Selfing, Inbreeding, and Inbreeding Depression
Explain how selfing changes genotype frequencies, exposes recessive alleles, increases line uniformity, and can reduce vigor or fertility.
Explain how selfing changes genotype frequencies, exposes recessive alleles, increases line uniformity, and can reduce vigor or fertility.
Core science
Selfing mates a plant with genetically female pollen derived from itself or otherwise allows self-fertilization. Under an ideal diploid single-locus model, heterozygosity is expected to halve with each selfed generation. The genome does not become instantly homozygous, and different regions fix at different times because of recombination, selection, viability, and structural variation.
Inbreeding exposes recessive alleles and can reveal hidden defects in vigor, fertility, development, disease response, sex expression, and seed quality. Inbreeding depression is the decline in performance caused by increased homozygosity for deleterious alleles and loss of favorable heterozygous combinations. Its magnitude differs among populations and traits. Selection can remove some severe defects, but “purging” is not guaranteed and may narrow diversity.
An S1 family is a population of selfed offspring from one parent, not a clone of that parent. It can segregate widely when the parent is heterozygous. Selfing can help reveal genotype, create mapping populations, and develop lines, but it should be paired with population size, family testing, and backup conservation. Do not interpret short-term uniformity in a few plants as genetic stability.
Why this matters in cultivation
- Track selfing generation separately from filial generation, compare each selfed family with the parent and outcrossed checks, and record losses rather than discarding them from the analysis.
Measure and record
Record 1
Record parent identity, starting heterozygosity information when available, selfing method, S generation, seed set, germination, family size, and all losses before phenotype scoring.
Record 2
Track vigor, fertility, abnormality rate, sex expression, trait variance, and molecular homozygosity if measured. Compare selfed families with the parent and, where useful, an outcrossed or noninbred reference under the same environment.
Record 3
At each generation, record selected families, discarded families, the reasons for removal, retained diversity, and backup seed or clones. Treat mortality and weak plants as data rather than silently excluding them from conclusions about inbreeding.
Common misconceptions
Misconception: S1 seed is genetically identical to the mother. Selfing reshuffles the mother’s alleles and can expose recessive variation, so S1 offspring can differ markedly from one another and from the parent.
Misconception: Selfing always creates a stable line in one generation. One selfed generation increases homozygosity but leaves substantial residual heterozygosity when the parent was heterozygous.
Misconception: Inbreeding depression affects every Cannabis population equally. Its magnitude depends on the starting genetic load, trait, population history, environment, and the alleles that become homozygous.
Evidence limits and uncertainty
Theoretical heterozygosity decline is an expectation, not a measurement of realized fixation in a particular line. Selection, viability, linkage, and structural variation can change the observed pattern.
Apparent recovery from inbreeding depression can reflect selection or environmental differences and should not be described as genetic purging without appropriate generational evidence.
Check your reasoning
- For "Selfing, Inbreeding, and Inbreeding Depression", explain the mechanism behind this objective: Explain how selfing changes genotype frequencies, exposes recessive alleles, increases line uniformity, and can reduce vigor or fertility. Which observation or measurement would best test whether that mechanism is operating in the real crop?
- A learner claims, "S1 seed is genetically identical to the mother." Use the lesson’s science and evidence limits to explain why that claim is unreliable, then name one observation or measurement that could separate the competing explanations.
- Applied case — Track selfing generation separately from filial generation, compare each selfed family with the parent and outcrossed checks, and record losses rather than discarding them from the analysis. Build a verification plan using the lesson’s record set (Parent ID and heterozygosity; selfing method; S generation; seed set and germination; family size; vigor, fertility, sex and abnormality rates; trait variance; molecular homozygosity; selected families and preserved backups.). What would you compare before and after the action, and what result would make you revise the original interpretation?
Require lesson-specific evidence, not memorized universal targets. Open the rationales after you have written or discussed your own answer.
