The principle of independent assortment is based on the fact that

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Question restatement: The principle of independent assortment is based on the fact that
- Option A: each set of sister chromatids is positioned independently at the metaphase plate of meiosis I.
- Option B: crossing over occurs within all pairs of homologous chromosomes.
- Option C: each homologous pair of chromosomes is positioned independently at the metaphase plate of meiosis I.
- Option D: each homologous pair of chromosomes is positioned independently at the metaphase plate of meiosis II.

Analysis of each option:
Option A: 'each set of sister chromatids is positioned independently at the metaphase plate of meiosis I.' This is incorrect because independent assortment concerns how paternal and maternal chromosomes (homologous pairs) align and separate, not the sister chromatids themselves. Sister chromatids are ultimately separated during anaphase II (and also during meiosis I for homologous chromosomes, but the concept of independent assortment refers to homologous chromosome orientation, not sister chromatid separation). The phrasing misattributes independence to sister chromatids and to meiosis I metaphase plate positioning.

Option B: 'crossing over occurs within all pairs of homologous chromosomes.' While crossing over does occur between nonsister chromatids of homologous chromosomes during prophase I, it is not the basis of the principle of independent assortment. Crossing over increases genetic recombination but independent assortment specifically refers to the random orientation of homologous chromosome pairs at metaphase I, not the occurrence of crossing over itself. Additionally, it is not accurate to claim it occurs within all pairs in every case.

Option C: 'each homologous pair of chromosomes is positioned independently at the metaphase plate of meiosis I.' This directly reflects the core idea of independent assortment: the orientation of one homologous pair is independent of the orientation of other pairs during metaphase I, leading to many possible combinations of maternal and paternal chromosomes in gametes. This aligns with Mendelian genetics and the mechanism by which genetic variation arises through independent alignment and separation.

Option D: 'each homologous pair of chromosomes is positioned independently at the metaphase plate of meiosis II.' This is inaccurate because independent assortment is classically described for metaphase I, where homologous pairs align and segregate. In meiosis II, the cells contain haploid sets, and there are no homologous chromosome pairs to orient independently in the same sense as metaphase I. Therefore, this statement misplaces the stage at which independent assortment operates.

In summary, option C correctly describes the foundational concept: the random, independent orientation of each homologous chromosome pair at metaphase I leads to independent assortment of alleles into gametes, whereas the other statements either misidentify the chromosomes involved (sister chromatids), attribute the concept to crossing over, or misstate the stage (meiosis II).

The principle of independent assortment is based on the fact thatSingle choice

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