Table of Contents
In Mendelian inheritance, different versions of a gene are called alleles. When an individual has two alleles for the same gene (one from each parent), how these alleles “interact” determines the trait (phenotype). Three fundamental patterns are:
- Dominant–recessive
- Co-dominant (treated in more detail elsewhere)
- Intermediate (often called incomplete dominance)
This chapter focuses on distinguishing dominant, recessive, and intermediate inheritance at the level of single-gene (monohybrid) traits and their typical patterns in offspring.
Dominant and Recessive Inheritance
Basic idea
For a gene with two alleles, we often use:
A= dominant allelea= recessive allele
The combination of alleles (genotype) can be:
AA– homozygous dominantAa– heterozygousaa– homozygous recessive
In strict dominant–recessive inheritance:
- The dominant allele
Ais expressed whenever it is present: - Phenotype of
AAandAais the same (dominant trait). - The recessive allele
ais only visible when paired with itself: - Phenotype of
aashows the recessive trait.
Thus, the heterozygote (Aa) “looks” like the dominant homozygote (AA).
Classical example: flower color
Suppose:
A= allele for purple flowers (dominant)a= allele for white flowers (recessive)
Genotype–phenotype relationship:
AA→ purpleAa→ purpleaa→ white
If you cross two pure-breeding lines:
- Parental generation (P):
AA(purple) ×aa(white)
F₁ (first filial generation):
- All offspring:
Aa→ all purple
If you then cross two F₁ plants:
- F₁:
Aa×Aa
The genotypes in F₂:
- 1/4
AA, 1/2Aa, 1/4aa
Phenotypes:
- 3/4 purple (dominant) : 1/4 white (recessive)
So, the classical 3 : 1 phenotypic ratio in F₂ is characteristic of strict dominant–recessive inheritance in a monohybrid cross.
Recognizing dominant–recessive inheritance
Key characteristics:
- Heterozygotes resemble one of the homozygotes (the dominant one).
- The recessive trait “skips” the F₁ generation when crossing two pure lines.
- In F₂, the recessive phenotype reappears in a 3 : 1 dominant : recessive ratio.
- Two parents with the dominant phenotype can have offspring with the recessive phenotype (if both are heterozygous).
Intermediate (Incomplete) Dominance
Basic idea
In intermediate (incomplete) dominance:
- Neither allele is completely dominant over the other.
- The heterozygote (
A a) shows a phenotype that is intermediate between the two homozygotes (A Aanda a).
Here, genotype and phenotype correspond more directly:
AA– one extreme phenotypeAa– intermediate (mixed) phenotypeaa– opposite extreme phenotype
This leads to three distinct phenotypes in a monohybrid cross.
Classical example: flower color with intermediate inheritance
Assume:
R= allele for red flowersr= allele for white flowers
Genotype–phenotype relationship:
RR→ redRr→ pink (intermediate)rr→ white
Cross of pure-breeding plants:
- P:
RR(red) ×rr(white)
F₁:
- All offspring:
Rr→ all pink
Cross F₁ with itself:
- F₁:
Rr×Rr
Genotypes in F₂:
- 1/4
RR, 1/2Rr, 1/4rr
Phenotypes in F₂:
- 1/4 red : 1/2 pink : 1/4 white
So the F₂ phenotypic ratio is 1 : 2 : 1, which mirrors the genotype ratio. This distinguishes intermediate dominance from strict dominant–recessive patterns.
Molecular/physiological idea (simplified)
A typical underlying mechanism is dosage of gene product:
RR: “full dose” of pigment → redRr: “half dose” → less pigment → pinkrr: no pigment → white
The intermediate phenotype arises because one functional allele does not produce enough product to reach the same level as two alleles.
Recognizing intermediate inheritance
Key characteristics:
- Heterozygotes have a phenotype distinct from both homozygotes.
- F₁ offspring of two contrasting pure lines show a new, intermediate trait.
- In F₂, the three phenotypes occur in a 1 : 2 : 1 ratio.
- The phenotypic ratio in F₂ is the same as the genotypic ratio.
Comparing Dominant–Recessive vs. Intermediate Inheritance
Phenotypes of genotypes
| Genotype | Dominant–recessive example | Intermediate example |
|---|---|---|
AA | dominant phenotype | phenotype 1 (extreme) |
Aa | dominant phenotype | intermediate phenotype |
aa | recessive phenotype | phenotype 2 (extreme) |
Result:
- Dominant–recessive: 2 phenotypes
- Intermediate: 3 phenotypes
F₂ phenotypic ratios after crossing pure lines
- Dominant–recessive (e.g.,
AA×aa): - F₂: 3 dominant : 1 recessive
- Intermediate (
RR×rr): - F₂: 1 phenotype of
RR: 2 phenotype ofRr: 1 phenotype ofrr
Mendel’s laws and these patterns
- Both dominant–recessive and intermediate inheritance obey Mendel’s laws (segregation and independent assortment).
- The difference lies in how the alleles affect the phenotype, not in how they are passed on.
- The heterozygote pattern (same as dominant homozygote vs. intermediate) tells you which type of inheritance you see.
Practical Consequences and Simple Predictions
Predicting outcomes (Punnett-square logic)
You can use Punnett squares for both patterns; only the interpretation of the heterozygote changes.
Example for intermediate inheritance, RR × Rr:
- Gametes:
RfromRR;RandrfromRr. - Offspring genotypes:
- 1/2
RR(red), 1/2Rr(pink)
So phenotypes: 1/2 red, 1/2 pink.
For dominant–recessive with AA × Aa:
- Genotypes: 1/2
AA, 1/2Aa - All offspring show dominant phenotype.
Hidden alleles vs. visible alleles
- In dominant–recessive inheritance, recessive alleles can be carried silently in heterozygotes (phenotypically invisible).
- In intermediate inheritance, each genotype produces a distinct phenotype, so carrier states are often directly recognizable (e.g., pink plants are always heterozygous).
This difference is important in breeding and in risk calculations for traits that follow simple Mendelian patterns.
Distinguishing Intermediate Dominance from Blending
Intermediate dominance resembles “mixing” of traits, but:
- Genes do not blend irreversibly.
- In F₂, the extreme parental phenotypes reappear (e.g., red and white), showing that the original alleles remain intact.
This reversibility and reappearance of parental types distinguish intermediate dominance from the outdated “blending inheritance” idea, in which hereditary material would mix permanently.
Summary of Key Points
- Dominant allele: determines phenotype in both
AAandAa. - Recessive phenotype appears only in
aa. - F₂ from pure-line cross: 3 : 1 phenotypic ratio.
- Intermediate (incomplete) dominance: heterozygote phenotype lies between the two homozygotes.
- Three distinct phenotypes (
AA,Aa,aa). - F₂ from pure-line cross: 1 : 2 : 1 phenotypic ratio.
- Both patterns follow Mendel’s laws; they differ in phenotype formation, not in allele segregation.
- Intermediate dominance is not true “blending inheritance,” because parental phenotypes reappear in later generations.