Law of Dominance — Revision Notes

⚡ 30-Second Revision

Law of Dominance: — One of Mendel's three laws.
Postulates:

1. Characters controlled by discrete units (genes/factors). 2. Factors occur in pairs (alleles). 3. In dissimilar pairs, one factor (dominant) masks the other (recessive).

Monohybrid Cross (P: Pure Dominant x Pure Recessive):

- F1 Generation: All heterozygous, express dominant phenotype. - F2 Generation (from F1 x F1): - Genotypic Ratio: 1 (Homozygous Dominant) : 2 (Heterozygous) : 1 (Homozygous Recessive) - Phenotypic Ratio: 3 (Dominant) : 1 (Recessive)

Key Terms: — Gene, Allele, Dominant, Recessive, Homozygous, Heterozygous, Phenotype, Genotype.
Exceptions: — Incomplete Dominance (intermediate F1, 1:2:1 F2 phenotypic ratio), Co-dominance (both expressed in F1, 1:2:1 F2 phenotypic ratio).
Example: — Pea plant height (Tall 'T' dominant over dwarf 't').

2-Minute Revision

The Law of Dominance is a foundational principle from Mendel's work, explaining how traits are expressed. It states that for a given trait, an organism inherits two 'factors' (now called alleles), one from each parent. When these two alleles are different (heterozygous state), one allele, the dominant one, completely masks the expression of the other, the recessive one. The recessive trait only appears if an individual inherits two copies of the recessive allele (homozygous recessive state).

In a monohybrid cross, starting with pure-breeding parents (e.g., pure tall TT x pure dwarf tt), the F1 generation will uniformly express the dominant trait (all tall Tt plants). This is a direct demonstration of dominance.

When these F1 individuals are self-pollinated, the F2 generation will show a phenotypic ratio of 3 dominant traits to 1 recessive trait (e.g., 3 tall : 1 dwarf). The corresponding genotypic ratio in F2 is 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.

Remember, this law describes complete dominance, and important exceptions like incomplete dominance and co-dominance exist where the heterozygous phenotype is either intermediate or expresses both alleles simultaneously.

5-Minute Revision

Mendel's Law of Dominance is one of the core principles governing how traits are passed down. It's based on three key ideas: firstly, hereditary characteristics are controlled by discrete units called 'factors' (genes); secondly, these factors exist in pairs (alleles) within an individual; and thirdly, in a pair of dissimilar factors (heterozygous state), one factor (the dominant allele) will express itself fully, masking the effect of the other (the recessive allele).

The recessive trait only manifests when an individual possesses two copies of the recessive allele.

Consider the classic example of pea plant height: Tall (T) is dominant over dwarf (t). If we cross a pure tall plant (TT) with a pure dwarf plant (tt), all the offspring in the F1 generation will have the genotype Tt. According to the Law of Dominance, since 'T' is dominant, all these F1 plants will be phenotypically tall. The dwarf trait is present genetically but not expressed.

Now, if these F1 (Tt) plants are self-pollinated or crossed with each other, the F2 generation will exhibit a specific pattern. Using a Punnett square for Tt x Tt:

	T	t
T	TT	Tt
t	Tt	tt

The genotypes will be TT, Tt, and tt in a 1:2:1 ratio. Phenotypically, both TT and Tt individuals will be tall, while only tt individuals will be dwarf. This results in a 3:1 phenotypic ratio of Tall:Dwarf in the F2 generation. This reappearance of the recessive trait (dwarfness) in F2 confirms that it was merely masked in F1, not lost.

It's crucial to remember that while fundamental, complete dominance isn't universal. Incomplete dominance (e.g., red x white flowers = pink F1) and co-dominance (e.g., $I^A I^B$ blood type = AB) are important exceptions where the heterozygous phenotype is either intermediate or expresses both alleles fully. For NEET, be adept at identifying these different modes of inheritance based on observed F1 and F2 phenotypic ratios.

Prelims Revision Notes

Definition: — In a cross between pure parents for contrasting traits, only one (dominant) trait appears in F1; the other (recessive) is masked.
Mendel's Postulates:

1. Factors (Genes): Traits are controlled by discrete units. 2. Paired Factors (Alleles): Each individual has two alleles for a trait. 3. Dominance: In a heterozygous pair, one allele (dominant) masks the other (recessive).

Alleles: — Alternative forms of a gene (e.g., T and t).
Homozygous: — Two identical alleles (TT or tt).
Heterozygous: — Two different alleles (Tt).
Phenotype: — Observable trait (e.g., tall).
Genotype: — Genetic makeup (e.g., TT, Tt, tt).
Monohybrid Cross (P: Homozygous Dominant x Homozygous Recessive):

- F1 Generation: All heterozygous (e.g., Tt), phenotypically express the dominant trait (e.g., all tall). - F2 Generation (from F1 x F1 cross): - Genotypic Ratio: 1 TT : 2 Tt : 1 tt - Phenotypic Ratio: 3 Dominant : 1 Recessive (e.g., 3 Tall : 1 Dwarf)

Test Cross: — Crossing an individual of unknown genotype (dominant phenotype) with a homozygous recessive individual. If any recessive offspring appear, the unknown parent is heterozygous.
Exceptions/Deviations:

- Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., Red x White -> Pink F1). F2 phenotypic ratio is 1:2:1. - Co-dominance: Both alleles express fully in heterozygote (e.g., $I^A I^B$ -> AB blood type). F2 phenotypic ratio is 1:2:1.

Significance: — Explains uniformity of F1 and reappearance of recessive trait in F2. Fundamental to understanding inheritance patterns.

Vyyuha Quick Recall

Dominant Always Masks Recessive In F1.