If both parents are affected with the same autosomal recessive disorder then the probability that each of their children will be affected equals

Correct Answer: D

Rationale: If both parents are homozygous recessive (aa x aa), all offspring will be aa (100% affected).

Correct Answer: C

Rationale: UAG is a stop codon; changing codon 91 to UAA (another stop) is nonsense, causing premature termination.

Correct Answer: C

Rationale: Amniotic fluid (C) is used in amniocentesis to study fetal chromosomes. Rationale: It contains fetal cells (e.g., amniocytes) that can be cultured for karyotyping, unlike lymphocytes (postnatal) or cheek swabs (not fetal). Skin biopsy and bone marrow are less common prenatally.

Correct Answer: B

Rationale: Giemsa stain (B) is standard for G-banding in karyotyping. Rationale: Giemsa produces dark (G-positive) and light bands, widely used for chromosome identification. Quinacrine (Q-banding) and trypsin (pre-treatment) are less common alone.

Correct Answer: C

Rationale: Trisomy 21 (Down syndrome, C) is most survivable. Rationale: Edwards (18) and Patau (13) are often lethal early; Turner (45, X) is viable but less common than Down syndrome, which has milder effects and higher live birth rates.