Pre AP Biology -GEN 4.3 Chromosomal Disorders- MCQ Exam Style Questions -New Syllabus

Pre AP Biology -GEN 4.3 Chromosomal Disorders- MCQ Exam Style Questions – New Syllabus 2025-2026

Pre AP Biology -GEN 4.3 Chromosomal Disorders- MCQ Exam Style Questions – Pre AP Biology – per latest Pre AP Biology Syllabus.

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Question

Which of the following could be an example of allopolyploidy?

a. Chromosome number in the offspring is exactly half that of the parents.
b. Gametes and somatic cells have the same number of chromosomes.
c. Chromosome number increases by one in a gamete and in the offspring it produces.
d. Chromosome number decreases by one in a gamete and in the offspring it produces.

▶️ Answer/Explanation

Correct Option: b

Detailed solution

Allopolyploidy involves the combination of chromosome sets from two different species.
In a typical hybrid, the resulting gametes are often sterile because chromosomes cannot pair properly.
If nondisjunction or chromosome doubling occurs, the gametes end up with a full somatic set.
This results in gametes and somatic cells having the same number of chromosomes, $2n = 2n$.
Option $a$ describes haploidy, while options $c$ and $d$ describe aneuploidy ($2n + 1$ or $2n – 1$).
Therefore, $b$ is the only scenario that aligns with the specialized doubling seen in polyploidy.

Question

Imagine that two protein-coding genes have very similar nucleotide sequences, and are located right next to each other on a chromosome. Which of these statements does this suggest?

a. One of them is a duplicate of the other, copied by a retrotransposon.
b. One of them is a pseudogene.
c. They were produced by unequal crossing-over.
d. They are transcribed in the same cell types.

▶️ Answer/Explanation

Detailed solution

The correct answer is c. They were produced by unequal crossing-over.
Unequal crossing-over occurs when homologous chromosomes misalign during meiosis.
This process results in one chromosome gaining an extra copy of a gene sequence.
The resulting duplicate genes are typically located tandemly (side-by-side) on the same chromosome.
Over time, these duplicates maintain high sequence similarity but may diverge in function.
Retrotransposons (option a) usually insert copies at distal, random locations, not necessarily adjacent.
While one could become a pseudogene (option b), the physical arrangement specifically suggests the mechanism of duplication.
Proximity does not strictly guarantee identical cell-specific transcription patterns (option d).

Question

Restriction enzymes are used in genetic engineering to precisely cut DNA. Which one of these methods tells us how they are used?

a. They digest DNA, one base at a time, from the $3’$ end of the DNA of interest.

b. They remove mismatched base pairs resulting from errors in ligation.

c. They break sugar–phosphate bonds in the DNA backbone between particular bases.

d. They cut PCR primers away from the template DNA.

▶️ Answer/Explanation

Detailed solution

The correct answer is c.

Restriction enzymes act as molecular scissors that recognize specific palindromic sequences.

They catalyze the hydrolysis of phosphodiester bonds within the sugar–phosphate backbone.

This cleavage occurs at precise locations between specific nucleotides on both DNA strands.

Option a describes exonucleases, which remove bases sequentially from ends, not internal cuts.

Option b refers to DNA repair or proofreading mechanisms rather than restriction digestion.

Option d is incorrect as primers are extended into the product, not removed by these enzymes.

Pre AP Biology -GEN 4.3 Chromosomal Disorders- MCQ Exam Style Questions -New Syllabus