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Q13(b):
Explain the following terms with example
(b) Incomplete dominance
Solution :
Theoretical Foundation: Incomplete Dominance
Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in the heterozygous condition that is a physical blend or intermediate of the dominant and recessive phenotypes [Per Mendelian deviation principles].
Step 1: Genetic Mechanism
Unlike the law of dominance where the dominant allele completely masks the recessive allele, in incomplete dominance, the dominant allele ($R$) produces a functional protein (enzyme) at a reduced quantity, insufficient to produce the full-blown dominant phenotype. Consequently, the heterozygous genotype ($Rr$) expresses a phenotype distinct from both the homozygous dominant ($RR$) and homozygous recessive ($rr$) parents.
Step 2: Illustrative Example - Mirabilis jalapa (Four O'Clock Plant)
In the snapdragon (*Antirrhinum majus*) or the Four O'Clock plant, flower color exhibits incomplete dominance. When a homozygous red-flowered plant ($RR$) is crossed with a homozygous white-flowered plant ($rr$):
| Generation | Parental Genotypes | Phenotypic Outcome |
|---|---|---|
| P | $RR$ (Red) $\times$ $rr$ (White) | - |
| $F_1$ | $Rr$ | Pink (Intermediate) |
Step 3: Visual Representation of the Cross
Step 4: Genotypic and Phenotypic Ratios
When the $F_1$ hybrids ($Rr$) are self-crossed:
Ratio Calculation: $Rr \times Rr \rightarrow 1 RR : 2 Rr : 1 rr$
- Genotypic Ratio: $1:2:1$ ($1$ Red : $2$ Pink : $1$ White)
- Phenotypic Ratio: $1:2:1$ ($1$ Red : $2$ Pink : $1$ White)
[Note: In incomplete dominance, the genotypic and phenotypic ratios remain identical, which deviates from the classical Mendelian $3:1$ phenotypic ratio].
Final Solution: Incomplete dominance is a genetic phenomenon where the heterozygote exhibits an intermediate phenotype because the dominant allele does not completely mask the expression of the recessive allele, resulting in a 1:2:1 phenotypic ratio in the F2 generation, as exemplified by the pink flower color in Mirabilis jalapa.
More Questions from Class 12 Biology Principles of Inheritance and Variation EXERCISES
- Q1: Mention the advantages of selecting pea plant for experiment by Mendel.
- Q1: Mention the advantages of selecting pea plant for experiment by Mendel.
- Q2(a): Differentiate between the following – (a) Dominance and Recessive
- Q2(a): Differentiate between the following – (a) Dominance and Recessive
- Q2(b): Differentiate between the following – (b) Homozygous and Heterozygous
- Q2(b): Differentiate between the following – (b) Homozygous and Heterozygous
- Q2(c): Differentiate between the following – (c) Monohybrid and Dihybrid.
- Q2(c): Differentiate between the following – (c) Monohybrid and Dihybrid.
- Q3: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
- Q3: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
- Q4: Explain the Law of Dominance using a monohybrid cross.
- Q4: Explain the Law of Dominance using a monohybrid cross.
- Q5: Define and design a test-cross.
- Q5: Define and design a test-cross.
- Q6: Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
- Q6: Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
- Q7(a): When a cross in made between tall plant with yellow seeds (\(TtYy\)) and tall plant with green seed (\(Ttyy\)), what proportions of phenotype in the offspring could be expected to be (a) tall and green.
- Q7(a): When a cross in made between tall plant with yellow seeds ((TtYy)) and tall plant with green seed ((Ttyy)), what proportions of phenotype in the offspring could be expected to be (a) tall and green.
- Q7(b): When a cross in made between tall plant with yellow seeds (\(TtYy\)) and tall plant with green seed (\(Ttyy\)), what proportions of phenotype in the offspring could be expected to be (b) dwarf and green.
- Q7(b): When a cross in made between tall plant with yellow seeds ((TtYy)) and tall plant with green seed ((Ttyy)), what proportions of phenotype in the offspring could be expected to be (b) dwarf and green.
- Q8: Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in \(F_{1}\) generation for a dibybrid cross?
- Q8: Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in (F_{1}) generation for a dibybrid cross?
- Q9: Briefly mention the contribution of T.H. Morgan in genetics.
- Q9: Briefly mention the contribution of T.H. Morgan in genetics.
- Q10: What is pedigree analysis? Suggest how such an analysis, can be useful.
- Q10: What is pedigree analysis? Suggest how such an analysis, can be useful.
- Q11: How is sex determined in human beings?
- Q11: How is sex determined in human beings?
- Q12: A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
- Q12: A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
- Q13(a): Explain the following terms with example (a) Co-dominance
- Q13(a): Explain the following terms with example (a) Co-dominance
- Q13(b): Explain the following terms with example (b) Incomplete dominance
- Q14: What is point mutation? Give one example.
- Q14: What is point mutation? Give one example.
- Q15: Who had proposed the chromosomal theory of the inheritance?
- Q15: Who had proposed the chromosomal theory of the inheritance?
- Q16: Mention any two autosomal genetic disorders with their symptoms.
- Q16: Mention any two autosomal genetic disorders with their symptoms.
CBSE Solutions for Class 12 Biology Principles of Inheritance and Variation
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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