🧬 Allelic Gene Interactions — Incomplete Dominance, Codominance & Lethal Genes
Study allelic interactions — incomplete dominance (1:2:1), codominance and lethal genes for CUET Agriculture. Snapdragon and ABO blood groups.
Gene interactions refer to the ways in which genes influence the expression of phenotypic traits. These interactions can be between alleles of the same gene (allelic/intragenic) or between alleles of different genes (non-allelic/intergenic/epistatic). Understanding gene interactions is essential because they explain why not all inheritance patterns follow the simple 3:1 Mendelian ratio.
Two types of gene interactions:
- Allelic gene interactions (Intragenic) — interaction between alleles of the same gene at the same locus
- Non-allelic gene interactions (Intergenic/Epistatic) — interaction between genes at different loci
NOTE
Allelic interactions modify the monohybrid ratio (3:1), while non-allelic interactions modify the dihybrid ratio (9:3:3:1). This lesson covers allelic interactions; epistasis is covered in the next lesson.
I. Allelic Gene Interactions
1. Incomplete Dominance (Blending Inheritance)
According to Mendel's law of dominance, all F1 offspring should show the dominant phenotype. However, in some organisms, F1 offspring from two contrasting parents show an intermediate phenotype — neither fully dominant nor fully recessive. This is called incomplete dominance.
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Gene interactions refer to the ways in which genes influence the expression of phenotypic traits. These interactions can be between alleles of the same gene (allelic/intragenic) or between alleles of different genes (non-allelic/intergenic/epistatic). Understanding gene interactions is essential because they explain why not all inheritance patterns follow the simple 3:1 Mendelian ratio.
Two types of gene interactions:
- Allelic gene interactions (Intragenic) — interaction between alleles of the same gene at the same locus
- Non-allelic gene interactions (Intergenic/Epistatic) — interaction between genes at different loci
NOTE
Allelic interactions modify the monohybrid ratio (3:1), while non-allelic interactions modify the dihybrid ratio (9:3:3:1). This lesson covers allelic interactions; epistasis is covered in the next lesson.
I. Allelic Gene Interactions
1. Incomplete Dominance (Blending Inheritance)
According to Mendel's law of dominance, all F1 offspring should show the dominant phenotype. However, in some organisms, F1 offspring from two contrasting parents show an intermediate phenotype — neither fully dominant nor fully recessive. This is called incomplete dominance.
- In incomplete dominance, the dominant allele cannot completely mask the recessive allele
- Both alleles are present but the dominant allele fails to express its effect fully
- The result is a blend or intermediate phenotype — for example, a cross between red and white flowers producing pink flowers
(a) Snapdragon / Antirrhinum majus (Dog Flower / Gul-e-Bans)
First discovered by Carl Correns in Mirabilis jalapa (4 O'Clock Plant). Also found in Antirrhinum majus (Snapdragon).
In Mirabilis, flower colour has 3 phenotypes based on colour: Red, White, and Pink.
Parents: Red (RR) × White (rr)
↓
F1: All Pink (Rr)
↓ (Self pollination)
F2: Red (RR) : Pink (Rr) : White (rr)
| Phenotypic Ratio | Genotypic Ratio | |
|---|---|---|
| F2 | 1 Red : 2 Pink : 1 White | 1 RR : 2 Rr : 1 rr |
| Ratio | 1 : 2 : 1 | 1 : 2 : 1 |
IMPORTANT
In incomplete dominance, the phenotypic ratio equals the genotypic ratio (1:2:1). The standard F2 ratio of 3:1 is modified to 1:2:1. This is because we can visually distinguish all three genotypes by their phenotypes.
- When pink (Rr) plants are crossed with pink (Rr), the offspring show the normal monohybrid ratio of 3:1 for flower colour (Red + Pink : White = 3:1), but the phenotypic ratio is 1:2:1 because the heterozygote is visually distinct.
