12. The Mendelian law of Independent assortment is due to arrangement of chromosome during
(1) Metaphase-I        (2) Anaphase-I
(3) Metaphase-II       (4) Anaphase-II

Explanation:

Mendel’s law of independent assortment states that alleles of different genes (on different chromosome pairs) segregate into gametes independently of one another. Cytologically, this comes from the way homologous chromosome pairs behave during meiosis.

• In metaphase I, each pair of homologous chromosomes (a bivalent) aligns on the equatorial plate, and the orientation of each pair is random relative to other pairs. One homolog may face one pole or the other with equal probability. This random alignment of multiple bivalents is what produces independent assortment of genes located on different chromosome pairs.

Option-wise:

1. Metaphase-I – correct

• Random orientation of each homologous pair at the metaphase I plate is the direct physical basis of Mendel’s law of independent assortment.

2. Anaphase-I

• Here homologous chromosomes actually move to opposite poles, but the “independence” has already been determined by how they were oriented in metaphase I. Anaphase I just executes the segregation.

3. Metaphase-II

• In metaphase II, sister chromatids (not homologous pairs) align; this stage relates more to the separation of chromatids, not to independent assortment of different homologous chromosome pairs.

4. Anaphase-II

• Sister chromatids separate here (basis for the law of segregation at the chromatid level), not for independent assortment of different chromosome pairs.

So, Mendel’s law of independent assortment specifically reflects the random arrangement of homologous pairs during metaphase I of meiosis, making option (1) the correct choice.