ROUTERA

Chapter 5 Principles of Inheritance and Variation

Class 12th Biology Chapter hots

1. Explain Mendel’s laws of inheritance with suitable examples. Why are they considered fundamental to genetics?

Answer:
Mendel's laws include:

  1. Law of Dominance: One allele masks the effect of another in a heterozygote. Example: In pea plants, tall (T) is dominant over dwarf (t).
  2. Law of Segregation: During gamete formation, alleles segregate randomly, ensuring only one allele is passed on.
  3. Law of Independent Assortment: Alleles of different genes assort independently, as seen in dihybrid crosses.

Importance:

  • Explains genetic inheritance patterns.
  • Forms the basis for modern genetic research.

2. Discuss the chromosomal theory of inheritance. How did Sutton and Boveri validate Mendel’s principles?

Answer:
Chromosomal Theory:

  • Genes are located on chromosomes.
  • Chromosomal behavior during meiosis mirrors Mendelian segregation and assortment.

Validation:

  • Sutton and Boveri demonstrated that chromosomes segregate during gamete formation, similar to Mendel’s factors.
  • They correlated chromosomal movement with inheritance patterns.

3. Compare incomplete dominance and codominance with examples. How do they deviate from Mendel’s observations?

Answer:

Principles of Inheritance and Variation

Deviation:

  • Mendel assumed complete dominance, but these patterns show exceptions where neither allele completely masks the other.

4. How does polygenic inheritance differ from pleiotropy? Illustrate with examples.

Answer:
Polygenic Inheritance: Multiple genes influence a single trait. Example: Human skin color involves at least three genes.
Pleiotropy: A single gene affects multiple traits. Example: Marfan syndrome gene affects the heart, eyes, and skeletal system.

Differences:

  • Polygenic: Many genes, one trait.
  • Pleiotropy: One gene, many traits.

5. Explain the inheritance pattern of ABO blood groups. Why is it an example of multiple alleles and codominance?

Answer:

  • ABO blood groups are determined by three alleles: IA, IB, and i.
  • IA and IB are codominant, while i is recessive.
  • Genotypes include IAIA (blood group A), IBIB (B), IAIB (AB), and ii (O).

Significance:

  • Demonstrates codominance (AB expresses both IA and IB) and multiple alleles (more than two alleles exist in the population).

6. Describe the significance of linkage and recombination in genetics. How did Morgan's experiments with Drosophila validate these concepts?

Answer:
Linkage: Genes located close on a chromosome are inherited together.
Recombination: Crossing over leads to new allele combinations.

Morgan’s Experiment:

  • Crossed flies with eye color and wing type traits.
  • Observed higher parental type frequencies due to linkage.
  • Identified recombinant types resulting from crossing over.

7. What is the role of sex chromosomes in determining gender? Explain with the human XX-XY system.

Answer:

  • Females (XX): Homogametic; all ova carry an X chromosome.
  • Males (XY): Heterogametic; half sperm carry X and half carry Y.
  • The Y chromosome carries the SRY gene, triggering male development.

8. Discuss the significance of pedigree analysis in studying human inheritance patterns.

Answer:
Significance:

  • Identifies inheritance patterns (dominant, recessive, X-linked).
  • Tracks genetic disorders through generations.
  • Helps predict genetic risks in offspring.

9. What are sex-linked traits? Explain with examples of X-linked recessive disorders.

Answer:
Sex-linked traits are associated with genes on sex chromosomes.
Examples of X-linked recessive disorders:

  • Hemophilia: Affects blood clotting.
  • Color blindness: Affects color vision.

Males (XY) are more affected due to the lack of a second X chromosome to mask the mutation.

10. How do mutations contribute to genetic variation? Classify mutations based on their effects.

Answer:
Contribution:

  • Alter DNA sequence, creating new alleles.
  • Drives evolution by introducing diversity.

Classification:

  1. Silent Mutation: No effect on protein function.
  2. Missense Mutation: Alters amino acid sequence.
  3. Nonsense Mutation: Produces a premature stop codon.
  4. Frameshift Mutation: Shifts the reading frame, altering downstream amino acids.

11. What is the significance of test crosses in genetics? How do they validate Mendelian predictions?

Answer:
A test cross involves crossing an individual with a homozygous recessive.
Significance:

  • Determines the genotype of an individual with a dominant phenotype.
  • Validates Mendelian segregation ratios.

12. How does genetic drift affect allele frequencies in a population? Discuss with examples.

Answer:
Genetic Drift: Random changes in allele frequencies, especially in small populations.
Examples:

  • Bottleneck Effect: Sudden population reduction (e.g., cheetah populations).
  • Founder Effect: A small group establishes a new population with limited genetic diversity.

13. Compare autosomal dominant and autosomal recessive inheritance with examples.

Answer:

Principles of Inheritance and Variation

14. What are lethal alleles? Explain their impact with examples.

Answer:
Lethal Alleles: Mutations causing death before reproduction.
Example: Yellow coat color in mice (homozygous yellow is lethal).
Impact: Alters Mendelian ratios by removing lethal homozygotes.

15. Discuss how environment influences gene expression using examples.

Answer:
Example: Himalayan rabbit fur color depends on temperature.
Mechanism: Certain genes are activated or suppressed based on environmental cues.

16. What is epistasis? Explain with an example.

Answer:
Epistasis: One gene masks the expression of another.
Example: In mice, the C gene controls pigmentation, and the A gene controls coat color pattern. If C is absent, no pigment is produced, regardless of the A gene.

17. Explain how human genetic disorders like Down syndrome arise due to chromosomal abnormalities.

Answer:
Down Syndrome: Trisomy of chromosome 21.
Cause: Nondisjunction during meiosis.
Symptoms: Cognitive impairment, facial features, heart defects.

18. Discuss the role of pleiotropy in genetic disorders like sickle-cell anemia.

Answer:

  • Sickle-cell gene: Affects hemoglobin structure.
  • Pleiotropic Effects: Impairs oxygen transport, damages organs, and causes anemia.

19. How does X-inactivation in females ensure dosage compensation?

Answer:

  • One X chromosome in females is randomly inactivated (Barr body).
  • Ensures equal expression of X-linked genes in males and females.

20. Explain Hardy-Weinberg equilibrium and the factors disrupting it.

Answer:
Equilibrium: Allele frequencies remain constant if no evolutionary forces act.
Disrupting Factors: Mutation, gene flow, genetic drift, selection, non-random mating.

21. How is sickle-cell anemia an example of balanced polymorphism?

Answer:

  • Heterozygotes: Resistant to malaria.
  • Homozygotes: Affected by sickle-cell disease.
  • Maintains both alleles in the population.

22. Explain why Mendel’s work went unrecognized during his lifetime.

Answer:

  • Complexity of statistical analysis.
  • Lack of knowledge about chromosomes.
  • Published in an obscure journal.

23. How do dihybrid crosses validate the law of independent assortment?

Answer:

  • Cross involving two traits results in a 9:3:3:1 phenotypic ratio.
  • Confirms genes assort independently during gamete formation.

24. Discuss the role of linkage maps in understanding gene locations.

Answer:

  • Linkage maps: Represent gene positions based on recombination frequencies.
  • Used to identify loci associated with genetic traits.

25. How does genetic counseling help manage hereditary disorders?

Answer:

  • Identifies carrier status.
  • Predicts risks for offspring.
  • Recommends preventive measures or treatment options.