Chapter 5: Principles of Inheritance and Variation

Q1. State Mendel's Law of Segregation and Law of Independent Assortment.

Solution

Law of Segregation: Allele pair separates during gamete formation, so each gamete carries only one allele.
Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Q2. A cross between Tt x Tt gives 3:1 ratio. Explain with a Punnett square.

Solution

T t
T | TT Tt
t | Tt tt

Genotypic ratio: 1 TT : 2 Tt : 1 tt
Phenotypic ratio: 3 Tall : 1 Dwarf

Q3. What is linkage? How does it affect Mendel's law of independent assortment?

Solution

Linkage is the tendency of genes located on the same chromosome to be inherited together. It violates the law of independent assortment because linked genes do not assort independently - they tend to stay together during gamete formation.

Q4. What is a test cross? Why is it performed?

Solution

A test cross involves crossing an organism with a dominant phenotype but unknown genotype with a homozygous recessive individual.

Purpose: To determine the genotype of the organism with dominant trait.
If all offspring show dominant trait → homozygous dominant
If 50% show recessive trait → heterozygous

Q5. What are the different types of chromosomal mutations?

Solution

Structural: Deletion, Duplication, Inversion, Translocation
Numerical: Aneuploidy (loss/gain of chromosomes), Euploidy (complete set changes)

Q6. What is incomplete dominance? Give an example.

Solution

Example: Snapdragon flower color
RR (Red) x rr (White) → Rr (Pink)
F₂ ratio: 1 Red : 2 Pink : 1 White

Q7. What is co-dominance? How is it different from incomplete dominance?

Solution

Co-dominance: Both alleles expressed equally in heterozygote (e.g., ABO blood group - IAIB shows both A and B antigens)
Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., pink flower)

Q8. What is pleiotropy? Give an example.

Solution

Example: Sickle cell anemia - HbS gene causes (1) Sickling of RBCs (2) Anemia (3) Organ damage (4) Malaria resistance in heterozygotes

Q9. What is a dihybrid cross? What phenotypic ratio does it produce?

Solution

Phenotypic ratio: 9:3:3:1
9 Yellow Round : 3 Yellow Wrinkled : 3 Green Round : 1 Green Wrinkled
Genotypic ratio has 9 genotypes

Q10. What is epistasis? Give an example.

Solution

Example: Coat color in dogs - Gene E (epistatic) controls pigment deposition. If ee present, coat is yellow regardless of B genotype. Modified ratio: 9:3:4

Q11. What is polygenic inheritance? Give an example.

Solution

Example: Human skin color - controlled by 3 genes (6 alleles)
Continuous variation observed (no distinct categories)

Q12. What is Down's syndrome? What are its characteristics?

Solution

Trisomy 21: Extra copy of chromosome 21 (47 chromosomes)
Symptoms: (1) Mental retardation (2) Short stature (3) Broad palm with single crease (4) Flat face (5) Protruding tongue (6) Congenital heart defects

Q13. What is sex determination in humans?

Solution

Cross: XX (Mother) x XY (Father)
Offspring: XX (50% female) or XY (50% male)
Sex of child depends on the sperm (father)

Q14. What is Turner's syndrome?

Solution

45, X0 - One X chromosome missing
Characteristics: (1) Female with 45 chromosomes (2) Sterile (3) Rudimentary ovaries (4) Short stature (5) Webbed neck (6) Shield-shaped chest

Q15. What is Klinefelter's syndrome?

Solution

47, XXY - Extra X chromosome in males
Characteristics: (1) Gynecomastia (2) Sterile (3) Reduced body hair (4) Tall stature (5) Small testes (6) No spermatogenesis

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