🌸 Chapter 4
Principles of Inheritance and Variation
(3 Marks)
1. Explain Mendel’s experiment on
inheritance of one gene (monohybrid cross).
Answer:
- Mendel crossed tall (TT) and
dwarf (tt) pea plants.
- F₁ generation: All tall (Tt).
- F₂ generation (Tt × Tt):
Phenotypic ratio = 3 tall : 1 dwarf; Genotypic ratio = 1 TT : 2 Tt : 1 tt.
Conclusion: Law of Segregation — factors separate during gamete formation.
2. Explain Mendel’s experiment on
inheritance of two genes (dihybrid cross).
Answer:
- Crossed yellow round (YYRR) ×
green wrinkled (yyrr).
- F₁: All yellow round (YyRr).
- F₂: 9 yellow round : 3 yellow
wrinkled : 3 green round : 1 green wrinkled.
Conclusion: Independent Assortment — traits assort independently.
3. What are the reasons for Mendel’s
success in his experiments?
Answer:
- Selected pea plant (Pisum
sativum) — easily cross-pollinated.
- Studied one character at a
time.
- Used large sample size and
mathematical analysis.
- Chose contrasting characters.
- Maintained pure lines.
4. Define the terms: homozygous,
heterozygous, and hemizygous.
Answer:
- Homozygous: Same alleles (TT or tt).
- Heterozygous: Different alleles (Tt).
- Hemizygous: Only one allele present (e.g.,
X-linked genes in males).
5. Differentiate between phenotype
and genotype with examples.
Answer:
- Phenotype: Observable character (Tall).
- Genotype: Genetic makeup (TT or Tt).
- Example: TT and Tt are
genotypically different but phenotypically tall.
6. State and explain Mendel’s Law of
Dominance.
Answer:
- In a heterozygote, only one
allele (dominant) expresses itself, while the other (recessive) remains
hidden.
Example: Tt (Tall × Dwarf) → Tall plant shows dominance.
7. Explain Mendel’s Law of
Segregation.
Answer:
- Allele pairs separate during
gamete formation.
- Each gamete receives one
allele.
- Segregation is pure and random
— no blending.
8. Explain Mendel’s Law of
Independent Assortment.
Answer:
- During gamete formation,
alleles of different genes assort independently.
- Demonstrated by dihybrid cross
— F₂ ratio 9:3:3:1.
9. What is incomplete dominance?
Explain with an example.
Answer:
- Neither allele is completely
dominant.
Example: Mirabilis jalapa:
Red (RR) × White (rr) → F₁: Pink (Rr).
F₂ ratio: 1 Red : 2 Pink : 1 White.
10. Explain co-dominance with an
example.
Answer:
- Both alleles express equally.
Example: Human blood group AB (IAIB) — both A and B antigens appear on RBCs.
11. How is multiple allelism
exhibited in ABO blood grouping?
Answer:
- Gene ‘I’ has three alleles: IA,
IB, i.
- IA and IB are co-dominant; i is
recessive.
- Genotypes:
- A → IAIA or IAi
- B → IBIB or IBi
- AB → IAIB
- O → ii
12. Define pleiotropy with one
example.
Answer:
- One gene affects multiple
traits.
Example: Sickle-cell anemia — single mutation affects RBC shape, oxygen transport, and health.
13. What is polygenic inheritance?
Give an example.
Answer:
- A trait controlled by multiple
genes with additive effects.
Example: Human skin color — determined by 3 pairs of genes.
14. Differentiate between monogenic
and polygenic inheritance.
Answer:
|
Feature |
Monogenic |
Polygenic |
|
Number of genes |
One |
Several |
|
Variation |
Discontinuous |
Continuous |
|
Example |
Tall/dwarf peas |
Human height, skin color |
15. What is a test cross? What is
its purpose?
Answer:
- Cross between F₁ hybrid and
homozygous recessive parent.
- Used to determine unknown
genotype and verify segregation law.
16. Differentiate between back cross
and test cross.
Answer:
|
Type |
Cross |
Purpose |
|
Back cross |
F₁ × any parent |
To maintain traits |
|
Test cross |
F₁ × recessive parent |
To determine genotype |
17. What is a Punnett square?
Answer:
- A grid to predict genotypes and
phenotypes of offspring.
- Helps visualize segregation and
recombination of alleles.
18. What is the Chromosomal Theory
of Inheritance?
Answer:
Proposed by Sutton and Boveri (1902) —
- Genes are located on
chromosomes.
- Chromosomes segregate and
assort independently like Mendelian factors.
- Thus, genes follow chromosomal
behavior.
19. What is linkage? Who discovered
it?
Answer:
- Tendency of genes on the same
chromosome to be inherited together.
- Discovered by Bateson and
Punnett in Lathyrus odoratus.
- Explained by Morgan in Drosophila.
20. What is recombination?
Answer:
- Exchange of genetic material
between homologous chromosomes during meiosis.
- Results in new gene
combinations in gametes.
21. What is crossing over? Explain
its significance.
Answer:
- Exchange of chromosomal
segments between non-sister chromatids during prophase I of meiosis.
Significance: Produces genetic variation.
22. What is a linkage group?
Answer:
- All genes present on one
chromosome form a linkage group.
Example: Humans have 23 linkage groups (22 autosomes + 1 sex chromosome).
23. How did Morgan’s experiment
prove linkage?
Answer:
- Crossed yellow-bodied
white-eyed female × brown-bodied red-eyed male Drosophila.
- Found more parental
combinations than recombinants → genes linked on same chromosome.
24. What determines strength of
linkage between genes?
Answer:
- Distance between genes on the
chromosome.
