Cytogenetics Multiple Choice Questions on “Chromosomal Abnormality : Inversion”.
1. Which of the following is not true about inversion?
A. Inverted chromosomes are generally viable
B. Inversion can cause chromosome breakage
C. Two DNA strands with an inverted segment will not pair
D. Inversion including centromere is known as paracenric
Answer: C
Clarification: Two DNA segments in inversion will pair but the pairing will be via formation of a loop such that appropriate gene loci can come close. Other options about inversion are true.
2. Consider this sequence A—O—B —C—D—E—F, be a DNA sequence where O is the centromere. Which of the following will be a pericentric inversion?
A. A—O—B—D—E—F
B. B—O—A—D—E—F
C. D—B—O—A—E—F
D. A—O—E—D—B—F
Answer: D
Clarification: Pericentric inversion is the inversion that doesn’t involve the centromere. Here in option A there is no change at all, option B and C are including the centromere. Only option D doesn’t involve the centromere in the inverted sequence, so it is pericentric.
3. ______________ inversions reduce crossing over in ______________
A. Paracentric, Heterozygous
B. Pericentric, Heterozygous
C. Paracentric, homozygous
D. Pericentric homozygous
Answer: B
Clarification: In heterozygous female the pairing is due to looping and then recombination in pericentric region can produce abnormal chromosomes which don’t separate properly in anaphase. Thus, it prevents recombination in heterozygous females.
4. What are the results of pericentric inversion with single crossing over?
A. Two normal, one dicentric and one acentric chromosome
B. 2 dicentric and 2 acentric chromosomes
C. Four normal chromosomes with centriomere in different positions
D. 3 normal chromosome, one abnormal acentric chromosome
Answer: A
Clarification: Recombination in the paracentric region will take place between only two strands, so the other two strands will be normal. This recombination will result in one dicentric chromosome and another acentric chromosome.
5. In Drosophila Crossing over in paracentric region is lethal and leads to cross-over suppression. This leads to reduction in fertility.
A. True
B. False
Answer: B
Clarification: In Drosophila only females undergo recombination. In the case of egg formation out of the 4 meiotic products, only one forms the egg and the 3 other forms the polar body. Thus, the cross over suppressed gamete simply forms the polar body so doesn’t affect fertility.
6. Long pericentric inversions generally don’t act as cross over suppressors. Why?
A. Long stretches of DNA recombination not recognized
B. Mechanism is different for short and long inversions
C. Two events of crossing over take place
D. Cross over product in this is viable
Answer: C
Clarification: Longer stretches of inversion favour formation of Double Crossover, then the DNA are resolved normally between all 4 meiotic products and no dicentric chromosome is formed. Thus, Long pericentric inversion generally don’t act as cross over suppressors.
7. Choose the wrong option.
A. Paracentric inversion cross over products is non-viable
B. Pacentric non-cross over gametes segregates normally
C. Double cross over in paracentric inversion is lethal
D. There is genetic imbalance in gametes produced by paracentric inversion cross over
Answer: C
Clarification: In paracentric inversion there is chromosomal imbalance in products of single cross over, but double cross over products lead to normal gene balance and viable gametes.
8. In drosophila male there is no cross over suppression due to pericentric inversion as___________
A. They have only one set of chromosome
B. They always have double cross over
C. They never recombine
D. They don’t undergo meiosis
Answer: C
Clarification: Drosophila males don’t recombine due to the absence of synaptonemal complex formation. Thus there is no question of recombination for it to be suppressed.
9. Which of the following is an example of inversion?
A. Chromosome 22 and 9
B. Chromosome 8
C. Chromosome 14
D. Chromosome 3
Answer: D
Clarification: In chromosome 3 of human there is duplication-deletion which leads to inversion. The rest of the options give examples of genes participating in translocation.
10. In pericentric inversion, the inversion loop involves __________ strands.
A. 1
B. 2
C. 3
D. 4
Answer: D
Clarification: In pericentric inversion there will be 4 strands in the inversion loop although only 2 will undergo recombination to give the recombination products. This is because the sister chromatids are bound by the centromere.
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