phenotypic ratio in F2, where the heterozygous phenotype is distinct and shows both parental traits. These distinctions are crucial for understanding the full spectrum of genetic inheritance.