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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 a variant form of a gene located at a specific locus on a chromosome.
2. Comparative Analysis of Dominance and Recessivity
The distinction between these two states is fundamental to understanding how traits are expressed in a diploid organism. The following table delineates these differences based on genetic behavior and phenotypic expression.
| Feature | Dominant Trait | Recessive Trait |
|---|---|---|
| Phenotypic Expression | Expressed in both homozygous ($AA$) and heterozygous ($Aa$) states. | Expressed only in the homozygous ($aa$) state. |
| Masking Effect | Masks the expression of the recessive allele in a heterozygote. | Masked/hidden in the presence of a dominant allele. |
| Symbolic Representation | Represented by a capital letter (e.g., $T$). | Represented by the corresponding lowercase letter (e.g., $t$). |
| Functional Basis | Usually codes for a functional, active protein/enzyme. | Often results in a non-functional enzyme or truncated protein. |
3. Mathematical Representation of Genetic Ratios
When considering a monohybrid cross between two pure-breeding parents ($TT$ and $tt$), the $F_1$ generation results in $100\%$ heterozygous offspring ($Tt$). Per the Law of Dominance, the dominant allele ($T$) overrides the recessive allele ($t$).
4. Theoretical Justification
- Law of Dominance: [Postulated by Gregor Mendel], this law states that in a heterozygote, one allele masks the presence of another. The allele that manifests the phenotype is dominant; the one that is suppressed is recessive.
- Genomic Basis: A dominant allele produces a normal, functional enzyme, whereas the recessive allele is often a mutated version that produces either no enzyme or a defective one. Consequently, the single functional allele in a heterozygote ($Aa$) is sufficient to produce the phenotype associated with the dominant trait [Per the Concept of Genetic Dosage].
Final Solution: Dominance is the phenomenon where one allele masks the phenotypic expression of its partner allele in a heterozygous genotype, whereas Recessivity is the condition where an allele's phenotypic expression is only manifested when the organism is homozygous for that specific 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(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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