- Closer genes → stronger linkage
→ fewer recombinants.
25. Define and explain sex-linked
inheritance.
Answer:
- Genes present on sex
chromosomes show sex-linked inheritance.
Example: Haemophilia and color blindness (X-linked recessive traits).
26. Why are males more affected by
X-linked disorders?
Answer:
- Males have only one X
chromosome; presence of a single defective gene leads to disease
expression.
27. Explain the inheritance pattern
of color blindness.
Answer:
- Caused by recessive gene on X
chromosome.
- A color-blind father transmits
it to daughters (carriers), who may transmit it to sons.
28. What is haemophilia? Why is it called
“Royal disease”?
Answer:
- X-linked recessive disorder
causing failure of blood clotting.
- Called Royal disease as it
appeared in Queen Victoria’s descendants.
29. Explain the genetic cause of
sickle-cell anemia.
Answer:
- Point mutation in β-globin gene
(GAG → GTG).
- Glutamic acid replaced by
valine → abnormal HbS → RBCs become sickle-shaped.
30. Differentiate between autosomal
and sex-linked inheritance.
|
Basis |
Autosomal |
Sex-linked |
|
Chromosome |
Non-sex (1–22) |
X or Y |
|
Example |
Sickle-cell anemia |
Haemophilia |
|
Expression |
Both sexes equally |
Mostly males |
31. What is mutation? Explain its
types.
Answer:
- Sudden heritable change in DNA.
Types: - Gene mutation: Change in nucleotide sequence
(e.g., point mutation).
- Chromosomal mutation: Structural or numerical
change.
32. Differentiate between gene and
chromosomal mutations.
|
Type |
Change |
Example |
|
Gene |
DNA sequence |
Sickle-cell anemia |
|
Chromosomal |
Structure or number |
Down’s, Turner’s syndromes |
33. Explain the genetic basis of
Down’s syndrome.
Answer:
- Caused by trisomy of
chromosome 21 (47 total).
- Features: Short stature, flat
face, mental retardation.
- Due to nondisjunction during
meiosis.
34. What are the symptoms and cause
of Klinefelter’s syndrome?
Answer:
- Males with XXY
chromosomes.
- Features: Sterile, small
testes, breast development (gynecomastia).
- Caused by nondisjunction of sex
chromosomes.
35. What are the symptoms and cause
of Turner’s syndrome?
Answer:
- Females with XO
condition (45 chromosomes).
- Features: Sterile, short
stature, undeveloped ovaries.
- Caused by absence of one X
chromosome.
36. Define aneuploidy and
polyploidy.
Answer:
- Aneuploidy: Loss or gain of one chromosome
(2n ± 1).
- Polyploidy: More than two sets of
chromosomes (3n, 4n).
37. What is a pedigree chart?
Explain its symbols.
Answer:
- Diagram showing inheritance of
a trait through generations.
Symbols:
⬛ = male; ⚪ = female; ● = affected; ☐–⚪ = mating; vertical line = offspring.
38. How does a pedigree chart help
in studying genetic disorders?
Answer:
- Identifies carriers,
inheritance pattern (dominant/recessive, X-linked), and risk prediction in
families.
39. Explain the inheritance of ABO
blood groups in humans.
Answer:
- Controlled by gene I with
alleles IA, IB, and i.
- IA and IB are co-dominant; i is
recessive.
- 4 phenotypes: A, B, AB, and O.
40. Differentiate between incomplete
dominance and co-dominance.
|
Feature |
Incomplete dominance |
Co-dominance |
|
Expression |
Intermediate phenotype |
Both alleles expressed |
|
Example |
Mirabilis jalapa (pink flower) |
Blood group AB |
41. Explain pleiotropy using
sickle-cell anemia as an example.
Answer:
- Mutation in one gene causes
multiple effects:
- Deformed RBCs
- Reduced oxygen transport
- Organ damage and anemia.
42. What is gene mapping?
Answer:
- Process of determining gene
positions on chromosomes using recombination frequency (1% = 1 map unit).
43. Explain how recombination
frequency is related to gene distance.
Answer:
- Greater distance → higher
recombination frequency.
- 1% recombination = 1 centimorgan
(cM).
44. What are the possible blood
group combinations in children if parents have blood groups A and B?
Answer:
- Possible genotypes of parents:
IAi × IBi.
- Offspring blood groups: A, B,
AB, or O.
45. Explain the concept of dominance
deviation using an example.
Answer:
- Difference between expected and
observed phenotype due to partial dominance.
Example: Incomplete dominance in Mirabilis jalapa.
46. How do mutations and
recombination contribute to evolution?
Answer:
- Mutation: Creates new alleles.
- Recombination: Rearranges existing alleles.
Together, they increase genetic variability → basis of evolution.
47. What is the difference between
genetic and chromosomal disorders?
|
Basis |
Genetic |
Chromosomal |
|
Cause |
Gene mutation |
Chromosome abnormality |
|
Example |
Sickle-cell anemia |
Down’s syndrome |
48. Explain Morgan’s contribution to
genetics.
Answer:
- Studied Drosophila;
discovered linkage and recombination.
- Explained that genes are
arranged linearly on chromosomes.
49. Why are genetic disorders more
dangerous than infectious diseases?
Answer:
- Inherited, present from birth,
cannot be cured.
- Affect multiple generations and
organs.
50. What are the key differences
between Mendelian and non-Mendelian inheritance?
|
Feature |
Mendelian |
Non-Mendelian |
|
Pattern |
Dominant/recessive |
Incomplete, co-dominance, multiple alleles |
|
Examples |
Pea plant traits |
Blood group, Mirabilis jalapa |
