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Genetics & Heredity

Mendelian genetics, chromosomal theory, sex determination, mutations, and DNA replication.

Class 11Class 12
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What You Will Learn in Genetics & Heredity

Genetics is the study of heredity — how traits are passed from parents to offspring through genes encoded in DNA.

  • Mendel's laws: Law of Segregation (alleles separate in gametes) and Law of Independent Assortment (genes on different chromosomes segregate independently).
  • Monohybrid cross: Tt × Tt → 1 TT : 2 Tt : 1 tt (genotype 1:2:1; phenotype 3:1).
  • Dihybrid cross: TtRr × TtRr → 9:3:3:1 phenotypic ratio.
  • DNA structure: double helix, antiparallel strands, A-T and G-C base pairs (Watson-Crick).
  • Mutations: point mutations (substitution, insertion, deletion), chromosomal mutations; cause genetic disorders.
  • Sex determination: XX female, XY male; sex-linked traits (haemophilia, colour blindness) on X chromosome.

Example

Mendel's pea plant experiments (1865) revealed dominant/recessive inheritance using tall (T) and short (t) traits.

Expected Exam Questions — Genetics & Heredity

Q1.In a monohybrid cross Tt × Tt, what is the phenotypic and genotypic ratio?
Answer: Phenotypic ratio: 3 Tall : 1 short (3:1). Genotypic ratio: 1 TT : 2 Tt : 1 tt (1:2:1). T is dominant (tall), t is recessive (short). The F₂ generation from selfing Tt (F₁) gives these ratios.
Q2.What is co-dominance? Give an example.
Answer: Co-dominance: both alleles are equally expressed in a heterozygote — neither is dominant. Example: ABO blood group (IAIb = AB blood type, both A and B antigens expressed). Sickle cell: HbAHbS has both normal and sickle-shaped cells.
Q3.Why is colour blindness more common in males than females?
Answer: Colour blindness is X-linked recessive. Males (XY) have only one X — if it carries the allele (Xᶜ Y), they are colour blind. Females (XX) need both X's to carry the recessive allele (XᶜXᶜ) to be colour blind; one normal X is enough to be a carrier. Females can be carriers (XᶜX) without showing the trait.

🔘 MCQ Practice — Genetics & Heredity

MCQ 1.What is the phenotypic ratio in a dihybrid cross?
A. 1:2:1
B. 3:1
C. 9:3:3:1 ✓
D. 1:1:1:1

✓ Correct Answer: 9:3:3:1

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Frequently Asked Questions — Genetics & Heredity

What is Genetics & Heredity in Biology?
Genetics is the study of heredity — how traits are passed from parents to offspring through genes encoded in DNA.
In a monohybrid cross Tt × Tt, what is the phenotypic and genotypic ratio?
Phenotypic ratio: 3 Tall : 1 short (3:1). Genotypic ratio: 1 TT : 2 Tt : 1 tt (1:2:1). T is dominant (tall), t is recessive (short). The F₂ generation from selfing Tt (F₁) gives these ratios.
What is co-dominance? Give an example.
Co-dominance: both alleles are equally expressed in a heterozygote — neither is dominant. Example: ABO blood group (IAIb = AB blood type, both A and B antigens expressed). Sickle cell: HbAHbS has both normal and sickle-shaped cells.
Why is colour blindness more common in males than females?
Colour blindness is X-linked recessive. Males (XY) have only one X — if it carries the allele (Xᶜ Y), they are colour blind. Females (XX) need both X's to carry the recessive allele (XᶜXᶜ) to be colour blind; one normal X is enough to be a carrier. Females can be carriers (XᶜX) without showing the trait.
How do I prepare Genetics & Heredity for exams?
To master Genetics & Heredity, start by reading the theory carefully, then go through solved examples step by step. Practice numericals (if applicable), revise key formulas, and attempt previous year questions. SII notes cover all these aspects in a structured manner.
Are these Genetics & Heredity notes free?
Yes! SII provides free access to Genetics & Heredity notes and introductory study materials. Enrolled students get full access to detailed notes, solved papers, and live doubt-clearing sessions.
Which exams ask questions from Genetics & Heredity?
Genetics & Heredity is an important topic tested in Class 11, Class 12 board exams, as well as NEET, CBSE Class 12 Boards, CUET. It frequently appears in both short-answer and long-answer sections.