All Free Biology MCQs with Answers

711. The visible cross shaped points at which homologous chromatids have exchanged segments are called

• A. chiasmata
• B. centromeres
• C. kinetochores
• D. telomeres

Explanation: Chiasmata can be seen under the microscope in prophase one and mark where crossing over has occurred. The centromere is the chromatid junction and the kinetochore its spindle attachment, neither of which involves exchange.

712. Distances between genes on a chromosome map are expressed in

• A. micrometres
• B. map units, or centiMorgans
• C. nanometres
• D. base pairs per gamete

Explanation: One map unit, one centiMorgan, equals one per cent recombination between two genes. The map is therefore a measure of recombination frequency rather than a physical length, though the two roughly correspond.

713. Two genes on the same chromosome that lie very far apart will show

• A. no recombination at all
• B. complete linkage in every meiosis
• C. a higher recombination frequency than genes lying close together
• D. fewer crossing overs than genes lying close together

Explanation: The longer the stretch of chromosome between two genes, the more likely a crossover will fall between them, so distant genes recombine more often. Genes very close together are strongly linked and rarely separated.

714. The chromosomal basis of linkage was worked out mainly through breeding experiments on Drosophila by

• A. Gregor Mendel
• B. Jean Baptiste Lamarck
• C. James Watson
• D. Thomas Hunt Morgan

Explanation: Morgan showed that certain Drosophila characters were inherited together and tied to the sex chromosomes, founding chromosome mapping. Mendel worked before chromosomes were understood, and Watson's fame rests on the structure of DNA.

715. In a test cross involving two linked genes, the offspring consist mostly of

• A. parental combinations of characters
• B. equal numbers of all four combinations
• C. recombinant combinations only
• D. non viable zygotes

Explanation: Because linked genes usually travel together, parental combinations outnumber recombinants, and the size of the minority reveals how far apart the genes lie. Equal numbers of all types would instead indicate independent assortment.

716. The number of linkage groups in human beings is

• A. 46
• B. 23
• C. 22
• D. 44

Explanation: One linkage group corresponds to each kind of chromosome in the haploid set, so humans have 23. The diploid number of 46 counts each homologue twice and is therefore a distractor.

717. The recombination frequency between two genes on the same chromosome can never exceed

• A. 25 per cent
• B. 40 per cent
• C. 50 per cent
• D. 100 per cent

Explanation: Once genes are far enough apart that a crossover happens between them in every meiosis, half the gametes are recombinant, the same figure as independent assortment. Frequencies above 50 per cent are therefore impossible to observe.

718. Traits controlled by genes located on the X chromosome are described as

• A. autosomal
• B. Y linked
• C. mitochondrial
• D. X linked

Explanation: The X chromosome carries many genes unrelated to sex, and their inheritance is distinctive because males have only one copy. Autosomal traits sit on chromosomes 1 to 22 and show no such difference between the sexes.

719. 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.

720. 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.