Free Inheritance MCQs with Answers

11. Genes situated on the same chromosome that tend to be inherited together are said to be

• A. allelic
• B. linked
• C. homologous
• D. codominant

Explanation: All the genes on one chromosome form a linkage group, and the number of linkage groups in a species equals its haploid chromosome number. Alleles are the alternative forms of one gene occupying the same locus on homologous chromosomes, which is a different relationship altogether. Linkage reduces the proportion of recombinant offspring below the Mendelian expectation.

12. Crossing over between homologous chromosomes takes place during

• A. metaphase I
• B. anaphase I
• C. prophase I
• D. telophase II

Explanation: During the pachytene stage of prophase I the homologues are paired as bivalents, and non sister chromatids exchange equal segments at points called chiasmata. Pairing must happen before the chromosomes line up on the spindle, so no later stage can allow the exchange. The chiasmata remain visible into metaphase I and hold the bivalent together.

13. Two linked genes show a recombination frequency of 10 per cent. The distance between them on the chromosome map is

• A. 10 map units
• B. 1 map unit
• C. 90 map units
• D. 100 map units

Explanation: One map unit, or centimorgan, is defined as a recombination frequency of one per cent, so ten per cent recombinants means ten map units apart. The logic is that the further apart two genes lie, the more likely a chiasma forms between them. Frequencies above fifty per cent are never observed, because at that point the genes assort as though unlinked.

14. The biological significance of crossing over is that it

• A. keeps linkage groups permanently intact
• B. halves the chromosome number of the gametes
• C. ensures that all gametes are genetically identical
• D. produces new combinations of alleles on the same chromosome

Explanation: Exchange between non sister chromatids creates chromosomes carrying combinations of alleles that neither parent had, and this recombination is a major source of the variation natural selection works on. Halving the chromosome number is the job of the reduction division itself, not of crossing over. Crossing over does the opposite of keeping linkage groups intact, since it breaks linkage up.

15. Red green colour blindness is far commoner in men than in women because

• A. a man has only one X chromosome, so a single recessive allele is expressed
• B. the gene lies on the Y chromosome
• C. the allele is dominant in men and recessive in women
• D. women lose the allele during meiosis

Explanation: The gene is on the X chromosome and a man is hemizygous, so there is no second copy that could mask a defective allele. A woman needs the recessive allele on both of her X chromosomes to be colour blind, which is a far less likely combination, and with only one she is a carrier with normal vision. The Y chromosome carries no corresponding gene.

16. A woman who is a carrier for colour blindness marries a man with normal colour vision. The probability that any son of theirs is colour blind is

• A. 0
• B. one in four
• C. one in two
• D. certain

Explanation: A son takes his single X from his mother, and half of her X chromosomes carry the defective allele, so half the sons are colour blind. The answer of one in four applies to the different question of the chance that a child of either sex is colour blind, which is the usual mistake here. None of the daughters is colour blind, since each receives a normal X from the father, but half are carriers.

17. Haemophilia A is caused by the deficiency of

• A. clotting factor VIII
• B. vitamin K
• C. platelets
• D. haemoglobin

Explanation: The X linked recessive gene codes for antihaemophilic factor VIII, and without it the clotting cascade stalls, so even a small injury bleeds for a very long time and bleeding into joints is common. Vitamin K deficiency also impairs clotting but does so by affecting several liver made factors and is dietary rather than inherited. The platelet count in haemophilia is normal.

18. A colour blind girl can be born only if

• A. her mother alone is colour blind
• B. her father alone is colour blind
• C. her father is colour blind and her mother is at least a carrier
• D. neither parent carries the allele

Explanation: A girl has two X chromosomes and needs the recessive allele on both, one from each parent. The father can only supply it if he is colour blind himself, and the mother must be either a carrier or colour blind. This is why affected women are rare, and it explains the pattern in which the condition seems to skip generations on the mother's side.

19. The sex of a human child is determined by

• A. the ovum, which may carry either an X or a Y chromosome
• B. the hormones of the mother during pregnancy
• C. the total number of chromosomes in the zygote
• D. the sperm, which carries either an X or a Y chromosome

Explanation: The mother is XX and so every ovum carries an X, while the father is XY and produces X bearing and Y bearing sperm in equal numbers, so it is the sperm that decides. A Y bearing sperm gives an XY male and an X bearing sperm an XX female, which also explains the roughly equal sex ratio at birth. Blaming the mother for the sex of a child has no biological basis whatsoever.

20. In X linked recessive inheritance, the criss cross pattern means that a trait typically passes from

• A. father to son directly
• B. mother to daughter only
• C. father to every one of his sons
• D. an affected man to his grandsons through his carrier daughters

Explanation: An affected father gives his X, and therefore the allele, to all of his daughters, who become carriers, and half of their sons then inherit it, so the trait reappears in the grandsons. A father can never pass an X linked allele to a son, because a son receives his Y from the father and his X from the mother. That impossibility is the quickest way to rule out X linkage in a pedigree.