🌸 Chapter 4
Principles of Inheritance and Variation
(5 Marks)
25 Questions (5 Marks Each) with Answers
1. Describe Mendel’s monohybrid
cross in detail. What conclusions did he draw from it?
Answer:
- Cross: Tall (TT) × Dwarf (tt) → F1:
All Tall (Tt).
- F2 Generation: F1 selfed → TT, Tt, tt → ratio
3 Tall : 1 Dwarf (Phenotypic).
Genotypic ratio = 1:2:1. - Conclusions:
1.
Traits
are controlled by factors (genes) occurring in pairs.
2.
One
factor (dominant) masks the other (recessive).
3.
Alleles
segregate during gamete formation — Law of Segregation.
4.
Each
gamete receives only one allele from each pair.
2. Explain Mendel’s dihybrid cross
experiment. What law did he derive from it?
Answer:
- Cross: Round Yellow (RRYY) × Wrinkled
Green (rryy).
- F1 Generation: RrYy (Round Yellow).
- F2 Generation: Produced 4 phenotypes –
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green. - Law Derived: Law of Independent
Assortment – Alleles of different genes assort independently during
gamete formation.
- Conclusion: Inheritance of one trait does
not affect another.
3. Explain all three Mendelian laws
of inheritance with suitable examples.
Answer:
1.
Law
of Dominance:
One allele expresses (dominant), other remains hidden (recessive). Example:
Tall (T) dominant over dwarf (t).
2.
Law
of Segregation:
Alleles separate during gamete formation. Example: Monohybrid cross of Tt →
1:2:1 ratio.
3.
Law
of Independent Assortment:
Alleles of different traits assort independently. Example: Dihybrid cross
giving 9:3:3:1 ratio.
4. What are the reasons for Mendel’s
success in his experiments?
Answer:
- Selection of Pisum sativum
(pea plant) with clear contrasting traits.
- Studied one or two traits
at a time.
- Used pure lines.
- Maintained accurate records
and used statistical analysis.
- Considered large samples
and generations.
- Performed reciprocal crosses
to confirm results.
- Observed quantitative ratios
and drew general conclusions.
5. Explain incomplete dominance and
co-dominance with examples.
Answer:
|
Feature |
Incomplete Dominance |
Co-dominance |
|
Expression |
Intermediate phenotype |
Both alleles express equally |
|
Example |
Snapdragon: Red (RR) × White (rr) → Pink (Rr) |
ABO blood group: IAIB = AB |
|
Ratio |
1 Red : 2 Pink : 1 White |
All AB show both antigens |
|
Genetic Mechanism |
Neither allele fully dominant |
Both alleles equally dominant |
6. Discuss the chromosomal theory of
inheritance. Who proposed it and what evidences support it?
Answer:
- Proposed by: Sutton and Boveri (1902).
- Postulates:
1.
Genes
are located on chromosomes.
2.
Homologous
chromosomes segregate during meiosis.
3.
Independent
assortment of chromosomes explains variation.
- Evidences:
o Parallel behavior between genes and
chromosomes.
o Each parent contributes one set of
chromosomes to offspring.
Conclusion: Chromosomes are physical carriers
of heredity.
7. What is linkage? Explain its
types with example.
Answer:
- Definition: Tendency of genes located
close together on the same chromosome to be inherited together.
- Types:
1.
Complete
linkage: No
recombination; genes inherited together (e.g., male Drosophila).
2.
Incomplete
linkage: Partial
recombination occurs due to crossing over (e.g., Morgan’s experiment in
Drosophila).
- Conclusion: Linkage strength inversely
proportional to distance between genes.
8. What is recombination? Explain
its importance and application.
Answer:
- Definition: Exchange of genetic material
between homologous chromosomes during meiosis (crossing over).
- Importance:
1.
Produces
genetic variation.
2.
Basis
for evolution.
3.
Used
in genetic mapping.
- Example: Morgan and Sturtevant used
recombination frequency to prepare gene maps in Drosophila.
9. What is pleiotropy? Explain with
the help of suitable examples.
Answer:
- Definition: A single gene influencing
multiple traits.
- Examples:
1.
Pea
plant: Gene for
starch synthesis affects seed shape (round/wrinkled).
2.
Human: Sickle cell anemia gene affects RBC
shape, oxygen transport, and causes anemia.
- Significance: Demonstrates that a gene may
have multiple phenotypic effects.
10. Explain the concept of multiple
allelism with an example.
Answer:
- Definition: A gene that has more than two
allelic forms in a population.
- Example: Human ABO blood group.
- Gene ‘I’ has three alleles:
IA, IB, i.
- IAIA / IAi → A group, IBIB /
IBi → B group, IAIB → AB group, ii → O group.
- Conclusion: Though multiple alleles exist,
an individual carries only two of them.
11. Describe polygenic inheritance
and its features.
Answer:
- Definition: Trait controlled by more than
one pair of genes.
- Example: Human skin color and height.
- Features:
1.
Genes
show additive effect.
2.
No
complete dominance.
3.
Continuous
variation in population.
4.
Follows
bell-shaped distribution curve.
- Conclusion: Polygenic inheritance explains
quantitative traits.
12. Explain sex determination in
humans with a neat diagram.
Answer:
- Males: XY; Females: XX.
- Mechanism:
- Ovum → X chromosome.
- Sperms → X or Y chromosome.
- Fertilization:
- X + X = female (XX).
- X + Y = male (XY).
- Conclusion: Father determines the sex of
the child.
(Diagram: XY sex determination chart)
13. Describe sex-linked inheritance
in humans using haemophilia as an example.
