OAT Genetics and Evolution 1 — Questions and Answers
Question 1: Which of the following best describes the law of independent assortment?
- Alleles of a gene segregate equally into gametes
- Genes on different chromosomes are inherited independently of one another (Correct answer)
- Dominant alleles always mask recessive alleles
- Linked genes are always inherited together
Correct answer: Genes on different chromosomes are inherited independently of one another
Mendel's law of independent assortment states that alleles of genes located on different (non-homologous) chromosomes segregate independently during gamete formation.
Question 2: A woman who is a carrier for an X-linked recessive disorder mates with an unaffected man. What is the probability that their son will be affected?
- 0%
- 25%
- 50% (Correct answer)
- 100%
Correct answer: 50%
A carrier mother (X^A X^a) passes either X^A or X^a to sons with equal probability; sons receiving X^a will be affected, giving a 50% chance.
Question 3: Which type of mutation results in a premature stop codon?
- Missense mutation
- Silent mutation
- Nonsense mutation (Correct answer)
- Frameshift mutation
Correct answer: Nonsense mutation
A nonsense mutation changes a codon that encodes an amino acid into a stop codon (UAA, UAG, or UGA), causing premature termination of translation.
Question 4: In a testcross, an individual with an unknown genotype for a single trait is crossed with a:
- Heterozygous individual
- Homozygous dominant individual
- Homozygous recessive individual (Correct answer)
- Wild-type individual
Correct answer: Homozygous recessive individual
A testcross uses a homozygous recessive individual so that only the gametes of the unknown-genotype parent determine the offspring phenotype ratio.
Question 5: Which of the following is an example of codominance?
- A heterozygous flower producing a pink color instead of red or white
- AB blood type in humans expressing both A and B antigens (Correct answer)
- A recessive allele being completely masked by a dominant allele
- Two alleles blending to produce an intermediate phenotype
Correct answer: AB blood type in humans expressing both A and B antigens
Codominance occurs when both alleles are fully and simultaneously expressed; AB blood type is the classic example because both A and B antigens appear on red blood cells.
Question 6: Crossing over between homologous chromosomes occurs during which phase of meiosis?
- Prophase I (Correct answer)
- Metaphase I
- Anaphase II
- Telophase II
Correct answer: Prophase I
Crossing over (recombination) occurs during Prophase I of meiosis when homologous chromosomes synapse and form the synaptonemal complex at chiasmata.
Question 7: Which evolutionary mechanism causes allele frequencies to change randomly due to chance events in small populations?
- Natural selection
- Gene flow
- Genetic drift (Correct answer)
- Mutation
Correct answer: Genetic drift
Genetic drift is the random fluctuation of allele frequencies due to chance, and its effects are most pronounced in small populations.
