Correct Answer: B

Rationale: The actual sets of traits that people exhibit are called phenotypes, not genotypes. Genotypes refer to the genetic makeup of an individual.

Correct Answer: C

Rationale: Down's syndrome, also known as trisomy 21, is caused by the presence of an extra chromosome on the 21st pair. Choice A is incorrect as alcohol abuse is not the cause of Down's syndrome. Choice B is incorrect because Down's syndrome is not related to sex-linked chromosomal abnormalities. Choice D is also incorrect as drug abuse by the mother during pregnancy is not the cause of Down's syndrome.

Correct Answer: D

Rationale: Hegar's sign, which is a softening of the lower uterine segment, is considered a probable sign of pregnancy as it indicates changes in the cervix and uterus that occur during pregnancy. Amenorrhea, the absence of menstruation, is a common early sign of pregnancy but can also be due to other factors. Morning sickness, nausea and vomiting, can be a sign of early pregnancy but is not as specific as Hegar's sign. Breast tenderness is a common symptom in early pregnancy due to hormonal changes, but it is not as definitive as Hegar's sign in indicating pregnancy.

Correct Answer: B

Rationale: The actual sets of traits that people exhibit are called phenotypes, not genotypes. Genotypes refer to the genetic makeup of an individual.

Correct Answer: B

Rationale: Human cells typically contain 46 chromosomes organized into 23 pairs. This is the correct number for a normal human cell. Choices A, C, and D are incorrect because they do not represent the typical chromosome count in human cells.