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):
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.
📚 Samacheer Kalvi 10th Science Guide — Buy Online
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.
