Heredity sits inside the CLEP exam's organismal and population biology domains and is one of the most predictable topics. Watch for terse stems like "Which of the following is an example of a testcross?" and simple ratio questions.
A Punnett square predicts offspring ratios.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Remember: these ratios are probabilities. Small groups of offspring may deviate from the exact ratio by chance.
To determine whether an individual showing the dominant phenotype is homozygous (AA) or heterozygous (Aa), cross it with a homozygous recessive (aa) individual. - If any recessive offspring appear, the unknown parent was heterozygous. - If all offspring show the dominant trait, the parent was likely homozygous.
Genes on the X chromosome show sex-linked patterns. For an X-linked recessive trait (e.g., red-green color blindness, hemophilia), males (XY) need only one recessive allele to be affected, so such traits appear more often in males.
Example: carrier mother (XᶜXᶜ, where the second is the recessive allele) × unaffected father (XᶜY). Half of the sons inherit the recessive X and are affected.
A pedigree charts a trait across generations. Patterns help identify inheritance mode: - Autosomal dominant: appears in every generation, affects both sexes, affected children usually have an affected parent. - Autosomal recessive: can skip generations; unaffected parents can produce affected children (both are carriers). - X-linked recessive: affects males more often.
[GRAPH: Three-generation pedigree — squares (males) and circles (females); filled shapes mark affected individuals; horizontal lines link mates and vertical lines link parents to offspring.]
Genes reside on chromosomes, and allele segregation mirrors chromosome separation in meiosis. Nondisjunction is the failure of chromosomes to separate properly, producing gametes with an extra or missing chromosome. Fertilization then yields an abnormal chromosome number — for example, three copies of chromosome 21 causes Down syndrome (trisomy 21).
1. B — Genotype. Fix: Genotype is the allele combination an organism carries for a trait. - A: Phenotype is the observable trait, not the genetic makeup. - C: Allele frequency is a population-level measure. - D: A karyotype is a picture of chromosomes, not a single-trait genotype. - E: A pedigree charts inheritance across a family. - Fix rule: Genotype = the letters (Aa); phenotype = what you see.
2. E — The two alleles for a trait separate during gamete formation. Fix: Segregation is about alleles parting so each gamete gets one. - A: That describes independent assortment, a different law. - B: Dominance does not mean numerical majority. - C: Blending inheritance is the idea Mendel disproved. - D: Genes are on chromosomes, not ribosomes. - Fix rule: Segregation = alleles split; assortment = genes sort independently.
3. B — 3:1. Fix: A monohybrid Aa × Aa gives three dominant to one recessive phenotype. - A: 1:1 is a testcross-type ratio, not Aa × Aa. - C: 9:3:3:1 is the dihybrid ratio. - D: 1:2:1 is the genotype ratio, not phenotype. - E: A recessive (aa) quarter appears, so not all dominant. - Fix rule: Aa × Aa → 3:1 phenotype, 1:2:1 genotype.
4. C — 9:3:3:1. Fix: Two independently assorting heterozygous genes give the classic 9:3:3:1 dihybrid ratio. - A: 3:1 is the monohybrid ratio. - B: 1:1:1:1 fits a dihybrid testcross, not AaBb × AaBb. - D: 1:2:1 is a single-gene genotype ratio. - E: 3:3:1:1 is not a standard dihybrid outcome. - Fix rule: Full dihybrid heterozygote cross → 9:3:3:1.
5. A — Cross with a homozygous recessive individual. Fix: A testcross to aa exposes a hidden recessive allele; any recessive offspring reveal heterozygosity. - B: A homozygous dominant partner masks the recessive allele, revealing nothing. - C: An identical dominant phenotype does not distinguish AA from Aa. - D: Self-crossing plants is possible, but the animal restriction makes this option wrong. - E: A testcross does determine the genotype. - Fix rule: Unknown dominant genotype → cross to homozygous recessive (testcross).
