CLEP Biology Mendelian Inheritance Patterns 3 — Questions and Answers
Question 1: In a testcross, an organism showing the dominant phenotype is crossed with a homozygous recessive individual. If the offspring ratio is 1:1 (dominant:recessive), what is the genotype of the dominant-phenotype parent?
- Heterozygous (Aa) (Correct answer)
- Homozygous dominant (AA)
- Homozygous recessive (aa)
- Cannot be determined
Correct answer: Heterozygous (Aa)
A 1:1 ratio in a testcross reveals that the unknown parent is heterozygous, contributing either A or a to offspring equally.
Question 2: Which statement correctly distinguishes Mendel's law of independent assortment from his law of segregation?
- Independent assortment applies to genes on different chromosomes, while segregation applies to alleles of a single gene (Correct answer)
- Segregation applies to sex chromosomes only, while independent assortment applies to autosomes
- Independent assortment describes how alleles blend in offspring, while segregation describes dominance
- Both laws describe the same phenomenon but in different organisms
Correct answer: Independent assortment applies to genes on different chromosomes, while segregation applies to alleles of a single gene
Segregation addresses separation of alleles at one locus; independent assortment addresses how genes at different loci (on different chromosomes) distribute independently.
Question 3: A woman is a carrier for an X-linked recessive condition. Her partner is unaffected. What is the probability that their daughter will be affected?
- 0% (Correct answer)
- 25%
- 50%
- 100%
Correct answer: 0%
The father contributes a normal X to all daughters, so daughters will be either carriers or unaffected — none will be homozygous recessive.
Question 4: In ABO blood typing, both I^A and I^B alleles are expressed in the I^A I^B individual (blood type AB). This exemplifies:
- Codominance (Correct answer)
- Incomplete dominance
- Epistasis
- Pleiotropy
Correct answer: Codominance
Codominance occurs when both alleles are fully and simultaneously expressed in heterozygotes, as seen in AB blood type.
Question 5: Two parents are each heterozygous for a condition caused by an autosomal dominant allele (Aa × Aa). What percentage of their offspring are expected to be unaffected?
- 25% (Correct answer)
- 50%
- 75%
- 100%
Correct answer: 25%
From Aa × Aa, offspring ratios are 1 AA : 2 Aa : 1 aa, so only aa (25%) are unaffected.
Question 6: Mendel chose pea plants for his experiments partly because they have:
- Distinct, easily scored traits with clear dominant-recessive relationships (Correct answer)
- Very long generation times allowing careful observation
- Only two chromosomes, simplifying genetic analysis
- Exclusively asexual reproduction for controlled crosses
Correct answer: Distinct, easily scored traits with clear dominant-recessive relationships
Pea plants offered discrete, contrasting traits (e.g., round vs. wrinkled seeds), short generation time, and easy controlled pollination.
Question 7: Which of the following ratios is characteristic of a monohybrid cross between two heterozygous parents (Aa × Aa) for a trait with complete dominance?
- 3:1 (Correct answer)
- 1:2:1
- 1:1
- 9:3:3:1
Correct answer: 3:1
A monohybrid cross Aa × Aa produces 3 dominant phenotype : 1 recessive phenotype (even though the genotype ratio is 1 AA : 2 Aa : 1 aa).
CLEP Biology Mendelian Inheritance Patterns 3
In a testcross, an organism showing the dominant phenotype is crossed with a homozygous recessive individual.
If the offspring ratio is 1:1 (dominant:recessive), what is the genotype of the dominant-phenotype parent?