In the structure of deoxyribonucleic acid (DNA), which of the following bases combines with thymine?

Correct Answer: B

Rationale: The correct answer is B: Adenine. In the structure of DNA, adenine pairs with thymine through hydrogen bonding, forming one of the complementary base pairs that make up the double helix structure of DNA. Cytosine pairs with guanine, not thymine. Rhodamine and serine are not DNA bases and do not participate in DNA base pairing. Therefore, choices A, C, and D are incorrect.

Correct Answer: C

Rationale: The correct answer is C. Mutations can occur due to environmental influences such as radiation, chemicals, or other external factors, leading to changes in the DNA sequence. Choice A is incorrect because mutations can also occur due to errors in DNA replication, not solely by chance. Choice B is incorrect as mutations affect the DNA sequence itself, not the production of specific cells. Choice D is incorrect because mutations involve changes in the DNA sequence and do not specifically involve the formation of the DNA structure.

Correct Answer: A

Rationale: The typical sex chromosome pattern for females is XX. Females have two X chromosomes, which is represented as XX. Choice B (XYY) is incorrect as it represents the sex chromosome pattern for males with an extra Y chromosome. Choice C (XY) is the sex chromosome pattern for males. Choice D (XXY) represents a genetic disorder known as Klinefelter syndrome, where males have an extra X chromosome.

Correct Answer: C

Rationale: A person is homozygous for a trait when they have two identical alleles for that trait. In this case, both alleles are the same, indicating a homozygous genotype. Choice A, 'monozygous,' is incorrect as it refers to identical twins originating from a single fertilized egg. Choice B, 'dizygous,' is also incorrect as it refers to fraternal twins or individuals that develop from two separate fertilized eggs. Choice D, 'hemizygous,' is incorrect because it describes a genetic condition where only one allele is present in a diploid organism, typically related to genes on the sex chromosomes.

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.