Free Inheritance MCQs with Answers

41. A male expresses an X linked recessive trait whenever he inherits one recessive allele because he

• A. has only one X chromosome
• B. has two Y chromosomes
• C. inactivates the normal allele
• D. lacks the gene altogether

Explanation: With no second X to supply a dominant allele, the single recessive allele has nothing to mask it, a state called hemizygosity. Females need two copies of the same allele to show the trait, which is why these conditions are commoner in men.

42. 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.

43. 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.

44. 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.

45. 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.

46. 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.

47. 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.

48. 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.

49. 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.

50. A test cross is used to determine

• A. The genotype of a dominant phenotype individual
• B. The number of chromosomes
• C. Mutation rates
• D. Linkage groups

Explanation: A test cross between an individual showing dominant phenotype and a homozygous recessive reveals whether the dominant is homozygous or heterozygous.