Claim-Evidence Writing
Learning Objectives
Apply the CER (Claim-Evidence-Reasoning) framework to genetics topics.
- Write clear claims about genetics concepts
- Support claims with genetic data and observations
- Explain reasoning using principles of inheritance
- Evaluate the strength of genetics arguments
CER Framework Review
- Claim: A statement that answers a genetics question
- Evidence: Data from crosses, pedigrees, or molecular genetics
- Reasoning: Connects evidence to claim using genetic principles
Example CER Response
Question: Is cystic fibrosis inherited as a dominant or recessive trait?
Claim: Cystic fibrosis is inherited as an autosomal recessive trait.
Evidence: Pedigree analysis shows that two unaffected parents can have an affected child. Additionally, when both parents are carriers, approximately 25% of their children are affected. The trait affects males and females equally.
Reasoning: For two unaffected parents to have an affected child, both parents must carry a copy of the disease allele without expressing it themselves. This is only possible with recessive inheritance, where the phenotype is only expressed when two copies of the recessive allele are present (homozygous recessive). The 25% affected ratio in offspring of two carriers matches the expected 1:2:1 genotypic ratio from a Punnett square (Cc x Cc). Equal occurrence in both sexes indicates the gene is autosomal rather than sex-linked.
Practice Quiz
Test your understanding of claim-evidence writing in genetics with these 10 questions.
1. What makes a strong claim in genetics?
Answer: A strong genetics claim is specific, addresses the question directly, can be supported with genetic data, and is testable. Example: "The trait for seed color in pea plants follows Mendelian inheritance patterns" is stronger than "Pea plants have different colored seeds."
2. Write a claim to answer: Why do siblings look different even though they have the same parents?
Answer: Siblings look different because they inherit different combinations of alleles from their parents due to independent assortment and crossing over during meiosis.
3. What type of evidence would best support a claim about a trait being X-linked?
Answer: A pedigree showing the trait appears more frequently in males, affected males pass the trait to carrier daughters but not to sons, and carrier females can have affected sons but rarely have affected daughters.
4. A student claims: "DNA replication is important." Is this a strong or weak claim? Why?
Answer: This is a weak claim because it is too vague. A stronger claim would specify what DNA replication does and why it matters, such as: "DNA replication ensures genetic information is accurately passed to daughter cells during cell division."
5. Write appropriate evidence for this claim: "Mendel's law of segregation is supported by monohybrid cross results."
Answer: When Mendel crossed heterozygous tall pea plants (Tt x Tt), he observed approximately 3:1 ratio of tall to short offspring (787 tall : 277 short). This matches the predicted ratio if each parent contributes one allele that segregates independently during gamete formation.
6. What is the role of reasoning in a genetics CER response?
Answer: Reasoning explains WHY the evidence supports the claim by connecting it to genetic principles like Mendel's laws, chromosome behavior during meiosis, or molecular genetics concepts. It demonstrates understanding of the mechanisms behind the observations.
7. Using CER, explain why identical twins have the same DNA but may have different traits.
Answer:
Claim: Identical twins can have different traits despite identical DNA due to epigenetic and environmental factors.
Evidence: Studies show identical twins can differ in disease occurrence, height, weight, and behavior. DNA methylation patterns differ between twins as they age.
Reasoning: While identical twins share the same DNA sequence, gene expression can be modified by environmental factors and epigenetic changes. These modifications don't change the DNA sequence but affect which genes are turned on or off, leading to phenotypic differences.
8. Identify the claim, evidence, and reasoning: "Huntington's disease is dominant because affected individuals always have at least one affected parent. Pedigrees show the trait in every generation. This pattern occurs because only one copy of the mutant allele is needed to express the disease phenotype."
Answer:
- Claim: Huntington's disease is dominant
- Evidence: Affected individuals always have at least one affected parent; pedigrees show the trait in every generation
- Reasoning: Only one copy of the mutant allele is needed to express the disease phenotype
9. What makes evidence "strong" when supporting a genetics claim?
Answer: Strong genetics evidence includes specific numbers or ratios, comes from multiple crosses or individuals, can be replicated, matches predicted outcomes based on genetic principles, and directly relates to the claim being made.
10. A pedigree shows that only males are affected with a condition. Write a CER response explaining the inheritance pattern.
Answer:
Claim: The condition is X-linked recessive.
Evidence: Only males in the pedigree are affected. Affected males have unaffected mothers. No father-to-son transmission is observed.
Reasoning: X-linked recessive traits primarily affect males because they have only one X chromosome (XY). If their single X carries the recessive allele, they express the trait. Females (XX) are usually carriers because they need two copies of the recessive allele to be affected. The lack of father-to-son transmission confirms X-linkage since fathers pass Y chromosomes to sons.
Next Steps
- Practice writing CER responses for other genetics topics
- Review pedigree analysis
- Continue to the Unit Checkpoint