CBSE Class 12 Biology – Principles of Inheritance and Variation
NCERT-aligned chapter resource | Biology Code 044
1. Mendel’s Experiments
Mendel studied pea plants and established principles of inheritance through controlled crosses. A monohybrid cross demonstrates segregation, while a dihybrid cross demonstrates independent assortment under appropriate conditions.
2. Monohybrid Cross
Genotypic ratio = 1:2:1 | Phenotypic ratio = 3:1 for complete dominance
The law of segregation states that the two alleles of a gene separate during gamete formation.
3. Dihybrid Cross
For AaBb × AaBb with independent assortment and complete dominance, the classic phenotypic ratio is 9:3:3:1. The ratio depends on the assumptions of the cross and should not be applied blindly to every inheritance problem.
4. Incomplete Dominance and Codominance
In incomplete dominance, the heterozygote shows an intermediate phenotype. In codominance, both alleles are expressed, as illustrated by the IA and IB alleles of the ABO blood group system.
5. Sex Determination
Humans have XX females and XY males. The sperm determines the sex chromosome combination of the offspring because sperm may carry either X or Y.
6. Linkage and Recombination
Genes located close together on the same chromosome tend to be inherited together. Crossing over can produce recombinant combinations. Recombination frequency provides a basis for genetic mapping.
7. Sex-Linked Inheritance
Traits associated with genes on sex chromosomes show characteristic inheritance patterns. Haemophilia and colour blindness are standard examples of X-linked recessive inheritance.
8. Chromosomal Disorders
Changes in chromosome number can result from nondisjunction. Down syndrome is associated with trisomy 21; Turner syndrome with XO; Klinefelter syndrome with XXY.
Worked Genetic Problem
If two heterozygous parents Aa × Aa are crossed, the probability of an aa offspring is 1/4. The probability follows from the four equally likely allele combinations under the standard Mendelian assumptions.
Chapter-Specific MCQs
- The law of segregation describes separation of: (A) chromosomes only (B) alleles during gamete formation (C) proteins (D) tissues. Answer: B.
- AB blood group demonstrates: (A) incomplete dominance (B) codominance (C) recessive inheritance only (D) polyembryony. Answer: B.
- Down syndrome is associated with: (A) trisomy 21 (B) XO (C) XXY (D) monosomy 21. Answer: A.
Competency Questions
- Predict offspring genotypes from a specified monohybrid cross and justify the ratio.
- Explain why linked genes do not always remain together.
- Use a pedigree or family data set to infer a likely inheritance pattern.