Free Mendel's Laws of Inheritance MCQs with Answers

31. A colour blind man marries a woman with normal vision whose father was colour blind. What proportion of their daughters will be colour blind?

• A. None
• B. Half
• C. A quarter
• D. All of them

Explanation: The woman must be a carrier since she inherited her affected father's X. The colour blind father passes his affected X to every daughter, and the mother passes her affected X to half of them, so half the daughters are colour blind and half carriers.

32. Haemophilia became known as the royal disease because it

• A. was cured by royal physicians
• B. is common in every royal family
• C. was traced through the descendants of Queen Victoria
• D. was first described by a king

Explanation: Victoria carried the allele and passed it into the royal houses of Russia and Spain, producing affected princes across Europe. The pedigree became the classic textbook example of X linked recessive inheritance.

33. A trait determined by a gene on the Y chromosome is transmitted

• A. from mother to sons
• B. from father to all his sons
• C. equally to sons and daughters
• D. to daughters only

Explanation: Only males carry a Y chromosome, and a father passes it to every son and no daughter, so Y linked traits run strictly down the male line. Nothing on the Y can ever appear in a female.

34. Which of the following human conditions is inherited as an X linked recessive trait?

• A. Red green colour blindness
• B. Sickle cell anaemia
• C. Down syndrome
• D. Albinism

Explanation: The genes for red and green pigments lie on the X chromosome, so colour blindness follows the X linked pattern. Sickle cell anaemia and albinism are autosomal recessive, and Down syndrome is a chromosome number disorder, not a single gene trait.

35. A carrier of haemophilia is typically

• A. a man suffering from the disease
• B. a heterozygous woman with one normal and one haemophilia allele
• C. a homozygous normal woman
• D. a person with an extra chromosome

Explanation: A carrier holds the recessive allele without symptoms because the normal allele on her other X is enough for clotting. A man with the allele is affected rather than a carrier, since he has no second X.

36. A normal man marries a woman whose father had haemophilia. The probability that their first son will have haemophilia is

• A. zero
• B. one quarter
• C. one half
• D. certain

Explanation: The woman inherited her haemophiliac father's X, making her a carrier. She passes that X to half her sons, who will be affected, and the father's normal X contribution is irrelevant to sons, who receive his Y.

37. In the cells of a female mammal, one of the two X chromosomes condenses into a dense body called the

• A. centromere
• B. nucleolus
• C. chiasma
• D. Barr body

Explanation: The inactivated X, or Barr body, equalises the gene dose between XX females and XY males. Males, with a single active X, show no Barr body, which is why the test was once used to check chromosomal sex.

38. Mendel's law of segregation states that

• A. Alleles separate during gamete formation
• B. Different genes assort independently
• C. Dominant alleles always mask recessive
• D. Genes are on chromosomes

Explanation: The law of segregation states that allele pairs separate during gamete formation, so each gamete carries one allele.

39. Incomplete dominance results in

• A. Only dominant phenotype
• B. Only recessive phenotype
• C. Intermediate phenotype
• D. Both phenotypes expressed

Explanation: In incomplete dominance, the heterozygote shows an intermediate phenotype, e.g., pink flowers from red and white parents.

40. In Hardy-Weinberg equilibrium, p² represents

• A. Heterozygote frequency
• B. Homozygous dominant frequency
• C. Homozygous recessive frequency
• D. Total alleles

Explanation: p² = frequency of homozygous dominant (AA), 2pq = heterozygous (Aa), q² = homozygous recessive (aa).