Principles of Inheritance - Previous Year Questions Solved Examples (Class 12 Biology)
Inheritance and genetics explain how traits pass from parents to offspring. These questions test understanding of Mendelian genetics, chromosomal inheritan
TL;DR: Inheritance and genetics explain how traits pass from parents to offspring. These questions test understanding of Mendelian genetics, chromosomal inhe…
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Inheritance and genetics explain how traits pass from parents to offspring. These questions test understanding of Mendelian genetics, chromosomal inheritan
Principles of Inheritance - Previous Year Questions — Solved Numerical Examples (Step by Step)
Example 1: State Mendel's Law of Segregation and explain it using a monohybrid cross. [4 marks]
Solution: Law of Segregation: During gamete formation, alleles of a gene separate so that each gamete receives only one allele. In monohybrid cross between homozygous tall (TT) and homozygous short (tt): P: TT × tt. F1: All Tt (tall). F2: From Tt × Tt = 1 TT : 2 Tt : 1 tt = 3 tall : 1 short. Segregation occurs during meiosis. During gamete formation, homologous chromosomes (carrying different alleles) separate into different gametes.
Example 2: Explain the difference between dominant and recessive traits. How would you determine the genotype of a dominant individual? [3 marks]
Solution: Dominant traits are expressed in both homozygous (AA) and heterozygous (Aa) individuals. Recessive traits appear only in homozygous (aa) individuals. To determine genotype of dominant individual: Perform a test cross with homozygous recessive (aa). If dominant individual is AA: all offspring will be Aa (dominant). If dominant individual is Aa: approximately half offspring will be Aa (dominant) and half aa (recessive). Phenotypic ratio of 1:1 in test cross indicates heterozygosity.
Example 3: What is incomplete dominance? Distinguish it from codominance with examples. [3 marks]
Solution: Incomplete Dominance: Heterozygote shows intermediate phenotype between two homozygotes. Example: Red flowers (RR) × White flowers (WW) = Pink flowers (RW) in F1. Codominance: Both alleles are equally expressed in heterozygote; both phenotypes appear. Example: Human ABO blood groups: AB blood type shows both A and B antigens. Difference: Incomplete dominance produces blended phenotype, codominance produces both distinct phenotypes simultaneously.
Example 4: Explain sex-linked inheritance using the example of color blindness. [4 marks]
Solution: Sex-linked traits are genes located on X chromosome. Color blindness is X-linked recessive. Notation: XB (normal), Xb (color blind). Males: XBY (normal), XbY (color blind). Females: XBXB (normal), XBXb (carrier), XbXb (color blind). Cross: Color blind male (XbY) × Carrier female (XBXb). Offspring: 25% XBY (normal males), 25% XbY (color blind males), 25% XBXB (normal females), 25% XBXb (carrier females). Males show trait more frequently because they have only one X chromosome.
Example 5: What is a dihybrid cross? Explain the law of independent assortment. [4 marks]
Solution: Dihybrid cross involves two genes on different chromosomes. Example: AABB × aabb. Law of Independent Assortment: Alleles of different genes segregate independently during gamete formation. AABB can produce gametes AB, Ab, aB, ab in equal proportions. F1 from AABB × aabb is AaBb (dihybrid). F2 from AaBb × AaBb produces 9:3:3:1 ratio: 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb. This 9:3:3:1 ratio proves independent assortment of two genes.
Example 6: Explain the chromosomal theory of inheritance and its evidence. [3 marks]
Solution: Chromosomal Theory: Genes are located on chromosomes and inheritance follows chromosome behavior. Evidence: (1) Parallel behavior of genes and chromosomes during meiosis, (2) Correspondence between genes segregating and chromosomes separating, (3) Sex-linked inheritance patterns matching X chromosome location, (4) Polytene chromosomes in Drosophila showing gene locations as bands, (5) Pedigree analysis confirming inheritance patterns expected from chromosome distribution.
Example 7: What is genetic variation? Explain its sources and significance in evolution. [4 marks]
Solution: Genetic variation is differences in genes/alleles within population. Sources: (1) Mutation creates new alleles, (2) Sexual reproduction shuffles existing alleles through crossing over and independent assortment, (3) Gene flow brings new alleles from other populations. Significance: (1) Provides raw material for natural selection, (2) Enables adaptation to changing environment, (3) Maintains genetic diversity preventing inbreeding, (4) Allows populations to survive environmental changes, (5) Drives evolution of species.
Tips
- Use Punnett squares consistently for cross problems; clearly label alleles.
- Remember that dominant alleles are expressed in both homozygous and heterozygous forms.
- For sex-linked traits, separately consider males and females as their inheritance patterns differ.
- Always perform test cross to determine if organism with dominant phenotype is homozygous or heterozygous.
Frequently Asked Questions
How do mutations contribute to genetic variation?
Mutations create new alleles that did not exist before, increasing genetic diversity within a population.
Why are some traits more common in males than females?
X-linked recessive traits appear more frequently in males because they have one X chromosome; females need two copies.
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