MCAT Genetics 1 — Questions and Answers
Question 1: In a dihybrid cross between two heterozygous parents (AaBb × AaBb), what is the expected phenotypic ratio in the offspring?
- 9:3:3:1 (Correct answer)
- 1:1:1:1
- 3:1
- 1:2:1
Correct answer: 9:3:3:1
A dihybrid cross between two double heterozygotes yields a 9:3:3:1 phenotypic ratio when the genes assort independently, producing four distinct phenotypic classes.
Question 2: According to Hardy-Weinberg equilibrium, which condition must be met for a population to remain in equilibrium?
- Natural selection is actively occurring
- Allele and genotype frequencies remain constant across generations (Correct answer)
- Mating is nonrandom within the population
- Population size is very small
Correct answer: Allele and genotype frequencies remain constant across generations
Hardy-Weinberg equilibrium describes a state where allele and genotype frequencies remain constant generation after generation in the absence of evolutionary forces.
Question 3: During DNA replication, which enzyme synthesizes new DNA strands in the 5' to 3' direction by adding nucleotides to the growing chain?
- DNA helicase
- DNA primase
- DNA polymerase III (Correct answer)
- DNA ligase
Correct answer: DNA polymerase III
DNA polymerase III is the primary replicative polymerase in prokaryotes, extending the new strand by adding deoxyribonucleotides to the 3'-OH end in the 5' to 3' direction.
Question 4: A missense mutation results in which of the following outcomes?
- A premature stop codon
- No change in the amino acid sequence
- A different amino acid incorporated into the protein (Correct answer)
- Deletion of an entire codon
Correct answer: A different amino acid incorporated into the protein
A missense mutation changes a single nucleotide, altering the codon so that a different amino acid is incorporated at that position in the polypeptide chain.
Question 5: In E. coli, when glucose is absent and lactose is present, which statement best describes the state of the lac operon?
- The repressor binds the operator and blocks transcription
- The operon is transcribed at high levels due to both positive and negative regulation (Correct answer)
- The catabolite activator protein (CAP) remains inactive
- The lac structural genes are not transcribed
Correct answer: The operon is transcribed at high levels due to both positive and negative regulation
With glucose absent, cAMP levels rise and activate CAP to stimulate transcription; with lactose present, allolactose inactivates the repressor, resulting in high-level transcription of the lac operon.
Question 6: Two genes are said to be genetically linked when they are:
- Located on different homologous chromosomes
- Always segregating independently during meiosis
- Located on the same chromosome (Correct answer)
- Showing a 9:3:3:1 ratio in dihybrid crosses
Correct answer: Located on the same chromosome
Linked genes reside on the same chromosome and tend to be inherited together unless separated by crossing over during meiosis.
Question 7: Which of the following correctly describes the central dogma of molecular biology?
- Protein → RNA → DNA
- DNA → Protein → RNA
- RNA → DNA → Protein
- DNA → RNA → Protein (Correct answer)
Correct answer: DNA → RNA → Protein
The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.
MCAT Genetics 1
In a dihybrid cross between two heterozygous parents (AaBb × AaBb), what is the expected phenotypic ratio in the offspring?