Mendelian Inheritance

Mendelian inheritance refers to the quantitative principles of particulate hereditary transmission established by Gregor Johann Mendel in his 1865 presentation and 1866 monograph (Versuche über Pflanzen-Hybriden) on hybridization experiments in the garden pea (Pisum sativum). Prior to Mendel's work, the prevailing biological model was blending inheritance, which posited that parental traits merged irreversibly in offspring like miscible fluids—a mechanism that Charles Darwin recognized would rapidly eliminate heritable variation required for natural selection. By tracking seven binary, true-breeding morphological characters across ~28,000 pea plants over eight years and applying combinatorial probability to large F₁, F₂, and F₃ progeny counts, Mendel demonstrated that traits are governed by discrete, immutable hereditary units ("Elemente," termed genes by Wilhelm Johannsen in 1909) that persist intact across generations even when phenotypically masked in heterozygotes.

Rediscovered independently in 1900 by Hugo de Vries, Carl Correns, and Erich von Tschermak, and integrated with cytological observations of homologous chromosome pairing by Walter Sutton and Theodor Boveri (1902–1903), Mendelian genetics forms the foundation of transmission genetics, population genetics (Hardy-Weinberg equilibrium), and causal genetic epidemiology (Mendelian randomization).

Mendel's Law of Segregation states that diploid organisms possess two alleles for each autosomal gene locus—one inherited from each parent—and that these two alleles separate cleanly from one another during gamete formation so that each haploid gamete (egg or sperm/pollen) receives exactly one allele with equal probability (p = 0.50).

Cytologically, segregation is executed during meiosis: homologous maternal and paternal chromosomes replicate during pre-meiotic S phase, pair as bivalents during Prophase I, and separate to opposite spindle poles during Anaphase I (for loci that have not undergone chiasma crossover relative to the centromere) or Anaphase II (for loci distal to a crossover event). When two true-breeding homozygous parental strains (P₁: AA × aa) are crossed, all first-filial (F₁) hybrid progeny inherit the heterozygous genotype Aa and display the dominant phenotype (the principle of uniformity). Self-fertilization or intercrossing of two F₁ heterozygotes (Aa × Aa) yields three zygotic combinations governed by the binomial expansion (0.5 A + 0.5 a)² = 0.25 AA + 0.50 Aa + 0.25 aa:

• Genotypic F₂ Ratio: 1 AA : 2 Aa : 1 aa (confirmed by Mendel via F₃ progeny testing, which showed that two-thirds of dominant-appearing F₂ plants segregated again while one-third bred true).
• Phenotypic F₂ Ratio (Complete Dominance): 3 Dominant (AA + 2Aa) : 1 Recessive (aa).
• Testcross Ratio (Aa × aa): Crossing a heterozygote to a homozygous recessive tester yields a 1 Aa : 1 aa (1:1) ratio, directly measuring the gametic output of the heterozygous parent.

Mendel's Law of Independent Assortment states that the alleles of two or more distinct gene loci segregate into gametes independently of one another, such that the inheritance of an allele at locus A does not influence the probability of inheriting an allele at locus B.

In a dihybrid cross between true-breeding parents differing in two traits (AABB yellow round × aabb green wrinkled seeds), the F₁ dihybrid (AaBb) produces four gametic classes in equal proportions (1/4 AB : 1/4 Ab : 1/4 aB : 1/4 ab). Random union of these gametes in an F₁ × F₁ self-cross generates a 16-cell Punnett square comprising nine distinct genotypes and four phenotypic classes in the product of two independent 3:1 monohybrid ratios: (3:1) × (3:1) = 9 A–B– : 3 A–bb : 3 aaB– : 1 aabb. In a dihybrid testcross (AaBb × aabb), independent assortment yields a 1:1:1:1 phenotypic ratio.

Cytologically, independent assortment arises from the random orientation of non-homologous chromosome bivalents along the metaphase I plate: in humans (n = 23 chromosome pairs), random metaphase I alignment alone generates 2²³ = 8,388,608 distinct haploid gametic combinations per parent. Loci located on different chromosomes always assort independently (recombination fraction r = 0.50). Loci situated on the same chromosome (synteny) violate independent assortment if they are physically close (genetic linkage, r < 0.50, discovered by Bateson, Saunders, and Punnett in 1905 and mapped linearly by Thomas Hunt Morgan and Alfred Sturtevant), whereas syntenic loci separated by ≥50 centimorgans (cM) undergo sufficient meiotic crossing over to assort pseudo-independently (r ≈ 0.50), as occurred for several of Mendel's seven Pisum sativum loci distributed across chromosomes 1, 4, 5, and 7.

