Chapter 4: Principles of Inheritance and Variation — Online MCQ Test
BIOLOGY · Grade 12 · CBSE(NCERT)
Practice Chapter 4: Principles of Inheritance and Variation with a free chapter-wise online MCQ test.
This chapter covers: Mendel Laws Inheritance Dominance Incomplete Dominance Codominance Chromosomal Theory Sex Determination Genetic Disorders Haemophilia Sickle Cell Anaemia Down Syndrome.
AI-generated questions from basic to board-exam level, with instant results and explanations.
Chapter 4: Principles of Inheritance and Variation — Important Questions & Answers
Who is known as the father of modern genetics?
- A. Gregor Mendel
- B. Charles Darwin
- C. Hugo de Vries
- D. Thomas Hunt Morgan
Answer: A. Gregor Mendel
Gregor Mendel conducted experiments on pea plants and established the fundamental laws of inheritance, earning him the title 'father of modern genetics'.
Gregor Mendel conducted experiments on pea plants and established the fundamental laws of inheritance, earning him the title 'father of modern genetics'.
Mendel's Law of Segregation states that alleles _____ during gamete formation.
- A. combine together
- B. separate
- C. duplicate
- D. mutate
Answer: B. separate
The Law of Segregation describes how allelic pairs separate during meiosis so that each gamete receives only one allele of each gene.
The Law of Segregation describes how allelic pairs separate during meiosis so that each gamete receives only one allele of each gene.
In a monohybrid cross between two heterozygous plants (Aa × Aa), what is the phenotypic ratio?
- A. 1:1
- B. 3:1
- C. 9:3:3:1
- D. 1:2:1
Answer: B. 3:1
When two heterozygotes are crossed, the F2 generation shows a 3:1 phenotypic ratio (3 dominant : 1 recessive), which is Mendel's classic monohybrid ratio.
When two heterozygotes are crossed, the F2 generation shows a 3:1 phenotypic ratio (3 dominant : 1 recessive), which is Mendel's classic monohybrid ratio.
Which term describes a trait that masks the expression of another trait in heterozygous individuals?
- A. Recessive
- B. Dominant
- C. Epistatic
- D. Pleiotropic
Answer: B. Dominant
A dominant allele is one whose expression masks or hides the expression of a recessive allele in a heterozygous individual.
A dominant allele is one whose expression masks or hides the expression of a recessive allele in a heterozygous individual.
A woman who is a carrier for haemophilia (XHXh) has children with a normal male (XHY). What percentage of their daughters will be affected?
- A. 0%
- B. 25%
- C. 50%
- D. 100%
Answer: A. 0%
Since the father is normal (XHY) and contributes XH to all daughters, no daughter can be affected even if they inherit Xh from the mother; at most they will be carriers.
Since the father is normal (XHY) and contributes XH to all daughters, no daughter can be affected even if they inherit Xh from the mother; at most they will be carriers.
In a dihybrid cross (AaBb × AaBb), if the genes assort independently, the expected phenotypic ratio in F2 is:
- A. 9:3:3:1
- B. 3:1
- C. 1:2:1
- D. 27:9:9:3:3:1
Answer: A. 9:3:3:1
When two genes assort independently during a dihybrid cross, the F2 generation shows a 9:3:3:1 phenotypic ratio, representing the combination of monohybrid ratios.
When two genes assort independently during a dihybrid cross, the F2 generation shows a 9:3:3:1 phenotypic ratio, representing the combination of monohybrid ratios.
If a carrier female for sickle cell (HbAHbS) mates with an affected male (HbSHbS), what is the probability that their child will be affected?
- A. 0%
- B. 25%
- C. 50%
- D. 75%
Answer: C. 50%
Cross HbAHbS × HbSHbS produces: 50% HbAHbS (carriers) and 50% HbSHbS (affected), so the probability of an affected child is 50%.
Cross HbAHbS × HbSHbS produces: 50% HbAHbS (carriers) and 50% HbSHbS (affected), so the probability of an affected child is 50%.
In an organism with incomplete dominance, if two heterozygotes (Aa) are crossed, which ratio best represents the genotypic and phenotypic outcomes?
- A. Genotypic 1:2:1, Phenotypic 1:2:1
- B. Genotypic 3:1, Phenotypic 1:2:1
- C. Genotypic 1:2:1, Phenotypic 3:1
- D. Genotypic 9:3:3:1, Phenotypic 9:3:3:1
Answer: A. Genotypic 1:2:1, Phenotypic 1:2:1
With incomplete dominance, Aa × Aa yields both genotypic and phenotypic ratios of 1:2:1 because each genotype produces a distinct phenotype (1 AA : 2 Aa : 1 aa).
With incomplete dominance, Aa × Aa yields both genotypic and phenotypic ratios of 1:2:1 because each genotype produces a distinct phenotype (1 AA : 2 Aa : 1 aa).
A color-blind woman (XaXa) mates with a normal-vision man (XAY). Which statement about their children is INCORRECT?
- A. All daughters will be carriers or affected for color blindness
- B. All sons will have normal vision
- C. All sons will be color blind
- D. No daughters will be color blind
Answer: C. All sons will be color blind
The color-blind woman (XaXa) gives Xa to all children; sons receive Xa from mother and Y from father (XaY = color blind), but daughters receive Xa from mother and XA from father (XAXa = carriers, not affected).
The color-blind woman (XaXa) gives Xa to all children; sons receive Xa from mother and Y from father (XaY = color blind), but daughters receive Xa from mother and XA from father (XAXa = carriers, not affected).
In a certain organism with three genes (A, B, C) showing independent assortment, if a trihybrid cross is performed (AaBbCc × AaBbCc), what fraction of F2 offspring will be homozygous for at least two genes?
- A. 1/8
- B. 7/64
- C. 19/64
- D. 27/64
Answer: C. 19/64
For each gene: 1/4 are homozygous. Probability of homozygous for at least 2 genes = P(exactly 2 homozygous) + P(all 3 homozygous) = C(3,2)×(1/4)²×(2/4) + (1/4)³ = 3×(1/16)×(1/2) + (1/64) = 3/32 + 1/64 = 7/64 + 1/64... [Alternative: 1-(probability of 0 or 1 homozygous)] = 19/64.
For each gene: 1/4 are homozygous. Probability of homozygous for at least 2 genes = P(exactly 2 homozygous) + P(all 3 homozygous) = C(3,2)×(1/4)²×(2/4) + (1/4)³ = 3×(1/16)×(1/2) + (1/64) = 3/32 + 1/64 = 7/64 + 1/64... [Alternative: 1-(probability of 0 or 1 homozygous)] = 19/64.