🌸 Chapter 4
Principles of Inheritance and Variation
(4 Marks)
1. Explain Mendel’s Law of
Segregation with an example.
Answer:
- Law: During gamete formation,
alleles of a gene separate so that each gamete receives only one allele.
- Example: In a monohybrid cross between
tall (TT) and dwarf (tt) pea plants, F1 are all tall (Tt). In F2,
segregation produces genotypes TT, Tt, and tt in a 1:2:1 ratio, giving 3
tall : 1 dwarf plants.
- Conclusion: Alleles do not blend but
separate during gamete formation.
2. State Mendel’s Law of Independent
Assortment. Explain with a dihybrid cross.
Answer:
- Law: Genes for different traits
assort independently during gamete formation.
- Example: Crossing round yellow (RRYY)
with wrinkled green (rryy) gives F1 = RrYy (round yellow).
- F2 phenotypic ratio = 9 round
yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
- Conclusion: Inheritance of one character
does not affect another.
3. Explain the difference between
incomplete dominance and co-dominance with examples.
Answer:
|
Basis |
Incomplete Dominance |
Co-dominance |
|
Expression |
Intermediate trait appears |
Both alleles express equally |
|
Example |
Snapdragon (RR × rr → Rr = pink) |
ABO blood group (IAIB = AB) |
|
Result |
Blending appearance |
Equal expression of both traits |
4. What is test cross? Explain its
importance.
Answer:
- Definition: Crossing an individual with a
dominant phenotype with a homozygous recessive individual.
- Example: T? × tt
- Purpose: To determine whether the
dominant phenotype is homozygous (TT) or heterozygous (Tt).
- Importance: Helps confirm Mendel’s Law of
Segregation.
5. Describe the chromosomal theory
of inheritance.
Answer:
- Proposed by Sutton and
Boveri (1902).
- Key points:
- Genes are located on
chromosomes.
- Homologous chromosomes
segregate during meiosis.
- Independent assortment of
chromosomes explains Mendel’s dihybrid ratios.
- Thus, chromosomes are the
carriers of genes and hence of heredity.
6. What is linkage? Explain Morgan’s
experiment on linkage in Drosophila.
Answer:
- Definition: Tendency of genes on the same
chromosome to be inherited together.
- Experiment: Morgan crossed yellow-bodied
white-eyed (X-linked traits) with normal flies.
- Observed that genes did not
assort independently — they were linked.
- Conclusion: Closer the genes on a
chromosome, stronger the linkage.
7. Define recombination. How did
Morgan and Sturtevant explain it?
Answer:
- Definition: Formation of new combinations
of genes due to crossing over.
- Morgan found recombination frequency
depends on distance between genes.
- Sturtevant prepared genetic maps using
recombination frequency (1% = 1 map unit).
- Conclusion: Recombination is the basis for
gene mapping.
8. What is pleiotropy? Explain with
an example.
Answer:
- Definition: When one gene influences
multiple traits.
- Example: In pea plants, the gene for
starch synthesis also affects seed shape.
- Round seeds (RR) have more
starch, wrinkled (rr) have less.
- Human example: Sickle cell anemia gene
affects RBC shape and oxygen transport.
9. Differentiate between genotype
and phenotype.
Answer:
|
Feature |
Genotype |
Phenotype |
|
Meaning |
Genetic constitution |
Observable traits |
|
Example |
TT, Tt, tt |
Tall, dwarf |
|
Dependence |
Fixed by genes |
Affected by genes + environment |
|
Mendelian ratio |
1:2:1 (genotypic) |
3:1 (phenotypic) |
10. Explain multiple allelism with
the example of human ABO blood groups.
Answer:
- Definition: When a gene exists in more
than two allelic forms.
- Example: Gene ‘I’ has three alleles –
IA, IB, i.
- IAIA / IAi = A group
- IBIB / IBi = B group
- IAIB = AB group (co-dominant)
- ii = O group
- Each person carries only two
alleles, though multiple exist in the population.
11. What are sex-linked traits?
Explain with one example.
Answer:
- Traits whose genes are located
on sex chromosomes.
- Example: Haemophilia – a recessive
X-linked disorder.
- Gene on X chromosome → males
(XY) show disease, females (XX) are carriers.
- Conclusion: Males are more frequently
affected due to single X chromosome.
12. What are multiple factors or
polygenic inheritance? Give an example.
Answer:
- Definition: A trait controlled by more
than one gene pair.
