Correct Answer: A

Rationale: The correct answer is A: monozygotic (MZ) twins. Monozygotic twins, also known as identical twins, occur when a single zygote splits into two embryos, leading to two genetically identical individuals. Choice B, fraternal twins, are twins that develop from two separate eggs fertilized by two different sperm cells, resulting in non-identical siblings. Choice C, non-identical twins, is not a common term used to describe this type of twinning. Choice D, dizygotic (DZ) twins, refer to twins that develop from two separate eggs fertilized by two different sperm cells, leading to non-identical twins.

Correct Answer: B

Rationale: Individuals with Klinefelter syndrome have an extra X chromosome (XXY), leading to reduced production of testosterone, the primary male sex hormone. This deficiency can result in various physical and developmental characteristics associated with the syndrome. Estrogen levels may actually be relatively elevated due to the imbalance between testosterone and estrogen. Choices C and D are incorrect as adenine and thymine are nucleotide bases found in DNA and are not related to hormone production.

Correct Answer: D

Rationale: Matt is exhibiting symptoms typical of Huntington's disease (HD), a hereditary condition characterized by involuntary movements, cognitive impairment, and behavioral changes. The fact that Matt's father and grandfather had similar issues supports the genetic nature of the disease. Phenylketonuria (PKU) is a metabolic disorder that affects amino acid metabolism, not presenting with the symptoms described. Cystic fibrosis primarily affects the respiratory and digestive systems, not causing the neurological symptoms described. Turner syndrome is a genetic condition affecting females and is not associated with the symptoms described in the case of Matt.

Correct Answer: D

Rationale: The correct answer is D. The likelihood of having a child with Down's syndrome increases as the age of the parents increases, particularly the mother's age. Choice A is incorrect because Down's syndrome is caused by an extra copy of chromosome 21, not a defect in the sex chromosomes. Choice B is incorrect as the symptoms of Down's syndrome and sickle-cell anemia are different. Choice C is also incorrect as Down's syndrome is not caused by a sexually transmitted infection during conception.

Correct Answer: B

Rationale: The correct answer is B - FALSE. Genotypes are solely based on genetic information and do not reflect environmental influences. Phenotypes, on the other hand, result from the interaction of genetic and environmental factors. Choices A, C, and D are incorrect because genotypes are not influenced by environmental factors, and they are determined by an individual's genetic makeup.