Polyploidy is involved in which of the following examples of chromosome number change?

Introduction / Context:
This question addresses the concept of polyploidy, which is an increase in the number of complete sets of chromosomes rather than gain or loss of individual chromosomes. Polyploidy is common in plants and can lead to larger fruits or seedless varieties. Recognising which example illustrates a change in whole sets of chromosomes helps you distinguish polyploidy from aneuploidy.

Given Data / Assumptions:
- Polyploidy involves three or more complete sets of chromosomes in a cell.- Aneuploidy involves gain or loss of individual chromosomes, not whole sets.- The options include sex chromosome changes, alternation of generations and differences in chromosome numbers in watermelons.- You must identify which example shows a polyploid increase in chromosome set number.

Concept / Approach:
Normal diploid watermelons with 22 chromosomes represent 2n, while seedless watermelons with 33 chromosomes represent 3n, a triploid condition. Having three sets of chromosomes is a clear example of polyploidy. In contrast, conditions like XYY or XO involve extra or missing single sex chromosomes and are examples of aneuploidy. Alternation between haploid and diploid phases in plant life cycles does not by itself represent polyploidy; it is a normal feature of many organisms. Therefore, the correct option is the one describing seedless watermelons with 33 chromosomes compared to normal watermelons with 22.

Step-by-Step Solution:
Step 1: Recall that diploid organisms have two sets of chromosomes, noted as 2n.Step 2: Recognise that triploid organisms have three sets, 3n, which is a type of polyploidy.Step 3: Examine each option and determine whether it involves extra or missing individual chromosomes or extra sets.Step 4: Notice that the example where normal watermelons have 22 chromosomes and seedless watermelons have 33 implies a 3n condition.Step 5: Select option D as it clearly illustrates polyploidy, while the others illustrate aneuploidy or normal life cycle alternation.

Verification / Alternative check:
Agricultural breeding programmes often create seedless fruit by producing triploid plants, which supports the idea that 33 chromosomes in watermelons represent polyploidy.Genetics texts explicitly describe XXY, XYY and XO as examples of aneuploidy involving sex chromosomes rather than changes in complete sets.This confirms that only the watermelon example matches the definition of polyploidy given in the question.

Why Other Options Are Wrong:
Option A is wrong because XYY males have an extra Y chromosome and are therefore an example of aneuploidy, not polyploidy.Option B is wrong because alternation between haploid and diploid phases is a normal life cycle pattern and does not necessarily involve extra sets beyond the usual 1n and 2n.Option C is wrong because XO females have one missing X chromosome, which is again aneuploidy.Option E is wrong because a missing chromosome 21 is also an example of aneuploidy and does not involve additional complete sets of chromosomes.

Common Pitfalls:
Students sometimes confuse any change in chromosome number with polyploidy, forgetting that polyploidy involves whole sets rather than single chromosomes.Another pitfall is to assume that alternation of generations must mean polyploidy, even though it may simply reflect normal haploid and diploid stages.Remember that in exam questions, polyploidy is commonly illustrated by triploid or tetraploid crop plants, such as seedless watermelons and some wheat varieties.

Final Answer:
Polyploidy is involved in the example where a normal watermelon has 22 chromosomes but seedless watermelons have 33 chromosomes.