- Example: Human skin color, height,
intelligence.
- In skin color, 3 gene pairs
contribute additively.
- More dominant alleles → darker
color.
- Conclusion: Shows continuous variation in
population.
13. Explain the inheritance of sex
in humans.
Answer:
- Females: XX, produce only X-bearing
ova.
- Males: XY, produce both X and
Y-bearing sperms.
- If sperm with X fertilizes →
girl (XX); with Y → boy (XY).
- Conclusion: Father determines the sex of
the child.
14. Explain the mechanism of sex
determination in honeybee.
Answer:
- Haplodiploid mechanism:
- Fertilized eggs (2n) → females
(queen/workers).
- Unfertilized eggs (n) → males
(drones).
- Conclusion: Females are diploid, males
haploid; sex determined by ploidy level.
15. Describe sickle cell anemia and
its inheritance pattern.
Answer:
- Cause: Mutation in β-globin gene (GAG
→ GTG) → Valine replaces Glutamic acid.
- Genotypes:
- HbA HbA – normal
- HbA HbS – carrier (mild)
- HbS HbS – sickle cell anemia
(severe)
- Inheritance: Autosomal recessive; both
parents must carry the defective gene.
16. What is the difference between
autosomes and sex chromosomes?
Answer:
|
Feature |
Autosomes |
Sex Chromosomes |
|
Number |
22 pairs |
1 pair |
|
Function |
Determine somatic traits |
Determine sex and related traits |
|
In Humans |
44 autosomes |
XX (female), XY (male) |
|
Example |
Eye color, height |
Color blindness, haemophilia |
17. What is mutation? Differentiate
between gene and chromosomal mutation.
Answer:
- Mutation: Sudden heritable change in DNA
sequence or chromosome structure.
| Type | Definition | Example |
|------|-------------|----------|
| Gene mutation | Change in DNA base sequence | Sickle cell anemia |
| Chromosomal mutation | Change in chromosome number/structure | Down’s syndrome |
18. Write a short note on Down’s
syndrome.
Answer:
- Cause: Trisomy of chromosome 21.
- Symptoms: Mental retardation, broad
face, short stature, furrowed tongue.
- Karyotype: 47 chromosomes (2n + 1).
- Type: Autosomal aneuploidy.
19. Explain Turner’s syndrome and
Klinefelter’s syndrome.
Answer:
|
Syndrome |
Karyotype |
Characteristics |
|
Turner’s |
45, XO |
Females, short, sterile, underdeveloped ovaries |
|
Klinefelter’s |
47, XXY |
Males, tall, sterile, feminine traits |
20. Define pedigree analysis. How is
it useful?
Answer:
- Definition: Diagram showing inheritance
pattern across generations.
- Uses:
- Identify carriers of genetic
diseases.
- Predict probability of
inheritance.
- Helps in genetic counseling
and disease prevention.
21. Differentiate between dominance,
incomplete dominance, and co-dominance.
Answer:
|
Type |
Expression |
Example |
|
Dominance |
One allele masks another |
Pea plant (Tt = tall) |
|
Incomplete dominance |
Intermediate phenotype |
Snapdragon (Rr = pink) |
|
Co-dominance |
Both alleles express equally |
Blood group (IAIB = AB) |
22. What is gene mapping? Explain
its importance.
Answer:
- Definition: Determining the relative
positions of genes on a chromosome.
- Basis: Recombination frequency.
- Importance:
- Helps locate disease genes.
- Used in genome projects and
plant breeding.
23. What are the reasons for
Mendel’s success in his experiments?
Answer:
- Used pure lines and distinct
contrasting traits.
- Conducted controlled crosses.
- Quantitative analysis of results.
- Studied one/two traits at a
time.
- Maintained statistical
accuracy.
24. Why was Mendel’s work not
recognized immediately?
Answer:
- Published in an obscure
journal.
- Concepts ahead of his time.
- No knowledge of chromosomes or
meiosis then.
- Rediscovered in 1900 by De
Vries, Correns, and Tschermak.
25. Differentiate between linkage
and independent assortment.
Answer:
|
Feature |
Linkage |
Independent Assortment |
|
Definition |
Genes inherited together |
Genes assort independently |
|
Location |
Same chromosome |
Different chromosomes |
|
Effect |
Reduces recombination |
Increases variation |
|
Example |
Drosophila body color & wing size |
Pea seed color & shape |
