The karyotype where euchromatic regions stain more darkly and the light regions are heretochromatin is:

Correct Answer: D

Rationale: R-banding is correct. Rationale: Euchromatin is gene-rich and active, staining dark in R-banding with reverse Giemsa or fluorescent dyes, while heterochromatin (repetitive, inactive) stains light.

Correct Answer: A

Rationale: CML is caused by t(9;22) (Philadelphia chromosome, A). Rationale: BCR-ABL1 fusion drives uncontrolled myeloid proliferation. Trisomy 18 and 13 cause Edwards and Patau syndromes, not CML.

Correct Answer: C

Rationale: 50% (C) is correct. Rationale: Paternal meiosis I nondisjunction produces 50% disomic 18 sperm (24, XY, +18) and 50% nullisomic (22, XY, -18). Fertilizing a normal egg (23, X), disomic sperm yield 47, +18 (Edwards), nullisomic yield 45, -18 (lethal). Thus, 50% viable zygotes are affected.

Correct Answer: B

Rationale: Complete mole (B) results. Rationale: Egg without nucleus (0 chromosomes) fertilized by sperm (23, X or Y) that duplicates (46, XX or YY, all paternal) forms a complete mole. No fetus, just placental tissue. Partial mole needs maternal DNA.

Correct Answer: D

Rationale: If a gamete has 16 chromosomes (n=16), the diploid cell (2n=32) before meiosis I has 32 chromosomes, each with 2 chromatids post-S phase, totaling 64 chromatids. 'D' is correct. Rationale: Before anaphase I, homologous pairs align; each chromosome has duplicated, so 32 x 2 = 64 chromatids.