Principles of Inheritance and Variation – Class 12 Biology notes, important questions and MCQs

Chapter overview

Mendel's experiments on pea plants showed that traits pass through factors (genes) following laws of dominance, segregation and independent assortment. Chromosomes carry genes; sex determination and genetic disorders follow from them.

Mendelian inheritance

• Monohybrid cross F₂: phenotype 3 : 1, genotype 1 : 2 : 1.
• Dihybrid cross F₂: 9 : 3 : 3 : 1.
• Incomplete dominance (snapdragon 1 : 2 : 1), co-dominance (ABO blood group), multiple alleles (Iᴬ, Iᴮ, i).

Chromosomes and disorders

• Linkage reduces recombination; Morgan worked with Drosophila.
• Sex determination: XX–XY in humans (father decides), XX–XO in grasshopper, ZW–ZZ in birds.
• Disorders: haemophilia and colour blindness (X-linked recessive), sickle-cell anaemia (autosomal recessive), Down's syndrome (trisomy 21), Turner's (XO), Klinefelter's (XXY).

Key terms

Allele

alternative form of a gene

Test cross

cross of an organism with a homozygous recessive parent to find its genotype

Co-dominance

both alleles express fully in the heterozygote

Formulas

• Monohybrid F₂: 3 : 1; dihybrid F₂: 9 : 3 : 3 : 1

Important questions with answers

Quick revision

• Test cross ratio for heterozygote is 1 : 1.
• Down's = trisomy 21.
• Haemophilia is X-linked recessive.

Exam tips

• For blood groups, IᴬIᴮ gives AB (co-dominance); ii gives O.
• In pedigree charts, squares are males and circles females; shaded symbols are affected individuals.

Practice MCQs

1. Mendel worked on

   1. Drosophila
   2. maize
   3. garden pea
   4. snapdragon
   Show answer

   Answer: (c) garden pea. Pisum sativum.

2. F₂ phenotypic ratio of a monohybrid cross is

   1. 1 : 2 : 1
   2. 9 : 3 : 3 : 1
   3. 3 : 1
   4. 1 : 1
   Show answer

   Answer: (c) 3 : 1. Dominance.

3. ABO blood group shows

   1. incomplete dominance
   2. linkage
   3. co-dominance and multiple alleles
   4. polygenic only
   Show answer

   Answer: (c) co-dominance and multiple alleles. Iᴬ, Iᴮ, i.

4. Down's syndrome is caused by

   1. trisomy 21
   2. XO
   3. XXY
   4. deletion of 5
   Show answer

   Answer: (a) trisomy 21. Extra chromosome 21.

5. A cross between Tt and tt gives the phenotypic ratio

   1. 3 : 1
   2. 1 : 2 : 1
   3. 1 : 1
   4. all tall
   Show answer

   Answer: (c) 1 : 1. Test cross.

6. F₂ dihybrid ratio is

   1. 3 : 1
   2. 1 : 2 : 1
   3. 9 : 3 : 3 : 1
   4. 1 : 1 : 1 : 1
   Show answer

   Answer: (c) 9 : 3 : 3 : 1. Independent assortment.

7. In snapdragon, F₂ of red × white shows red : pink : white as

   1. 3 : 0 : 1
   2. 9 : 6 : 1
   3. 1 : 2 : 1
   4. 2 : 1 : 1
   Show answer

   Answer: (c) 1 : 2 : 1. Incomplete dominance.

8. A colour-blind man marries a normal (non-carrier) woman. Their daughters will be

   1. all colour blind
   2. half carriers
   3. all carriers
   4. all normal non-carriers
   Show answer

   Answer: (c) all carriers. Each daughter gets the father's Xᶜ.