Answer rationale 1: Mechanism / workflow rationale
- A strong answer should connect the response to the lesson objective: Explain how selfing changes genotype frequencies, exposes recessive alleles, increases line uniformity, and can reduce vigor or fertility.
- Selfing mates a plant with genetically female pollen derived from itself or otherwise allows self-fertilization. Under an ideal diploid single-locus model, heterozygosity is expected to halve with each selfed generation. The genome does not become instantly homozygous, and different regions fix at different times because of recombination, selection, viability, and structural variation.
- Inbreeding exposes recessive alleles and can reveal hidden defects in vigor, fertility, development, disease response, sex expression, and seed quality. Inbreeding depression is the decline in performance caused by increased homozygosity for deleterious alleles and loss of favorable heterozygous combinations. Its magnitude differs among populations and traits. Selection can remove some severe defects, but “purging” is not guaranteed and may narrow diversity.
- The most useful verification evidence includes Record parent identity, starting heterozygosity information when available, selfing method, S generation, seed set, germination, family size, and all losses before phenotype scoring.
- Keep this limit explicit: Theoretical heterozygosity decline is an expectation, not a measurement of realized fixation in a particular line. Selection, viability, linkage, and structural variation can change the observed pattern.
Answer rationale 2: Misconception rationale
- The shortcut is unreliable because the lesson explicitly teaches a more conditional explanation.
- Representative misconception: S1 seed is genetically identical to the mother. Selfing reshuffles the mother’s alleles and can expose recessive variation, so S1 offspring can differ markedly from one another and from the parent.
- Selfing mates a plant with genetically female pollen derived from itself or otherwise allows self-fertilization. Under an ideal diploid single-locus model, heterozygosity is expected to halve with each selfed generation. The genome does not become instantly homozygous, and different regions fix at different times because of recombination, selection, viability, and structural variation.
- A useful discriminator is Track vigor, fertility, abnormality rate, sex expression, trait variance, and molecular homozygosity if measured. Compare selfed families with the parent and, where useful, an outcrossed or noninbred reference under the same environment.
- Do not overextend the conclusion beyond this limit: Theoretical heterozygosity decline is an expectation, not a measurement of realized fixation in a particular line. Selection, viability, linkage, and structural variation can change the observed pattern.
Answer rationale 3: Applied verification rationale
- In practice: Track selfing generation separately from filial generation, compare each selfed family with the parent and outcrossed checks, and record losses rather than discarding them from the analysis.
- Record before action: Record parent identity, starting heterozygosity information when available, selfing method, S generation, seed set, germination, family size, and all losses before phenotype scoring.
- Also record: Track vigor, fertility, abnormality rate, sex expression, trait variance, and molecular homozygosity if measured. Compare selfed families with the parent and, where useful, an outcrossed or noninbred reference under the same environment.
- After the action, repeat the same measurement or observation so the comparison is valid.
- Revise the interpretation if the result conflicts with the lesson limit or the expected response: Theoretical heterozygosity decline is an expectation, not a measurement of realized fixation in a particular line. Selection, viability, linkage, and structural variation can change the observed pattern.
Related lessons
Sources and evidence
- Allard — Principles of Plant BreedingV20-SRC-004
Foundational mating systems, selection, population improvement, backcrossing, and line development; general plant breeding, not Cannabis-specific.
- Falconer and Mackay — Introduction to Quantitative GeneticsV20-SRC-006
Inbreeding, variance, heritability, selection response, and quantitative-trait foundations; model assumptions must be stated.
- Leckie et al. 2024 — Genetic basis of daylength-insensitive floweringV20-SRC-016
Cannabis-specific evidence for loss of photoperiod sensitivity; flowering remains background and environment dependent.
- Flajšman et al. 2021 — Feminized seed production by manipulation of sex expressionV20-SRC-018
Cannabis-specific STS/colloidal-silver studies and progeny context; no universal operating recipe.
- Challenges and potentials of new breeding techniques in Cannabis, 2023V20-SRC-033
Review of Cannabis diversity, breeding, selfing, markers, transformation, and editing limitations.
Downloads
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