45. Autotetraploids arise by the doubling of 2n complement to 4m There are three different pairing possibilities at meiosis in tetraploids as given below:
A. Two bivalents
B. One quadrivalent
C. One univalent + one trivalent
Which of the above pairings can lead to production of diploid?
(1) only A           (2) B and c
(3) A and C        (4) A and B

Basic idea

An autotetraploid has four homologous copies of each chromosome (AAAA).
Normal reduction to balanced gametes is n = 2x (one pair worth), but certain mis‑pairings can produce 2n gametes (diploid for that chromosome set).

The question asks which pairing configurations at meiosis can lead to production of diploid gametes.

Option A: Two bivalents – can give diploid gametes

• Configuration: AA | AA (two separate pairs).

• If each bivalent segregates reductionally but to the same pole (both A’s from each pair go together), a gamete can receive two homologous chromosomes from each bivalent, i.e. 2n for that set.

• Thus, mis‑segregation of two bivalents can yield diploid (2n) gametes, so A qualifies.

Option B: One quadrivalent – generally gives reduced gametes

• All four homologs form a single quadrivalent.

• Typical segregation patterns (alternate or adjacent) tend to give gametes with two chromatids total for that chromosome set (balanced or with duplication/deletion), but not a full 2n complement of homologs for that set.

• While quadrivalents are a major source of aneuploid gametes, they are not the usual route to clean diploid gametes for that chromosome set.

• For exam purposes, B is not taken as leading to diploid gametes.

Option C: One univalent + one trivalent – can give diploid gametes

• One chromosome fails to pair (univalent A) and three form a trivalent (AAA).

• The univalent may lag or move randomly to one pole; the trivalent can segregate 2:1.

• A gamete that receives:

  • 2 chromosomes from the trivalent side plus

  • the univalent
    can end up with three copies, and complementary gametes may carry two copies (2n) depending on how the rest of the genome behaves.

• Such irregular segregation is recognized in classical cytogenetics as a path to diploid or higher‑ploidy gametes from autotetraploids.

• Hence C is counted as a configuration that can produce diploid gametes.

Option D: Answer sets

• (1) only A – incomplete, ignores the possibility from univalent + trivalent.

• (2) B and C – includes B, which is not accepted for clean 2n gamete production.

• (3) A and C – correct, both can generate diploid gametes through mis‑segregation.

• (4) A and B – again wrongly includes B.

Therefore, the pairing configurations in an autotetraploid that can lead to production of diploid gametes are two bivalents (A) and one univalent + one trivalent (C), so the correct option is (3) A and C.