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Q9:
Briefly mention the contribution of T.H. Morgan in genetics.
Solution :
1. Foundation of Chromosomal Theory of Inheritance
Thomas Hunt Morgan (1866–1945) utilized the fruit fly, Drosophila melanogaster, to provide experimental evidence for the Chromosomal Theory of Inheritance. His work transitioned genetics from purely mathematical probability to a physically observable science linked to cellular structures [Per the Sutton-Boveri Chromosomal Theory].
2. The Drosophila melanogaster Model System
Morgan selected Drosophila for several analytical advantages:
- Life Cycle: Short life cycle (approximately 12–14 days), allowing for multiple generations within weeks.
- High Fecundity: A single mating produces hundreds of offspring.
- Cytological Advantage: Clear differentiation between male and female and a low diploid chromosome number ($2n = 8$).
- Variability: Existence of distinct, heritable morphological variations.
3. The Discovery of Sex-Linked Inheritance
Morgan observed a white-eyed male mutation in a population of red-eyed flies. Upon crossing, he noted that the inheritance pattern of the white-eye trait correlated specifically with the $X$ chromosome, effectively proving that genes are located on specific chromosomes [By application of Mendelian principles to cytological observation].
4. Linkage and Recombination
Morgan’s most significant contributions involve the behavior of genes located on the same chromosome:
- Linkage: The physical association of genes on the same chromosome. He observed that traits like "body color" and "wing size" did not assort independently, contradicting Mendel’s Law of Independent Assortment for linked genes.
- Recombination: He identified that the frequency of recombination between linked genes is proportional to the physical distance between them on the chromosome.
5. Analytical Framework: Genetic Mapping
Morgan’s student, Alfred Sturtevant, used Morgan's recombination frequencies to map the relative positions of genes on chromosomes. This methodology established the concept of the Map Unit or Centimorgan (cM), where $1$ cM is equivalent to a $1\%$ recombination frequency [By the Principle of Genetic Linkage Mapping].
| Concept | Morgan's Contribution |
|---|---|
| Linkage | Genes on the same chromosome travel together during meiosis. |
| Recombination | Physical crossing-over generates new allelic combinations. |
| Gene Mapping | Recombination frequency determines the linear order of genes. |
Final Solution: Thomas Hunt Morgan established the field of experimental genetics by proving that genes are physically located on chromosomes, identifying the phenomenon of sex-linkage, demonstrating the principles of genetic linkage and recombination, and providing the framework for the construction of genetic maps.
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.
- 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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