DP Biology (first assessment 2025)

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Question 22N.2.HL.TZ0.8b

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Date November 2022 Marks available [Maximum mark: 5] Reference code 22N.2.HL.TZ0.8b
Level HL Paper 2 Time zone TZ0
Command term Describe Question number b Adapted from N/A
b.
[Maximum mark: 5]
22N.2.HL.TZ0.8b
(b)

Describe polyploidy and how it can lead to speciation.

[5]

Markscheme
  1. polyploidy is having more than two (complete) sets of chromosomes/3n/4n/other specific example of polyploidy;
  2. can be due to errors in meiosis/production of diploid gametes;
  3. can be due to DNA replication without mitosis/cytokinesis;
  4. polyploidy causes reproductive isolation;
  5. diploids crossed with tetraploids produce infertile (triploid) offspring / triploid offspring are infertile;
  6. tetraploids are therefore a new species/failure to interbreed/reproductive isolation leads to speciation;
  7. (many) examples in the onion family/Allium/other valid example of speciation by polyploidy;
  8. infertile interspecific hybrids can become fertile by becoming polyploid;

Reject non-disjunction as a cause of polyploidy as it usually applies to a single bivalent.

Examiners report

A minority of answers defined polyploidy correctly and then explained how it results in speciation through reproductive isolation. Weaker responses muddled up aneuploidy with polyploidy so claimed that non-disjunction of a pair of chromosomes causes polyploidy. The average score for this question was low, because it tended to be weaker candidates who were answering it, and also because it is a relatively difficult part of the program.

Syllabus sections