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Q2(a):
Differentiate between the following –
(a) Dominance and Recessive
Solution :
1. Conceptual Definitions
In classical Mendelian genetics, the interaction between alleles of a single gene determines the phenotype of an organism. An allele is defined as a variant form of a given gene located at a specific locus on a chromosome [Per the Law of Segregation].
- Dominance: A relationship between alleles where one allele masks the expression of the other in a heterozygous state. The allele that expresses itself is termed the "dominant" allele.
- Recessive: A relationship where the phenotype of an allele is only expressed when the organism is homozygous for that trait, meaning it is masked in the presence of a dominant allele.
2. Comparative Analysis Table
| Characteristic | Dominant Trait | Recessive Trait |
|---|---|---|
| Genotypic Expression | Expressed in both homozygous ($AA$) and heterozygous ($Aa$) conditions. | Expressed only in the homozygous ($aa$) condition. |
| Protein Product | Usually produces a functional, active protein or enzyme. | Often results in a non-functional protein or absence of the gene product. |
| Masking Effect | Masks the expression of the recessive allele in $Aa$ genotypes. | Remains unexpressed (hidden) in the presence of a dominant allele. |
| Notation | Represented by a capital letter (e.g., $T$). | Represented by the corresponding lowercase letter (e.g., $t$). |
3. Visual Representation of Allelic Interaction
4. Theoretical Justification
The distinction between dominant and recessive traits is grounded in the Law of Dominance. This law states that in a dissimilar pair of factors (alleles), one member of the pair dominates (dominant) the other (recessive). Biologically, this often occurs because the dominant allele produces a sufficient amount of the functional protein required for the trait's expression, whereas the recessive allele is often a "loss-of-function" mutation that produces either a defective protein or no protein at all [Per Molecular Biology of the Gene].
Final Solution: Dominance is the phenotypic expression of an allele in both heterozygous and homozygous states, whereas Recessivity is the phenotypic expression of an allele restricted strictly to the homozygous state where no dominant allele is present.
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(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
- 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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