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Q4:
Explain the Law of Dominance using a monohybrid cross.
Solution :
Step 1: Theoretical Foundation of the Law of Dominance
The Law of Dominance, proposed by Gregor Mendel, states that in a heterozygous organism (possessing two different alleles for a trait), one allele expresses itself in the phenotype (the dominant allele), while the other is masked (the recessive allele). This law is based on the postulate that characters are controlled by discrete units called "factors," which occur in pairs [Per Mendel’s Postulate of Unit Factors].
Step 2: Defining the Monohybrid Cross Parameters
To demonstrate this law, consider the height of pea plants ($T$ for tall, $t$ for dwarf):
- Parental Generation ($P$): Pure-breeding Tall ($TT$) $\times$ Pure-breeding Dwarf ($tt$).
- Gametes: $TT$ produces only $T$ gametes; $tt$ produces only $t$ gametes.
- $F_1$ Generation: The fusion of gametes results in $Tt$ genotype.
Step 3: Visual Representation (Monohybrid Cross)
Step 4: Analytical Breakdown of the $F_1$ Phenotype
Upon crossing homozygous parents ($TT$ and $tt$), the resulting offspring ($F_1$ generation) all exhibit the genotype $Tt$. Despite the presence of the recessive allele ($t$), all $F_1$ individuals display the "Tall" phenotype. [By Law of Dominance, the $T$ allele masks the phenotypic expression of the $t$ allele].
Step 5: Comparative Analysis Table
| Generation | Genotype | Phenotype | Observation |
|---|---|---|---|
| Parental ($P$) | $TT$ / $tt$ | Tall / Dwarf | Homozygous traits |
| $F_1$ | $Tt$ | Tall | Dominant allele masks recessive |
Conclusion of Principles
The Law of Dominance is validated here because the phenotype of the $F_1$ hybrid is identical to one of the parents, not an intermediate blend. The recessive trait ($t$) is not lost but remains latent, as it reappears in the $F_2$ generation when self-pollinated, proving the particulate nature of inheritance [Per Law of Segregation].
Final Solution: The Law of Dominance is demonstrated by the observation that all $F_1$ progeny of a cross between homozygous tall ($TT$) and dwarf ($tt$) plants are phenotypically tall ($Tt$), confirming that the tall allele is dominant over the dwarf allele.
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.
- 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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