6. E — Incomplete dominance. Fix: A blended intermediate (pink) between two homozygotes signals incomplete dominance. - A: Complete dominance would give all red or all white, not pink. - B: Codominance shows both colors distinctly, not a blend. - C: Flower color here is not sex-linked. - D: Only two alleles are involved, not a multiple-allele system. - Fix rule: Blended intermediate = incomplete dominance; both traits shown together = codominance.
7. A — Codominance. Fix: In type AB, the A and B alleles are each fully expressed, the definition of codominance. - B: Incomplete dominance would blend into an intermediate, not show both. - C: A frameshift is a mutation type, unrelated to allele expression. - D: Polygenic blending is not what ABO demonstrates. - E: ABO is autosomal, not sex-linked. - Fix rule: Both alleles fully visible together = codominance.
8. A — 1/2. Fix: Sons get their single X from the carrier mother; half receive the recessive allele and are affected. - B: Zero would require the mother to carry no recessive allele. - C: Only half of sons inherit the affected X. - D: 1/4 confuses this with an autosomal or two-carrier cross. - E: 3/4 has no basis in this single-carrier-mother cross. - Fix rule: X-linked recessive + carrier mother → half of sons affected.
9. C — Autosomal dominant. Fix: Appearing every generation in both sexes with an affected parent each time points to autosomal dominant. - A: X-linked recessive skews toward males and can skip generations. - B: Y-linked traits affect only males. - D: Mitochondrial traits pass from mothers and are not "males only." - E: Codominant blood typing is not a pedigree disease pattern like this. - Fix rule: Every generation + both sexes + affected parent → autosomal dominant.
10. E — Down syndrome (trisomy 21). Fix: Three copies of chromosome 21 from nondisjunction cause Down syndrome. - A: Turner syndrome is a single X (45,X), not trisomy 21. - B: Color blindness is an X-linked gene mutation, not a whole-chromosome error. - C: Sickle-cell anemia is a point mutation in a gene. - D: Klinefelter syndrome is XXY (a sex-chromosome trisomy), not chromosome 21. - Fix rule: Extra chromosome 21 → trisomy 21 = Down syndrome.
11. D — Incorrect; 3:1 is a probability, so small samples may deviate. Fix: Genetic ratios are expected averages; chance makes small groups vary. - A: The ratio is not guaranteed exact in any single group of four. - B: Short (tt) offspring can and do appear. - C: The expected ratio is 3:1, not 1:1. - E: Most offspring are tall, so "all short" is wrong. - Fix rule: Punnett ratios are probabilities, not guarantees for small samples.
12. D — Both parents are heterozygous carriers. Fix: Two unaffected parents producing an affected child indicates both carry a hidden recessive allele. - A: A homozygous recessive parent would show the disorder, contradicting "unaffected." - B: The pattern points to recessive, not dominant, inheritance. - C: Carrier parents explain it without invoking a new mutation. - E: Misdiagnosis is an unfounded leap when carrier status fits. - Fix rule: Unaffected parents + affected child → both are recessive carriers.
1. B — Genotype. Fix: Genotype is the allele combination an organism carries for a trait. - A: Phenotype is the observable trait, not the genetic makeup. - C: Allele frequency is a population-level measure. - D: A karyotype is a picture of chromosomes, not a single-trait genotype. - E: A pedigree charts inheritance across a family. - Fix rule: Genotype = the letters (Aa); phenotype = what you see.
2. E — The two alleles for a trait separate during gamete formation. Fix: Segregation is about alleles parting so each gamete gets one. - A: That describes independent assortment, a different law. - B: Dominance does not mean numerical majority. - C: Blending inheritance is the idea Mendel disproved. - D: Genes are on chromosomes, not ribosomes. - Fix rule: Segregation = alleles split; assortment = genes sort independently.
3. B — 3:1. Fix: A monohybrid Aa × Aa gives three dominant to one recessive phenotype. - A: 1:1 is a testcross-type ratio, not Aa × Aa. - C: 9:3:3:1 is the dihybrid ratio. - D: 1:2:1 is the genotype ratio, not phenotype. - E: A recessive (aa) quarter appears, so not all dominant. - Fix rule: Aa × Aa → 3:1 phenotype, 1:2:1 genotype.
