“Principles of Inheritance and Variation” is a core Class 12 Biology chapter because it builds the foundation for understanding how traits pass from parents to offspring (Mendelian patterns), how genes and alleles interact (complementary/incomplete dominance, codominance), and how genetic variation arises and can be measured (Hardy–Weinberg, linkage, recombination, interference). It is heavily asked in both board exams and competitive tests due to frequent use of probability, genotype–phenotype mapping, and numerical reasoning.
15
Minutes
10
Questions
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Marking
Q1. In a plant species alleles (red) and (white) show incomplete dominance so that heterozygote is pink. Two pink F plants () are selfed to produce F. What fraction of the F progeny will show the pink phenotype?
Q2. Genes and exhibit complementary interaction for flower colour: at least one dominant allele at both loci ( and ) is required for purple pigment; any other genotype is white. A dihybrid cross is performed. What fraction of the offspring will be purple?
Q3. A man with blood group and a woman with blood group have a child with blood group . Assuming ABO alleles (, , ), what is the probability that their next child will have blood group ?
Q4. Genes , and lie in that linear order on a chromosome. Recombination frequency between – is and between – is . In a sample of test-cross progeny, double recombinants (recombination between both – and –) are observed. What are the coefficient of coincidence () and the interference ()?
Q5. Assertion (A): If a mother carries a pathogenic mitochondrial DNA mutation, all her children inherit the mutant mitochondrial genome but may show variable severity of disease. Reason (R): Mitochondrial inheritance is maternal and, due to heteroplasmy, different offspring receive different proportions of mutant and wild‑type mitochondria, producing variable expressivity among them.
Both A and R are true but R is NOT a correct explanation of A
Both A and R are true and R is a correct explanation of A
A is true but R is false
A is false but R is true
Q6. In a large randomly mating population at Hardy–Weinberg equilibrium the frequency of individuals showing the recessive phenotype is given as . What percentage of the population are heterozygous carriers ()?
Q7. In a test cross of a dihybrid heterozygote (AaBb) with the double recessive (aabb) the progeny phenotypes counted were: parental types (AB and ab) total = 400 and recombinant types (Ab and aB) total = 200. Using , what is the map distance between the two loci (in cM)?
Q8. Two unaffected parents have an affected child with an autosomal recessive disorder. Their unaffected daughter marries an unrelated man from the general population where the carrier frequency for the disease allele is . What is the probability that their first child will be affected? (Use: probability that an unaffected sibling of an affected child is a carrier = .)
Q9. In a test cross between a female heterozygote for three linked genes (ABC/abc) and a homozygous recessive tester (abc/abc) the following offspring counts were obtained: . Based on these data, which of the following gives the most likely gene order and the map distances between A–B and B–C?
Order: A–C–B;
Order: B–A–C;
Order: A–B–C;
Order: A–B–C;
Q10. In a three-locus mapping experiment total offspring = 2000. Observed recombination frequencies are and . The observed number of double crossover offspring is 10. Calculate the interference , where coefficient of coincidence .