21. Which of the following representations of chromosomal arrangement in meiotic metaphase I best explains the Law of Independent Assortment?

Concept: independent assortment and metaphase I

• Consider a dihybrid $AaBb$ where A/a and B/b are on different homologous chromosome pairs.

• In metaphase I, each homologous pair (A vs a, B vs b) aligns independently on the spindle.​

• Two equally probable orientations exist:

  • A and B toward the same pole; a and b toward the opposite pole.

  • A with b and a with B.

• Because each orientation occurs in 50% of meiotic cells, the four gamete types (AB, Ab, aB, ab) appear in equal frequency, which is the essence of the law of independent assortment.

Option (1) shows exactly this:

• Left cell: A and B on the top side, a and b on the bottom (one orientation, 50%).

• Right cell: A with b on top, a with B on bottom (alternate orientation, 50%).

Why the other options are incorrect

2. Option (2)

• The diagrams do not place the A/a and B/b homologues in truly independent random orientations.

• The way the chromosomes are grouped would bias which alleles travel together, so it does not correctly portray independent assortment with two equally likely orientations.

3. Option (3)

• Shows one chromosome pair oriented vertically and the other horizontally in both cells, but the allele combinations relative to poles are not swapped between the two diagrams.

• This fails to depict the complementary 50% orientations that generate all four gamete types in equal numbers.

4. Option (4)

• Labeled “OR 100%”, it implies only one fixed arrangement in all cells.

• That would violate independent assortment, because there would be no random orientation and certain gamete types would be missing.

Thus, the metaphase I representation that best explains Mendel’s law of independent assortment—random, 50:50 orientation of homologous chromosome pairs—is option (1).