Mendelian Inheritance and Its Limits
Use segregation ratios as testable models while recognizing linkage, incomplete penetrance, multiple loci, structural variation, and measurement error.
Educational reference · evidence, sources, and limits shown below
Use segregation ratios as testable models while recognizing linkage, incomplete penetrance, multiple loci, structural variation, and measurement error.
Terms to know
- Dominant
- An allele whose effect is expressed in a heterozygote under the defined scoring conditions.
- Recessive
- An allele whose defined effect is masked in a heterozygote by another allele.
- Segregation ratio
- Expected distribution of scored classes under a specified inheritance model.
- Penetrance
- Proportion of individuals with a genotype that display the defined phenotype.
Core science
Mendelian models are powerful because they make falsifiable predictions. A single locus with two alleles and complete dominance can yield familiar F2 ratios, while codominance can reveal all three genotypic classes. The ratio applies to a specified cross and phenotype definition, not to every characteristic of the plant.
Cannabis offers useful examples and warnings. Major cannabinoid-ratio classes can often be predicted from functional synthase genotypes, yet the synthase region contains linked genes, pseudogenes, rearrangements, and expression differences. Absolute cannabinoid concentration, aroma, architecture, yield, and stress response are not explained by a one-locus ratio.
Departures from expectation may reflect linkage, epistasis, lethality, gametic or seedling selection, incomplete penetrance, environmental misclassification, small samples, pollen contamination, or incorrect parent identity. A chi-square result does not decide which mechanism caused a departure.
Why this matters in cultivation
- Use simple inheritance models for traits that can be scored clearly and tied to validated markers. Keep chemical ratio, absolute concentration, total yield, and legal compliance as separate outcomes.
- Breeders should publish family size and all scored classes, not only selected winners. Removing weak or nonconforming plants before scoring can distort the apparent segregation.
Measure and record
Hypothesis
Locus model, allele symbols, dominance relationship, linkage assumption, and predicted classes.
Parents
Verified genotypes and phenotypes, sample dates, assay methods, and cross direction.
Scoring
Trait definition, threshold, stage, observer or laboratory, blind status, and missing-data rule.
Results
Counts by class, total n, expected counts, statistical method, and confidence interval.
Exceptions
Unscored plants, mortality, selection, contamination checks, and alternative models.
Common misconceptions
Correction: Dominance describes heterozygote expression, not quality, frequency, or vigor.
Correction: 1-looking family proves one gene: Several mechanisms and small samples can mimic the same pattern.
Correction: Major ratio class and absolute accumulation are different phenotypes.
Evidence limits
Validated major-locus models are valuable but do not eliminate assay error or synthase-locus complexity. Mendelian language should not be applied to loosely defined traits such as quality, stability, or effect.
Related encyclopedia topics
- THC-ENC-141-145, THC-ENC-151-152, THC-ENC-181-200, and THC-GROW-030.
Source notes
- Wenger J.P. et al. (2020). Validating a predictive model of cannabinoid inheritance with feral, clinical, and industrial Cannabis sativa. American Journal of Botany 107:1423-1432.
- Welling M.T. et al. (2016). Characterisation of cannabinoid composition in a diverse Cannabis sativa germplasm collection. Euphytica 208:463-475.
- Laverty K.U. et al. (2019). A physical and genetic map of Cannabis sativa identifies extensive rearrangements at the THC/CBD acid synthase loci. Genome Research 29:146-156.
- van Velzen R. and Schranz M.E. (2021). Origin and evolution of the cannabinoid oxidocyclase gene family. Genome Biology and Evolution 13:evab130.
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.