Principles of Inheritance and Variation — Core Principles

Core Principles

The Principles of Inheritance and Variation form the core of genetics, explaining how traits are passed from parents to offspring (inheritance) and the differences observed among individuals (variation).

Gregor Mendel's experiments with pea plants established fundamental laws: the Law of Dominance states that one allele masks another; the Law of Segregation explains that alleles separate during gamete formation; and the Law of Independent Assortment describes how different genes assort independently.

Key terms include gene (unit of heredity), allele (alternative form of a gene), genotype (genetic makeup), and phenotype (observable trait). Deviations from Mendelian inheritance include incomplete dominance (intermediate phenotype), co-dominance (both alleles expressed), and multiple alleles (more than two alleles for a gene).

The Chromosomal Theory of Inheritance links genes to chromosomes. Linkage describes genes on the same chromosome inherited together, while recombination creates new combinations. Sex determination mechanisms vary across species.

Genetic disorders can be Mendelian (single gene) or chromosomal (chromosome number/structure changes), and pedigree analysis helps trace their inheritance patterns.

Important Differences

vs Incomplete Dominance vs. Co-dominance

Aspect	This Topic	Incomplete Dominance vs. Co-dominance
Definition	Heterozygote exhibits an intermediate phenotype, a blend of the two parental traits.	Both alleles express themselves fully and simultaneously in the heterozygote, resulting in a combined phenotype.
Phenotypic Expression	Neither allele is completely dominant; the dominant allele cannot fully mask the recessive one.	Both alleles are equally dominant and contribute to the phenotype without one masking the other.
F2 Phenotypic Ratio (Monohybrid)	1:2:1 (e.g., Red:Pink:White)	1:2:1 (e.g., A:AB:B blood types, or Red:Roan:White in cattle)
F2 Genotypic Ratio (Monohybrid)	1:2:1 (same as phenotypic ratio)	1:2:1 (same as phenotypic ratio)
Example	Flower color in Mirabilis jalapa (four o'clock plant) – Red x White -> Pink.	ABO blood group in humans ($I^A I^B$ genotype results in AB blood type); Roan coat color in cattle.

While both incomplete dominance and co-dominance represent deviations from simple Mendelian dominance, their key distinction lies in the heterozygous phenotype. In incomplete dominance, the heterozygote shows an intermediate trait, like pink flowers from red and white parents. In co-dominance, both parental traits are fully expressed simultaneously in the heterozygote, such as the AB blood type where both A and B antigens are present. Both result in a 1:2:1 phenotypic ratio in the F2 generation of a monohybrid cross, which is identical to the genotypic ratio, unlike the 3:1 phenotypic ratio seen in complete dominance.