Inheritance MCQs
MDCAT · Biology · 6 questions
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PMDC MDCAT syllabus: Inheritance
Mendel's laws are the core of this chapter. In a monohybrid cross of two heterozygotes (Tt × Tt), the F2 genotypes are 1 TT : 2 Tt : 1 tt and, with complete dominance, the phenotypes are 3 : 1. Two genes on different chromosomes assort independently, so a dihybrid F2 gives 9 : 3 : 3 : 1. A test cross with a homozygous recessive reveals an unknown genotype (all dominant offspring means homozygous; half and half means heterozygous). Beyond simple dominance: in codominance both alleles show (ABO blood groups: Iᴬ and Iᴮ are codominant and both dominant over i, so Iᴬi is group A and IᴬIᴮ is AB); in incomplete dominance the heterozygote is intermediate. Sex-linked traits such as haemophilia and colour blindness are carried on the X chromosome and are recessive, so they appear far more often in males. Practise by drawing the Punnett square every time; most mistakes are arithmetic, not concepts.
Questions with answers
Written by Parcha from the syllabus. Samples until a subject reviewer checks them; report anything wrong from the practice screen.
Q1 · Parcha original · Sample
In a monohybrid cross with complete dominance, the F2 phenotypic ratio is:
- A. 1:2:1
- B. 9:3:3:1
- C. 3:1
- D. 1:1
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Answer: C. 3:1
Crossing two heterozygotes (Tt × Tt) gives TT, Tt, Tt and tt. Three of the four show the dominant trait, so the phenotypic ratio is 3:1 (the genotypic ratio is 1:2:1).
Open this question on its own pageQ2 · Parcha original · Sample
The F2 phenotypic ratio of a dihybrid cross between two independently assorting genes is:
- A. 9:3:3:1
- B. 3:1
- C. 1:1:1:1
- D. 12:3:1
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Answer: A. 9:3:3:1
For two genes on different chromosomes, the 3:1 ratios multiply: (3:1) × (3:1) = 9:3:3:1. A 1:1:1:1 ratio comes from a dihybrid test cross; 12:3:1 is a case of epistasis.
Q3 · Parcha original · Sample
A test cross is a cross between an individual of unknown genotype and one that is:
- A. Homozygous dominant
- B. Heterozygous
- C. Homozygous recessive
- D. From the F2 generation
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Answer: C. Homozygous recessive
Crossing with a homozygous recessive (tt) shows the unknown genotype directly: all offspring dominant means TT; half dominant and half recessive means Tt.
Q4 · Parcha original · Sample
In the ABO blood group system, the alleles Iᴬ and Iᴮ show:
- A. Incomplete dominance
- B. Codominance
- C. Epistasis
- D. Sex linkage
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Answer: B. Codominance
A person with Iᴬ Iᴮ makes both A and B antigens and has blood group AB, so both alleles are fully expressed: codominance. Both are dominant over the i allele.
Q5 · Parcha original · Sample
Haemophilia A is inherited as a trait that is:
- A. Autosomal dominant
- B. Autosomal recessive
- C. Y-linked
- D. X-linked recessive
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Answer: D. X-linked recessive
The gene for clotting factor VIII is on the X chromosome and the faulty allele is recessive. Males (XY) need only one copy to be affected, so the disorder is far more common in males; carrier mothers pass it to half their sons on average.
Q6 · Parcha original · Sample
A person with genotype Iᴬi has blood group:
- A. A
- B. B
- C. AB
- D. O
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Answer: A. A
Iᴬ is dominant over i, so Iᴬi makes only the A antigen and the blood group is A. Group O needs ii.