A genetic law stating that each diploid organism carries two alleles for a given locus and transmits only one of these alleles to each gamete because the alleles segregate during gamete formation (meiosis).

Principle

Alleles at a single locus separate into different gametes during meiosis so that each gamete receives a single allele; the diploid genotype is thus partitioned into haploid contributions to offspring according to Mendelian segregation.

Demonstration

Illustrative scenario: In a monohybrid cross of a single-locus dominant/recessive trait, F1 heterozygotes (Aa) produce gametes A and a in approximately equal proportions; selfing F1 individuals yields F2 genotypes in a 1:2:1 ratio (AA:Aa:aa) and phenotypes in 3:1 if one allele is fully dominant.

Misapplication

Applying segregation without regard to exceptions—meiotic nondisjunction, gene conversion, segregation distorters, linked loci with no recombination, uniparental inheritance, or polyploidy—will produce incorrect predictions; treating multiallelic systems or interacting loci as if simple segregation applies is also erroneous.

Consequence

The law explains predictable ratios of genotypes and underlies inheritance patterns for single Mendelian loci, enabling predictions of carrier probabilities, mapping of traits, and breeding strategies when its assumptions are met.

Reversal

Non-segregation (e.g., meiotic nondisjunction, cytoplasmic inheritance, or segregation distortion) leads to unequal transmission of alleles and can produce aneuploidy or biased allele frequencies in offspring populations—directly opposing Mendelian expectations.

Boundary

Applies to single Mendelian loci in diploid sexually reproducing organisms with regular meiosis; it excludes haploid organisms, loci inherited cytoplasmically, cases of frequent gene conversion, polyploidy without disomic segregation, and situations where gamete contributions are nonrandomly transmitted.

Semantic Tension

Tension exists between segregation and related concepts—independent assortment (which concerns different loci) and dominance (which concerns phenotype expression)—leading to confusion if locus-independence or phenotype mapping is assumed without checking conditions.

Synthesis

Mendel's law of segregation states that the two alleles at a diploid locus separate into different gametes during meiosis so each gamete carries one allele; it is a core rule for predicting single-locus inheritance, valid when standard meiotic segregation and diploidy apply, and it must be qualified in the presence of known exceptions.