How many double stranded DNA molecules are in a somatic human cell that is in present G2 phase:

Correct Answer: C

Rationale: In G2 phase, after DNA replication (S phase), each of the 46 chromosomes consists of 2 sister chromatids (double-stranded DNA molecules). Thus, 92 double-stranded molecules exist. Answer 'C' (92) is correct. Rationale: Each chromosome’s chromatids are identical double helices, totaling 92 in a diploid human cell (2n=46).

Correct Answer: C

Rationale: 45, XY, t(14,21) (C) is a balanced Robertsonian translocation, phenotypically normal. Rationale: 46, 0Y is inviable (no X), trisomy 14 is lethal, but t(14;21) carriers have normal gene content despite 45 chromosomes.

Correct Answer: D

Rationale: 1/2 (D) is approximate. Rationale: Balanced translocation (e.g., t(11;22)) with normal partner yields 4 gamete types via adjacent/alternate segregation: 50% balanced (normal or carrier), 50% unbalanced (duplication/deletion), risking abnormal fetuses. Exact risk varies, but 1/2 is typical.

Correct Answer: B

Rationale: Mitosis post-fertilization (B) causes mosaicism. Rationale: 46, XX/47, XX, +13 arises when nondisjunction in an early mitotic division of a 46, XX zygote loses or gains a 13, creating two cell lines. Meiotic errors produce uniform aneuploidy.

Correct Answer: A

Rationale: Recombination (crossing over) occurs in pachytene of prophase I. 'A' is correct. Rationale: In meiosis I, leptotene condenses chromosomes, zygotene pairs them, pachytene facilitates crossing over via chiasmata, diplotene reveals chiasmata, and diakinesis prepares for separation.