NCERT Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation
NCERT Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation covers Mendelian principles of heredity, deviations from Mendelism (Incomplete Dominance, Co-dominance, Multiple Allelism), Chromosomal Theory of Inheritance, Sex Determination, Linkage, Recombination, and Genetic Disorders.
Overview of Topics & Subtopics (Click to Jump)
1. Mendel Laws of Inheritance & Deviations
Short Note:
• Gregor Mendel: Known as the Father of Genetics. Worked on Garden Pea (Pisum sativum) for 7 years (1856-1863).
• Mendel Laws:
1. Law of Dominance: Characters are controlled by discrete units called factors (genes) which occur in pairs. In a dissimilar pair, one expresses itself (Dominant) and the other remains hidden (Recessive).
2. Law of Segregation: Alleles do not show any blending and both characters are recovered in F2 generation. Gametes receive only one allele (Law of Purity of Gametes).
3. Law of Independent Assortment: During dihybrid cross, segregation of one pair of traits is independent of another pair (Phenotypic Ratio 9:3:3:1).
• Deviations from Mendelism:
- Incomplete Dominance: F1 phenotype does not resemble either parent (e.g., Antirrhinum majus / Dog flower: Red x White = Pink, Ratio 1:2:1).
- Co-dominance: F1 progeny resembles both parents equally (e.g., ABO blood groups in humans controlled by gene I).
- Multiple Allelism: Presence of more than two alleles governing the same character (e.g., ABO Blood Group alleles: IA, IB, i).
2. Chromosomal Theory, Linkage & Sex Determination
Short Note:
• Chromosomal Theory of Inheritance: Proposed by Walter Sutton and Theodore Boveri (1902). Stated that chromosome behavior is parallel to gene behavior during meiosis. Experimentally verified by T.H. Morgan using fruit fly Drosophila melanogaster.
• Linkage & Recombination: Linkage is the physical association of genes located on the same chromosome. Tightly linked genes show low recombination frequency, whereas loosely linked genes show higher recombination frequency.
• Sex Determination Types:
1. XX-XY Type: Humans, Drosophila (Male heterogamety: XY, Female homogamety: XX).
2. XX-XO Type: Insects like Grasshopper (Male heterogamety: XO, Female homogamety: XX).
3. ZZ-ZW Type: Birds (Female heterogamety: ZW, Male homogamety: ZZ).
4. Haplodiploidy: Honeybees (Females diploid 2n=32, Males haploid n=16 development via parthenogenesis).
3. Mutations, Pedigree Analysis & Genetic Disorders
Short Note:
• Mutation: Sudden inheritable change in genetic material. Point mutation occurs due to a single base pair change (e.g., Sickle cell anemia).
• Mendelian Disorders: Determined by alteration or mutation in a single gene.
- Hemophilia: Sex-linked recessive disorder where blood clotting is delayed.
- Sickle-cell Anemia: Autosomal recessive disorder caused by substitution of Glutamic acid by Valine at 6th position of beta-globin chain.
- Phenylketonuria (PKU): Inborn error of metabolism, autosomal recessive disorder.
- Thalassemia: Autosomal recessive blood disorder leading to reduced synthesis of globin chains.
• Chromosomal Disorders: Caused by absence, excess, or abnormal arrangement of chromosomes.
- Down Syndrome: Trisomy of 21st chromosome (47 chromosomes).
- Klinefelter Syndrome: Karyotype 47, XXY (Male with feminine development like Gynaecomastia).
- Turner Syndrome: Karyotype 45, XO (Sterile female with rudimentary ovaries).Intext & Conceptual Questions
| Feature | Incomplete Dominance | Co-dominance |
|---|---|---|
| Expression in F1 | F1 phenotype is an intermediate blending between both parental traits. | F1 phenotype expresses both parental traits fully and simultaneously. |
| Blending | Effect of both alleles blends to produce a middle variant. | No blending occurs; both alleles manifest independently. |
| Example | Flower color in Snapdragons / Antirrhinum majus (Red x White = Pink). | ABO Blood Grouping (IAIB produces AB blood group). |
T.H. Morgan chose Drosophila melanogaster (Fruit fly) because:
1. They could easily be grown on simple synthetic medium in laboratory.
2. They complete their entire life cycle in about two weeks.
3. A single mating could produce a large number of progeny flies.
4. There was a clear differentiation of sexes (male and female are easily distinguishable).
5. It has many types of hereditary variations that can be seen with low power microscopes.
Textbook Chapter End Exercise Questions
Law of Independent Assortment: States that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.
Dihybrid Cross Example:
Cross between Round Yellow seeds (RRYY) and Wrinkled Green seeds (rryy):
• P Generation: RRYY x rryy
• F1 Generation: RrYy (All Round Yellow)
• F2 Generation Phenotypic Ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green (9:3:3:1).
This 9:3:3:1 ratio proves that seed shape and seed color inherit independently of each other.
Pedigree Analysis: The study of inheritance of genetic traits in several generations of a human family represented through a standardized family tree chart using symbols.
Significance / Utility:
1. Provides a valuable tool in human genetics to trace genetic disorders across generations.
2. Helps genetic counselors predict the risk of genetic diseases in offspring.
3. Helps identify whether a trait is dominant, recessive, autosomal, or sex-linked.
• Down's Syndrome (Trisomy 21):
- Cause: Presence of an additional copy of chromosome number 21 (Total = 47).
- Symptoms: Short statured with small round head, furrowed tongue, partially open mouth, broad palm with characteristic palm crease, and physical/mental development is retarded.
• Turner's Syndrome (45, XO):
- Cause: Absence of one X chromosome in females (Total = 45).
- Symptoms: Females are sterile with rudimentary ovaries, webbed neck, lack of secondary sexual characteristics, and short stature.
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