What is the difference between genotype and phenotype?

Genotype: The genetic makeup (allele combination) of an organism. Written as letters e.g. TT, Tt, tt. Phenotype: The observable physical characteristic resulting from the genotype. e.g. Tall or Short. Same phenotype can have different genotypes (TT and Tt are both tall). Same genotype always gives same phenotype in same environment.

Samacheer Kalvi 10th Science Chapter 17 Short Answer Questions — Heredity with Answers

Short answer questions from Samacheer Kalvi Class 10 Science Chapter 17 Heredity with detailed answers for 2 and 3 mark questions.

Updated 24 May 20265 min read

Chapter 17 — Heredity: Short Answer Questions

1. What is heredity? Define the key terms in genetics.

Answer

Heredity is the transmission of traits (characteristics) from parents to offspring through genes.

Key terms:

Term	Definition
Gene	A segment of DNA that codes for a specific protein/trait; unit of inheritance
Allele	Alternative forms of a gene occupying the same locus on homologous chromosomes
Dominant	An allele that expresses itself even in the presence of a recessive allele (written as capital letter: T)
Recessive	An allele expressed only when two copies are present (written as lowercase: t)
Homozygous	Having two identical alleles for a trait (TT or tt)
Heterozygous	Having two different alleles for a trait (Tt)
Genotype	The actual genetic makeup (allele combination) of an organism
Phenotype	The observable physical trait/characteristic
F₁ generation	First filial generation — offspring of original cross
F₂ generation	Second filial generation — offspring obtained by crossing F₁ × F₁

2. Explain Mendel's monohybrid cross with a Punnett square.

Answer

Mendel crossed tall (TT) pea plants with short (tt) pea plants — studying one trait at a time.

P generation (Parents):

Tall plant: TT
Short plant: tt

Gametes from tall parent: T, T Gametes from short parent: t, t

F₁ generation (all offspring): Tt (tall) — 100% tall Observation: All F₁ plants were tall. Shortness "disappeared."

F₁ × F₁ cross: Tt × Tt

Punnett Square:

	T	t
T	TT	Tt
t	Tt	tt

F₂ results:

TT : Tt : tt = 1 : 2 : 1 (genotype ratio)
Tall : Short = 3 : 1 (phenotype ratio)

Conclusion: Shortness was not lost — it was masked in F₁ (recessive). Mendel called tall "dominant" and short "recessive."

3. State and explain Mendel's Laws of Inheritance.

Answer

Law 1 — Law of Dominance: In a cross between two pure-breeding individuals differing in one character, the character that appears in the F₁ generation is dominant; the other is recessive. Both alleles are present in the F₁ hybrid but only the dominant one is expressed.

Law 2 — Law of Segregation (Law of Purity of Gametes): Each organism carries two alleles for each trait. During gamete formation, the two alleles of each gene separate (segregate) so each gamete carries only one allele. The alleles reunite randomly at fertilisation.

Law 3 — Law of Independent Assortment: When two pairs of traits are studied simultaneously (dihybrid cross), each pair of alleles segregates independently of the other pairs during gamete formation. This gives F₂ dihybrid ratio of 9:3:3:1.

4. What is sex determination in humans?

Answer

Sex determination refers to the genetic mechanism that determines whether an offspring will be male or female.

In humans:

Females have XX sex chromosomes (homogametic)
Males have XY sex chromosomes (heterogametic)
Total chromosome number: 46 (44 autosomes + 2 sex chromosomes)

Mechanism:

All eggs produced by females carry X chromosome
Males produce two types of sperm: 50% carry X and 50% carry Y
Fertilisation is random:

Egg (X) +	Sperm (X)	→ XX = Female
Egg (X) +	Sperm (Y)	→ XY = Male

Conclusion: The father's sperm determines the sex of the child — not the mother. Equal chance (50:50) of male or female offspring.

Note: Insects like grasshopper use XO system (females XX, males XO). Birds use ZW system (females ZW, males ZZ).

5. Explain a dihybrid cross and state the F₂ ratio obtained.

Answer

A dihybrid cross studies the inheritance of two pairs of contrasting traits simultaneously.

Mendel's dihybrid cross (Round Yellow × Wrinkled Green seeds):

Round (R) is dominant over Wrinkled (r)
Yellow (Y) is dominant over Green (y)

Parents: RRYY (Round Yellow) × rryy (Wrinkled Green)

F₁: All RrYy — Round Yellow (dominant phenotype)

F₁ × F₁: RrYy × RrYy

F₂ Phenotypic ratio: 9 : 3 : 3 : 1

9 Round Yellow
3 Round Green
3 Wrinkled Yellow
1 Wrinkled Green

This demonstrates the Law of Independent Assortment — R/r alleles assort independently of Y/y alleles.

Two new combinations appear in F₂ (Round Green and Wrinkled Yellow) that were not seen in the parents — showing independent assortment creates new combinations.

6. Differentiate between homozygous and heterozygous organisms. Give examples.

Answer

Feature	Homozygous	Heterozygous
Alleles	Both alleles for a trait are identical	Two alleles for a trait are different
Notation	TT (dominant homozygous) or tt (recessive homozygous)	Tt
Gametes produced	Only one type of gamete (T or t)	Two types of gametes (T and t)
Phenotype	Expresses phenotype of that allele	Expresses phenotype of dominant allele (masks recessive)
Also called	Pure breeding / true breeding	Hybrid
Examples	TT (tall, pure), tt (short, pure) in pea	Tt (tall, hybrid)

A homozygous organism always breeds true — its offspring always show the same trait. A heterozygous organism does not breed true — its offspring show segregation of traits.

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Frequently Asked Questions

Mendel's Law of Segregation (Law of Purity of Gametes): Each organism has two alleles for each trait. During gamete formation, these two alleles separate (segregate) so that each gamete carries only one allele for each trait. The alleles unite again randomly during fertilisation.