CBSE Class 12 Genetics — Inheritance & Variation Complete Guide with Punnett Squares

In short: Principles of Inheritance and Variation (Chapter 5) is one of the most important chapters in CBSE Class 12 Biology, carrying 7-8 marks out of 70. It covers Mendel's laws, monohybrid and dihybrid crosses, incomplete dominance, co-dominance, sex-linked inheritance, pedigree analysis, and chromosomal disorders. Expect at least one 5-mark question involving a genetic cross or pedigree analysis. This chapter blends concepts with problem-solving — once you master Punnett squares, scoring is straightforward.

Marks Distribution

For the full Biology marks breakdown, see CBSE Class 12 Biology chapter-wise marks weightage. For diagram practice, check the Biology diagrams complete list.

Mendel's Laws of Inheritance

Law of Dominance

When two organisms with contrasting traits are crossed, only the dominant trait appears in F1 generation. The recessive trait is masked but not lost.

Law of Segregation (Law of Purity of Gametes)

During gamete formation, the two alleles of a gene segregate from each other so that each gamete receives only one allele.

Law of Independent Assortment

When two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair.

Monohybrid Cross

Cross: Tt × Tt (Tall × Tall, both heterozygous)

Punnett Square:

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

Test Cross

Cross of F1 (unknown genotype) with homozygous recessive (tt).

• If all offspring are tall → F1 is TT (homozygous)
• If 1:1 tall:dwarf → F1 is Tt (heterozygous)

Dihybrid Cross

Cross: RrYy × RrYy (Round Yellow × Round Yellow)

Gametes from each parent: RY, Ry, rY, ry

Punnett Square (4×4):

Phenotypic ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green (9:3:3:1)

Genotypic ratio: 1:2:1:2:4:2:1:2:1

Board exam tip: Draw the Punnett square neatly. Examiners award marks for the cross diagram, so even if your ratio has a minor error, you get partial credit for the correct setup.

Deviations from Mendelian Genetics

Incomplete Dominance

Neither allele is completely dominant. F1 shows an intermediate (blended) phenotype.

Example: Snapdragon (Antirrhinum) flower colour

• RR = Red, Rr = Pink, rr = White
• F2 phenotypic ratio: 1 Red : 2 Pink : 1 White (1:2:1)

Co-dominance

Both alleles express themselves equally in the heterozygote.

Example: ABO Blood Groups

• Iᴬ and Iᴮ are co-dominant (both express in AB blood group)
• Iᴬ and Iᴮ are both dominant over i

Multiple Allelism

ABO blood groups are controlled by gene I which has three alleles: Iᴬ, Iᴮ, and i. Though any individual carries only two alleles, the population has three.

Sex-Linked Inheritance

Sex Determination in Humans

• Females: XX (homogametic — produce only X-type eggs)
• Males: XY (heterogametic — produce X and Y type sperm)
• Father's sperm determines the sex of the child

X-Linked Recessive Disorders

Colour Blindness:

• Gene located on X chromosome
• Carrier female (XᶜX) × Normal male (XY)

Result: 50% sons colour blind, 50% daughters carriers

Haemophilia:

• Similar X-linked pattern
• "Royal disease" — Queen Victoria was a carrier
• Affected males (XʰY) bleed excessively due to lack of clotting factor

Key pattern: X-linked recessive traits show criss-cross inheritance — carrier mother passes the gene to sons, not daughters.

Pedigree Analysis

Pedigree charts track inheritance of a trait through generations.

How to identify the pattern:

Board exam tip: If you see a pedigree chart in the exam, first check if the trait skips generations (recessive) or appears in every generation (dominant). Then check if males are more affected (X-linked) or both sexes equally affected (autosomal).

Chromosomal Disorders

Aneuploidy — Abnormal Chromosome Number

Cause: Non-disjunction during meiosis — chromosomes fail to separate properly.

Key Terms

Solved Examples

Example 1: Monohybrid Cross

Q: In pea plants, yellow seed colour (Y) is dominant over green (y). Cross a heterozygous yellow plant with a green plant. What are the phenotypic and genotypic ratios?

Solution: Cross: Yy × yy

Genotypic ratio: 1 Yy : 1 yy Phenotypic ratio: 1 Yellow : 1 Green (1:1) — This is a test cross.

Example 2: Blood Group Inheritance

Q: A mother has blood group A (Iᴬi) and father has blood group B (Iᴮi). What blood groups are possible in their children?

Solution:

Possible blood groups: A, B, AB, O — all four blood groups are possible.

Example 3: Sex-Linked Inheritance

Q: A colour-blind man marries a woman who is a carrier. What proportion of their children will be colour blind?

Solution: Father: XᶜY, Mother: XᶜX

1/4 colour blind females + 1/4 colour blind males = 1/2 children colour blind.

Board Exam Important Questions

1. Explain Mendel's Law of Independent Assortment with a dihybrid cross. Draw the Punnett square and give the phenotypic ratio. (5 marks) — High frequency
2. Explain co-dominance with the example of ABO blood groups. (3 marks)
3. What is incomplete dominance? Explain with a cross in snapdragon. (3 marks)
4. A colour-blind man marries a normal-vision woman whose father was colour blind. Show the possible genotypes and phenotypes of the offspring. (3 marks)
5. Differentiate between Turner Syndrome and Klinefelter Syndrome. (2 marks)
6. Draw a pedigree chart showing autosomal recessive inheritance for 3 generations. (2 marks)
7. Explain the chromosomal theory of inheritance. (3 marks)
8. What is aneuploidy? How does it arise? Give two examples. (3 marks)

Frequently Asked Questions

How many marks does Genetics carry in the board exam?

7-8 marks out of 70 in the theory paper. Combined with Molecular Basis of Inheritance (Chapter 6), the Genetics unit carries about 12-14 marks.

Is a Punnett square compulsory in genetic cross answers?

Yes. CBSE marking scheme awards specific marks for the Punnett square diagram. Even if your final ratio has a small mistake, the square itself earns you marks.

Which genetic cross is most frequently asked?

The dihybrid cross (9:3:3:1 ratio) is the most common 5-mark question. Sex-linked inheritance crosses (colour blindness, haemophilia) are the next most frequent.

How do I solve pedigree analysis questions?

First, check if the trait appears in every generation (dominant) or skips generations (recessive). Then, check if more males are affected (X-linked) or both sexes equally (autosomal). Finally, trace the alleles through each individual.

What is the difference between incomplete dominance and co-dominance?

In incomplete dominance, the heterozygote shows a blended intermediate phenotype (e.g., pink from red × white). In co-dominance, both alleles express fully and simultaneously (e.g., AB blood group shows both A and B antigens).

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