Unit 2 of 5
Study guide for CLEP CLEP Biology — Unit 2: Genetics and Molecular Biology. Practice questions, key concepts, and exam tips.
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In pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). If a heterozygous tall plant is crossed with a homozygous recessive short plant, what proportion of the offspring would be expected to have short stems?
Answer: B — This question tests the student's ability to apply Mendelian genetics principles using a Punnett square. The cross is Tt (heterozygous tall) × tt (homozygous recessive short). When worked out in a Punnett square, the possible offspring genotypes are: Tt (tall) and tt (short) in a 1:1 ratio. Therefore, 50% of offspring would be short (tt). Option A (25%) is incorrect because this would result from a Tt × Tt cross (monohybrid cross of two heterozygotes). Option C (75%) would result from a TT × tt or Tt × TT cross. Option D (100%) would only occur if both parents were homozygous recessive, which contradicts the question setup. The correct answer demonstrates understanding that heterozygous individuals carry and can pass on recessive alleles.
A cross between two heterozygous organisms for a single trait produces offspring in a 3:1 phenotypic ratio. Which of the following best explains why the recessive phenotype appears in only 25% of the offspring?
Answer: A — The correct answer is B. In a cross between two heterozygotes (Aa × Aa), the Punnett square produces a 1:2:1 genotypic ratio (1 AA : 2 Aa : 1 aa). The recessive phenotype only appears when an organism is homozygous recessive (aa), which occurs in 25% of offspring. This demonstrates that recessive alleles must be present in two copies to be expressed, since the heterozygous (Aa) individuals display the dominant phenotype. Option A is incorrect because both parents contribute one allele each; the recessive allele appears in 50% of offspring genotypically, but only 25% phenotypically. Option C is incorrect because dominance is not about alleles being 'stronger' or preventing inheritance—it's about which phenotype is expressed when both alleles are present. Option D is incorrect because this 3:1 ratio is a consistent genetic outcome based on Mendelian inheritance, not environmental suppression. This question tests understanding of why recessive traits skip generations and appear less frequently than dominant traits.
A couple has a child with cystic fibrosis, an autosomal recessive disorder. If both parents are carriers of the disease, what is the probability that their next child will also have cystic fibrosis?
Answer: C — The correct answer is A) 25% because when both parents are carriers of an autosomal recessive disorder, there is a 25% chance that each child will have the disorder (homozygous recessive), a 50% chance that each child will be a carrier like the parents (heterozygous), and a 25% chance that each child will neither have the disorder nor be a carrier (homozygous dominant). The other options are incorrect because they do not accurately reflect the probability of the child having cystic fibrosis given that both parents are carriers.
In a species of flowering plant, petal color is controlled by a single gene with two alleles. Homozygous individuals with the R allele produce red petals, homozygous individuals with the W allele produce white petals, and heterozygous individuals produce pink petals. A gardener crosses a pink-petaled plant with a red-petaled plant and obtains 47 red-petaled offspring and 53 pink-petaled offspring out of 100 total plants. Based on these results, which of the following best explains the genetic basis of petal color in this species?
Answer: C — The correct answer is A. The observed ratio of approximately 50% red to 50% pink petals matches the expected 1:1 Mendelian ratio, which results from a testcross between a heterozygote (RW, pink phenotype) and a homozygous dominant (RR, red phenotype). The RW individual produces equal proportions of R and W gametes, while the RR individual produces only R gametes. This produces offspring that are either RR (red) or RW (pink) in equal proportions. The question establishes incomplete dominance because heterozygotes produce an intermediate pink phenotype rather than displaying either parent phenotype. Option B is incorrect because it describes complete dominance (which doesn't match the pink phenotype) and would be a cross between two homozygotes (WW × RR), producing all heterozygotes—not the observed 1:1 ratio. Option C is incorrect because a cross between two heterozygotes (RW × RW) under incomplete dominance would produce a 1:2:1 ratio (red:pink:white), not the observed 1:1 ratio. Option D is incorrect because the 47:53 ratio is very close to the expected 1:1 ratio (expected would be 50:50), and minor deviations from expected ratios are normal due to random sampling variation. A chi-square test would confirm this observed ratio is not significantly different from 1:1.
In a certain snapdragon plant species, flower color exhibits incomplete dominance. Red flowers (RR) and white flowers (WW) are homozygous true-breeding lines, while heterozygous plants (RW) produce pink flowers. A researcher crosses a pink-flowered snapdragon with a red-flowered snapdragon and observes the offspring. If 47% of the offspring produced pink flowers and 53% produced red flowers, which of the following best explains this deviation from the expected 1:1 Mendelian ratio?
Answer: B — This question tests the student's understanding of both incomplete dominance AND the distinction between biological principles and statistical variation. The correct answer is D because a 47:53 ratio is remarkably close to the expected 1:1 (50:50) ratio predicted from crossing RW × RR (which should produce 50% RW pink and 50% RR red). A 6% deviation is well within the range of random sampling error and does not indicate a genetic problem. This tests whether students understand that small deviations from expected ratios are normal and do not necessarily indicate violations of genetic principles. Answer A is incorrect because it invokes unsubstantiated explanations (differential survival and environmental masking of phenotype) when a simpler explanation exists. It also demonstrates misunderstanding—the question states pink offspring were observed, so environmental factors would not explain the deviation. Answer B is incorrect because it assumes the parents' genotypes must be wrong without evidence. The question stem establishes that the researcher crossed a pink × red, and the results (47% pink, 53% red) are consistent with the expected heterozygote × homozygote cross. Answer C is incorrect because it misrepresents incomplete dominance. Incomplete dominance is a well-understood genetic phenomenon that follows Mendelian principles—it simply means heterozygotes show an intermediate phenotype rather than dominant masking. The inheritance pattern is still predictable and ratios follow expected Mendelian outcomes; the confusion is about phenotypic expression, not inheritance itself.
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