Answer:
- Definition: Transmission of traits through
sex chromosomes.
- Example: Haemophilia (X-linked
recessive).
- Males (XY) → one X only →
disease expressed if X carries gene.
- Females (XX) → disease only if
both X carry the gene.
- Carrier females can pass to
sons.
- Conclusion: Sex-linked disorders show
unequal inheritance between genders.
14. Explain the inheritance pattern
of sickle cell anemia.
Answer:
- Cause: Mutation in β-globin gene (GAG
→ GTG) → Glutamic acid replaced by Valine.
- Genotypes:
- HbA HbA → Normal
- HbA HbS → Carrier (mild)
- HbS HbS → Disease (sickle cell
anemia)
- Inheritance: Autosomal recessive.
- Effect: RBCs become sickle-shaped →
blocked vessels → anemia.
- Advantage: Carriers resistant to malaria.
15. Differentiate between
sex-linked, autosomal dominant, and autosomal recessive disorders with
examples.
Answer:
|
Type |
Gene Location |
Example |
Characteristic |
|
Sex-linked |
On X or Y chromosome |
Haemophilia, Color blindness |
Males more affected |
|
Autosomal Dominant |
On autosome, single copy causes disease |
Huntington’s chorea |
50% chance if one parent affected |
|
Autosomal Recessive |
On autosome, both copies needed |
Sickle cell anemia, Cystic fibrosis |
Carriers normal, disease in homozygotes |
16. Explain the human genetic
disorders – Down’s, Turner’s, and Klinefelter’s syndromes.
Answer:
|
Syndrome |
Cause |
Karyotype |
Symptoms |
|
Down’s |
Trisomy of chromosome 21 |
47, 21+ |
Short, broad face, mental retardation |
|
Turner’s |
Absence of one X chromosome |
45, XO |
Female sterile, short stature |
|
Klinefelter’s |
Extra X chromosome in males |
47, XXY |
Male sterile, feminine features |
17. What is pedigree analysis? How
is it useful in human genetics?
Answer:
- Definition: Diagrammatic record of
inheritance of traits across generations.
- Uses:
1.
Study
inheritance of genetic disorders.
2.
Identify
carriers of diseases.
3.
Helps
in genetic counseling.
4.
Predicts
probability of future occurrence.
- Symbols: Males – squares; Females –
circles; shaded = affected; half-shaded = carriers.
18. What is mutation? Discuss
different types of mutations with examples.
Answer:
- Definition: Sudden heritable change in DNA
or chromosome structure.
- Types:
1.
Gene
mutation: Change in
base sequence (e.g., sickle cell anemia).
2.
Chromosomal
mutation: Change in
chromosome number or structure.
§ Aneuploidy: e.g., Down’s syndrome (Trisomy 21).
§ Polyploidy: e.g., 3n in plants.
- Significance: Creates genetic variation;
important for evolution and plant breeding.
19. Explain Morgan’s experiments on
linkage and recombination in Drosophila.
Answer:
- Morgan (1910): Crossed yellow-bodied,
white-eyed (X-linked traits) with normal flies.
- Expected 9:3:3:1 ratio, but
obtained deviations due to linkage.
- Found two types: Parental
and Recombinant offspring.
- Sturtevant used recombination frequency
to map genes.
- Conclusion: Linked genes are inherited
together; recombination frequency indicates distance between genes.
20. Explain the chromosomal basis of
sex-linked inheritance.
Answer:
- Genes on sex chromosomes
(X/Y) show sex-linked inheritance.
- X-linked recessive: Haemophilia, color blindness →
more in males.
- Y-linked: Transmitted from father to son
only (e.g., hairy ears).
- Conclusion: Inheritance depends on sex
chromosome composition.
21. Compare Mendelian inheritance
with non-Mendelian inheritance.
Answer:
|
Basis |
Mendelian |
Non-Mendelian |
|
Genes involved |
Single gene |
Multiple genes |
|
Expression |
Complete dominance |
Incomplete or co-dominance |
|
Ratio |
3:1, 9:3:3:1 |
Variable (1:2:1 etc.) |
|
Example |
Pea traits |
ABO blood group, Snapdragon color |
22. What are the limitations of
Mendel’s laws?
Answer:
- Not applicable to:
1.
Linked
genes (do not assort independently).
2.
Polygenic
traits.
3.
Incomplete
and co-dominance.
4.
Gene
interaction (epistasis).
5.
Mutation
and environment influence.
- Conclusion: Mendel’s laws are ideal for
simple traits, not complex ones.
23. Describe the types of chromosomal
abnormalities.
Answer:
1.
Numerical
Abnormalities:
o Aneuploidy (2n ± 1): Down’s,
Turner’s, Klinefelter’s syndromes.
o Polyploidy (3n, 4n): Common in
plants.
2.
Structural
Abnormalities:
o Deletion, duplication, inversion,
translocation.
Result: Genetic imbalance leads to developmental and reproductive
issues.
24. Describe the concept and
importance of gene mapping.
Answer:
- Concept: Determining relative positions
of genes on a chromosome.
- Based on: Recombination frequency.
- Steps:
1.
Cross
individuals for linked traits.
2.
Calculate
% of recombinants.
3.
1%
recombination = 1 map unit (centimorgan).
- Importance: Used in locating disease
genes, breeding programs, genome studies.
25. Explain the significance of
variations in evolution.
Answer:
- Definition: Differences among individuals
due to recombination or mutation.
- Types:
- Somatic (non-heritable).
- Germinal (heritable).
- Significance:
1. Basis of natural selection.
2. Helps species adapt and evolve.
3. Maintains biodiversity.
4. Prevents extinction under changing
environment.
