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
Diploid cells contain chromosome pairs called homologous chromosomes. Homologues carry corresponding loci in corresponding regions, but they can carry different alleles. Before meiosis, DNA is replicated, so each chromosome consists of two sister chromatids.
Meiosis produces cells with one chromosome from each homologous pair. This reduction division is important because fertilization combines chromosome sets. Without reduction, chromosome number would tend to double each generation.
Vocabulary: diploid; haploid; homologous chromosomes; sister chromatids; locus; meiosis; gamete; fertilization; reduction division.
Worked example with reasoning
A fictional species has 2n = 6. A diploid cell has three homologous pairs. After DNA replication it still has 6 chromosomes, now made of 12 sister chromatids. After meiosis is complete, each typical gamete contains n = 3 chromosomes—one from each homologous pair.
Guided question
Why are homologous chromosomes not the same thing as sister chromatids?
Independent work
- Draw a 2n = 4 cell before replication, after replication, after meiosis I, and after meiosis II.
- Label homologues and sister chromatids.
- Track chromosome number and chromatid number at each stage.
- Explain why fertilization restores diploidy.
Misconception check
Meiosis does not begin with a haploid cell. It begins with a diploid germ-line precursor whose chromosomes have replicated before the two meiotic divisions.
Formative understanding check
A species has 2n = 8. What is the usual chromosome number in a gamete?
Show the worked response
n = 4. The gamete receives one chromosome from each homologous pair.
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 color-coded homolog cards and a four-stage counting table. Core: track homologues, sister chromatids, and chromosome number through both meiotic divisions. Extended: compare DNA-content change with chromosome-number change and explain where those quantities diverge.
Preserve learning evidence
Keep the chromosome model, calculations, variation analysis, correction note, and limitations in the matching Week 19 evidence field.
Return to monday Evidence Workspace