(b) Andalusian Fowls (Feather Colour)
Incomplete dominance is also seen in Andalusian fowls for feather colour:
Parents: Black × White (Splashed White)
F1: All Blue (Andalusian Blue)
F2: 1 Black : 2 Blue : 1 Splashed White
- When a black-feathered fowl is crossed with a splashed white one, the F1 generation has blue feathers (intermediate colour)
- F1 is heterozygous — the blue colour is a result of neither the black nor the white allele being fully dominant
- This is a classic example used in CUET exams
(c) Snapdragon (Antirrhinum majus) — Flower Shape
Incomplete dominance is also seen for flower shape: normal flowers crossed with peloric flowers give intermediate flowers in F1. This shows that incomplete dominance is not limited to colour — it can affect any trait.
(d) Mend Kapni (Mandible shape)
When large-jawed and small-jawed organisms are crossed, F1 shows intermediate mandible shape. When medium-shaped F1 are crossed among themselves, the F2 ratio is 1:2:1.
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Gene interactions — two types | Allelic (intragenic) = same gene's alleles interact Non-allelic (intergenic/epistatic) = different genes interact |
| Allelic interactions modify | Monohybrid ratio (3:1) |
| Non-allelic interactions modify | Dihybrid ratio (9:3:3:1) |
| Incomplete dominance | F1 shows intermediate phenotype; dominant allele cannot fully mask recessive |
| Incomplete dominance discovered by | Carl Correns in Mirabilis jalapa (4 O'Clock Plant) |
| Incomplete dominance F2 ratio | 1 : 2 : 1 (phenotypic = genotypic) |
| Snapdragon flower colour | Red (RR) × White (rr) → F1 all Pink (Rr) → F2: 1 Red : 2 Pink : 1 White |
| Andalusian fowls | Black × Splashed White → F1 Blue → F2: 1 Black : 2 Blue : 1 Splashed White |
| Codominance | Both alleles express independently and simultaneously in heterozygote; distinct from blending |
| Cattle coat colour (Roan) | Black (R1R1) × White (R2R2) → F1 Roan (R1R2); F2 = 1 Black : 2 Roan : 1 White |
| ABO blood group | 3 alleles (I^A, I^B, i); 4 phenotypes (A, B, AB, O); 6 genotypes |
| ABO — codominance | I^A and I^B are codominant to each other; both dominant over i |
| AB blood group | Universal Recipient (genotype I^A I^B) |
| O blood group | Universal Donor (genotype ii) |
| Multiple alleles genotype formula | n(n+1)/2 (e.g., 3 alleles → 6 genotypes; 4 alleles → 10 genotypes) |
| MN blood group | Genotype L^M L^N — both M and N antigens present; classic codominance |
| Sickle cell anaemia | Pleiotropic gene; carrier (Hb^A Hb^S) shows both normal and sickle RBCs (codominance) |
| Sickle cell molecular basis | Beta-globin chain position 6: Glutamic acid → Valine DNA: GAG → GTG; mRNA: GAG → GUG |
| Sickle cell carrier advantage | Carriers (Hb^A Hb^S) are resistant to malaria (Plasmodium falciparum) |
| Multiple alleles | Gene exists in >2 allelic forms in population; only 2 alleles per diploid individual |
| Rabbit coat colour hierarchy | C > c^ch > c^h > c (Full colour > Chinchilla > Himalayan > Albino); 4 alleles → 10 genotypes |
| Lethal genes | Homozygous condition causes death; modifies 3:1 ratio to 2:1 |
| Yellow mice (Cuenot) | YY = lethal; ratio = 2 Yellow : 1 Brown; yellow mice always heterozygous (Yy) |
| Lethal genes in Snapdragon | E. Baur; GG = lethal (no chlorophyll); ratio = 2 Golden : 1 Green |
| Pleiotropic genes | One gene affects multiple phenotypic traits |
| PKU (Phenylketonuria) | Single gene mutation → mental retardation, light hair, light skin, reduced melanin |
| Pleiotropy in pea | SBE gene: BB = round seeds + large starch grains; bb = wrinkled + small starch grains |
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