At the molecular level, dominance and recessiveness are not intrinsic properties of a DNA sequence itself, but emergent properties of how a diploid genotype maps onto a biochemical or cellular phenotype through transcription, translation, and enzyme kinetics. As formulated by Sewall Wright (1934) and Henrik Kacser and James Burns (1981) in metabolic control analysis, because metabolic pathway flux is shared across multiple serial enzymes, a 50% reduction in the concentration of a single enzyme in a heterozygote (Aa) typically reduces steady-state metabolic flux by less than 5%, leaving the wild-type phenotype intact (haplosufficiency). Consequently, most loss-of-function mutations in enzyme-coding genes are recessive.

For instance, Mendel's seed-shape locus (R vs. r, round vs. wrinkled peas, cloned by Bhattacharyya et al. in 1990) encodes starch-branching enzyme I (SBEI): the recessive r allele contains an 800 bp Ac-like transposon insertion that abolishes amylopectin branching. In rr homozygotes, unbranched sucrose and fructose accumulate osmotically, drawing water into immature cotyledons that subsequently wrinkle upon desiccation; in Rr heterozygotes, a single functional R allele synthesizes sufficient amylopectin to maintain normal seed turgor. Similarly, the stem-length locus (Le) encodes gibberellin 3β-hydroxylase (GA3ox1), where a single alanine-to-threonine missense substitution in the recessive le allele reduces bioactive gibberellin GA₁ synthesis by 95%, producing dwarf plants.

Conversely, dominant mutant alleles arise through three primary molecular mechanisms:

1. Haploinsufficiency: A 50% gene dosage (Aa) is insufficient for normal function (common in stoichiometric structural proteins like fibrillin-1 in Marfan syndrome or transcription factors).
2. Dominant-Negative (Antimorphic) Mutation: A mutant subunit assembles into a multimeric complex (such as triple-helical Type I collagen in osteogenesis imperfecta or homotetrameric p53) and poisons the wild-type subunits.
3. Gain-of-Function (Hypermorphic or Neomorphic) Mutation: Constitutive receptor activation (e.g., FGFR3 in achondroplasia) or toxic protein aggregation (e.g., polyglutamine expansion in HTT causing Huntington disease).

Classical Mendelian ratios are systematically modified when allelic interactions deviate from complete dominance or when two loci act within the same biochemical pathway (epistasis):

To evaluate whether observed progeny counts (O_i) across k phenotypic classes conform to a theoretical Mendelian ratio (E_i = N × p_i, where N is total progeny), Karl Pearson's chi-squared (χ²) goodness-of-fit test is applied:

χ² = Σ [ (O_i − E_i)² / E_i ] with df = k − 1 degrees of freedom.

If the calculated χ² value is less than the critical threshold at α = 0.05 (e.g., χ² < 3.841 for df = 1 in a monohybrid cross, or χ² < 7.815 for df = 3 in a dihybrid cross), deviations between observed and expected counts are consistent with random sampling variation; values exceeding the threshold indicate genetic linkage, differential viability (segregation distortion / meiotic drive), or an alternative epistatic model.

1. Mendel, G. (1866). Versuche über Pflanzen-Hybriden. Verhandlungen des naturforschenden Vereines in Brünn, 4, 3–47. https://doi.org/10.5962/bhl.title.61004
2. Kacser, H., & Burns, J. A. (1981). The molecular basis of dominance. Genetics, 97(3–4), 639–666. https://doi.org/10.1093/genetics/97.3-4.639
3. Bhattacharyya, M. K., Smith, A. M., Ellis, T. H. N., Hedley, C., & Martin, C. (1990). The wrinkled-seed character of pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branching enzyme. Cell, 60(1), 115–122. https://doi.org/10.1016/0092-8674(90)90721-P90721-P)
4. Ellis, T. H. N., Hofer, J. M. I., Timmerman-Vaughan, G. M., Coyne, C. J., & Hellens, R. P. (2011). Mendel, 150 years on. Trends in Plant Science, 16(11), 590–596. https://doi.org/10.1016/j.tplants.2011.06.006