Safety and privacy boundary
Use fictional organisms, chromosome cards, beads, paper models, or teacher-approved simulations. Do not require family genetic histories, fertility information, ancestry testing, reproductive information, medical records, or personal genetic data. Human worth is never ranked by chromosome combinations or genetic variation.
Explicit science instruction
During early meiosis I, homologous chromosomes pair closely. Non-sister chromatids can exchange corresponding DNA segments through crossing over. This recombination creates chromosomes containing new combinations of alleles already present on the homologues.
In meiosis I, homologous chromosomes—not sister chromatids—separate. Sister chromatids remain joined until meiosis II. This distinction is central to understanding chromosome-number reduction.
Vocabulary: synapsis; tetrad; crossing over; recombination; non-sister chromatids; chiasma; meiosis I; homolog segregation.
Worked example with reasoning
Suppose one homolog carries A—B and the other carries a—b at two linked loci. A crossover between the loci can produce chromatids with A—b and a—B combinations. The genes remain at corresponding loci; recombination changes which allele combinations travel together.
Guided question
Why does crossing over usually change allele combinations without changing the number of chromosome types?
Independent work
- Build a two-homolog paper model with two loci.
- Model one crossover between non-sister chromatids.
- List parental and recombinant chromatid types.
- Explain what separates in meiosis I.
- State one limitation of the paper model.
Misconception check
Crossing over does not normally occur between sister chromatids as the main source of new allele combinations in standard meiosis models. It is modeled between homologous non-sister chromatids after homolog pairing.
Formative understanding check
What is the key segregation event of meiosis I?
Show the worked response
Homologous chromosomes move to opposite cells while sister chromatids remain associated.
Open-book formative practice only; not an official mastery attempt or grade.
Mentor evidence conference
Ask Archaemenes to challenge chromosome-counting logic, homolog versus sister-chromatid distinctions, probability reasoning, and model limitations. The mentor does not infer personal genetics or reveal protected assessment answers.
Open Archaemenes for this lessonTeacher guidance
Foundation: use pre-aligned chromosome strips and one marked crossover site. Core: construct and explain recombinant versus parental chromatids. Extended: analyze how crossover position affects linkage between loci and why recombination frequency has upper interpretive limits.
Preserve learning evidence
Keep the chromosome model, calculations, variation analysis, correction note, and limitations in the matching Week 19 evidence field.
Return to tuesday Evidence Workspace