4. C — 9:3:3:1. Fix: Two independently assorting heterozygous genes give the classic 9:3:3:1 dihybrid ratio. - A: 3:1 is the monohybrid ratio. - B: 1:1:1:1 fits a dihybrid testcross, not AaBb × AaBb. - D: 1:2:1 is a single-gene genotype ratio. - E: 3:3:1:1 is not a standard dihybrid outcome. - Fix rule: Full dihybrid heterozygote cross → 9:3:3:1.
5. A — Cross with a homozygous recessive individual. Fix: A testcross to aa exposes a hidden recessive allele; any recessive offspring reveal heterozygosity. - B: A homozygous dominant partner masks the recessive allele, revealing nothing. - C: An identical dominant phenotype does not distinguish AA from Aa. - D: Self-crossing plants is possible, but the animal restriction makes this option wrong. - E: A testcross does determine the genotype. - Fix rule: Unknown dominant genotype → cross to homozygous recessive (testcross).
6. E — Incomplete dominance. Fix: A blended intermediate (pink) between two homozygotes signals incomplete dominance. - A: Complete dominance would give all red or all white, not pink. - B: Codominance shows both colors distinctly, not a blend. - C: Flower color here is not sex-linked. - D: Only two alleles are involved, not a multiple-allele system. - Fix rule: Blended intermediate = incomplete dominance; both traits shown together = codominance.
7. A — Codominance. Fix: In type AB, the A and B alleles are each fully expressed, the definition of codominance. - B: Incomplete dominance would blend into an intermediate, not show both. - C: A frameshift is a mutation type, unrelated to allele expression. - D: Polygenic blending is not what ABO demonstrates. - E: ABO is autosomal, not sex-linked. - Fix rule: Both alleles fully visible together = codominance.
8. A — 1/2. Fix: Sons get their single X from the carrier mother; half receive the recessive allele and are affected. - B: Zero would require the mother to carry no recessive allele. - C: Only half of sons inherit the affected X. - D: 1/4 confuses this with an autosomal or two-carrier cross. - E: 3/4 has no basis in this single-carrier-mother cross. - Fix rule: X-linked recessive + carrier mother → half of sons affected.
9. C — Autosomal dominant. Fix: Appearing every generation in both sexes with an affected parent each time points to autosomal dominant. - A: X-linked recessive skews toward males and can skip generations. - B: Y-linked traits affect only males. - D: Mitochondrial traits pass from mothers and are not "males only." - E: Codominant blood typing is not a pedigree disease pattern like this. - Fix rule: Every generation + both sexes + affected parent → autosomal dominant.
10. E — Down syndrome (trisomy 21). Fix: Three copies of chromosome 21 from nondisjunction cause Down syndrome. - A: Turner syndrome is a single X (45,X), not trisomy 21. - B: Color blindness is an X-linked gene mutation, not a whole-chromosome error. - C: Sickle-cell anemia is a point mutation in a gene. - D: Klinefelter syndrome is XXY (a sex-chromosome trisomy), not chromosome 21. - Fix rule: Extra chromosome 21 → trisomy 21 = Down syndrome.
11. D — Incorrect; 3:1 is a probability, so small samples may deviate. Fix: Genetic ratios are expected averages; chance makes small groups vary. - A: The ratio is not guaranteed exact in any single group of four. - B: Short (tt) offspring can and do appear. - C: The expected ratio is 3:1, not 1:1. - E: Most offspring are tall, so "all short" is wrong. - Fix rule: Punnett ratios are probabilities, not guarantees for small samples.
12. D — Both parents are heterozygous carriers. Fix: Two unaffected parents producing an affected child indicates both carry a hidden recessive allele. - A: A homozygous recessive parent would show the disorder, contradicting "unaffected." - B: The pattern points to recessive, not dominant, inheritance. - C: Carrier parents explain it without invoking a new mutation. - E: Misdiagnosis is an unfounded leap when carrier status fits. - Fix rule: Unaffected parents + affected child → both are recessive carriers.