27. The following statements were made regarding chromosome pairing (shown in the figure below) and subsequent segregation during meiosis-I in the reciprocal translocation heterozygote:

A. Three ways of segregation tn Anaphase would be adjacent 1 (vertically in the above figure) adjacent 2 (horizontally) and alternate.
B. Gametes resulting from adjacent 1 and adjacent 2 segregation will be nonwiable because of deletions and duplications of several genes.
C. All gametes resulting from alternate segregation Will be viable as they wilt carry both normal chromosomes or both chromosomes having trans locations in the two poles, respectively.
D. A dicentric and an acentric chromosome will be generated following alternate segregation.
Which of the following combination of statements Willmost appropriately explain the consequence?
(1) A, B and D               (2) A, B and C
(3) Only A and B       (4) Only A and C

Key concepts

In a reciprocal translocation heterozygote, two normal and two translocated chromosomes form a quadrivalent cross at meiosis I. This structure can segregate in three main ways:

• Adjacent-1 (vertical): each pole receives one normal and one translocated chromosome with nonhomologous centromeres.

• Adjacent-2 (horizontal): each pole receives one normal and one translocated chromosome but with homologous centromeres going together (rarer).

• Alternate: one pole gets both normal chromosomes, the other both translocated chromosomes; these are balanced.

Adjacent segregations produce gametes with duplications and deletions; alternate produces balanced gametes.

Statement-wise analysis

A. Three ways of segregation … adjacent 1 (vertical), adjacent 2 (horizontal) and alternate. – Correct

• Accurately lists the three main segregation patterns of a reciprocal translocation quadrivalent.

B. Gametes resulting from adjacent 1 and adjacent 2 segregation will be nonviable because of deletions and duplications of several genes. – Correct

• In both adjacent patterns, each gamete gets one normal + one translocated chromosome, giving partial monosomy and partial trisomy (duplication/deletion), typically lethal or severely unbalanced.

C. All gametes resulting from alternate segregation will be viable as they will carry both normal chromosomes or both chromosomes having translocations in the two poles, respectively. – Correct

• Alternate segregation sends both normals to one pole and both translocated derivatives to the other, giving two types of balanced gametes (normal and balanced translocation carrier), which are usually viable.

D. A dicentric and an acentric chromosome will be generated following alternate segregation. – Incorrect

• Acentric/dicentric chromatids arise from a paracentric inversion crossover, not from segregation of a reciprocal translocation quadrivalent. Alternate segregation preserves one centromere per chromosome.

Correct option

Since A, B and C accurately describe the meiotic behavior and outcomes of the reciprocal translocation heterozygote shown, the correct choice is:

• (2) A, B and C.