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Q13(b):
Explain the following terms with example
(b) Incomplete dominance
Solution :
Definition and Theoretical Foundation
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 which the expressed physical trait is a combination of the phenotypes of both alleles. [Per Mendelian Genetics, this violates the Law of Dominance, which states that the dominant allele should completely mask the recessive allele in the heterozygous condition.]
Mechanism of Action
In incomplete dominance, the phenotypic ratio in the $F_2$ generation deviates from the classic Mendelian ratio of $3:1$. Instead, it follows a genotypic and phenotypic ratio of $1:2:1$ ($1$ homozygous dominant : $2$ heterozygous : $1$ homozygous recessive).
Biological Example: Antirrhinum majus (Snapdragon)
In Antirrhinum majus, flower color is determined by a pair of alleles: $R$ (Red) and $r$ (White). When a pure-breeding red-flowered plant ($RR$) is crossed with a pure-breeding white-flowered plant ($rr$), the $F_1$ generation produces pink flowers ($Rr$).
| Generation | Parental/Offspring Genotype | Phenotype |
|---|---|---|
| Parental ($P$) | $RR \times rr$ | Red $\times$ White |
| $F_1$ Generation | $Rr$ | Pink (Intermediate) |
| $F_2$ Generation | $1RR : 2Rr : 1rr$ | $1$ Red : $2$ Pink : $1$ White |
Visual Representation of the Cross
Genetic Rationale
- Genotype $RR$: The dominant allele produces functional enzymes necessary for full red pigment synthesis.
- Genotype $rr$: The recessive allele produces a non-functional enzyme, resulting in no pigment (White).
- Genotype $Rr$: A single copy of the dominant allele is insufficient to produce the full intensity of the red pigment, leading to a diluted, intermediate pink color. This is an example of dosage-dependent gene expression.
Final Solution: Incomplete dominance is the phenomenon where the dominant allele does not completely mask the recessive allele, resulting in an intermediate phenotype. In the snapdragon (Antirrhinum majus), a cross between red-flowered ($RR$) and white-flowered ($rr$) plants results in pink ($Rr$) $F_1$ offspring, demonstrating this inheritance pattern.
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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