Week 10 — Halve, Shuffle, Restore: The Meiosis Counting Report — Traditional alternative (no AI)

Instructors: publish this instead of the adaptive version if you prefer, and unpublish or delete the adaptive one.

Assignment 9 of the term — follow the model cell through meiosis and count what it makes, then explain to a cousin why two siblings differ.

This week you learned that meiosis runs as two divisions in order — meiosis I, where homologous chromosomes separate, then meiosis II, where sister chromatids separate — after a single round of DNA copying, and that crossing over, independent assortment, and random fertilization make every gamete different. In this assignment you work four short problems, interpret each one, and explain to someone who has never studied biology why two children of the same parents differ. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles.

The task

The three-pair model cell is a classroom paper model, illustrative and constructed for this course. It has a long, a medium, and a short chromosome pair, so it holds 6 chromosomes. In each pair, B stands for the copy from one parent and R stands for the copy from the other parent. A human body cell, for comparison, has 46 chromosomes in 23 pairs.

Work these four problems, showing your arithmetic:

1. In the three-pair model cell (illustrative), the cell holds 6 chromosomes before meiosis, and the S phase copies each chromosome into 2 sister chromatids. How many chromatids does the cell hold after the S phase?
2. The three-pair model cell (illustrative) has a long, a medium, and a short chromosome pair, and each pair sends either its B copy or its R copy into a gamete. From independent assortment alone, how many genetically different gametes can it make?
3. A human body cell has 46 chromosomes. If a human egg and a human sperm each carried 46 chromosomes instead of being halved, how many chromosomes would the fertilized egg carry?
4. Two parents of the three-pair model (illustrative) can each make 8 kinds of gametes from independent assortment alone. Any egg can meet any sperm. How many different egg-and-sperm pairings are possible?

Beside each answer, add one brief interpretation:

• Problem 1: explain why each duplicated chromosome still counts as one chromosome, naming its two sister chromatids, and say when the DNA is copied relative to meiosis I and meiosis II.
• Problem 2: list all eight gametes, written long, medium, short, with no repeats, and name the process and the phase that produce them, saying why the count is neither 3 nor 6.
• Problem 3: explain how meiosis and fertilization keep the human count at 46, with the arithmetic, and name the division that halves the number of chromosome sets.
• Problem 4: name random fertilization and the second shuffle, crossing over, with the phase it happens in, and connect the variety they make to natural selection.

Explain it to a non-expert

Your cousin, expecting a second child, asks, "Will our second baby look exactly like our first, since they have the same parents?" Write a 3–5 sentence message that explains in everyday words why two children of the same parents usually differ, and names one limit, including that the counts in your report come from a paper model with only 3 pairs and that the human count from independent assortment alone is a floor that crossing over raises. Explain any technical term in everyday words. More than one good answer is possible.

For a different, ungraded example, imagine a friend asking why salted lettuce goes limp. Model answer: "Salt on the leaf makes the water outside the leaf's cells saltier than the water inside. Water moves across a cell's thin outer layer toward the saltier side, a movement called osmosis, so the cells lose water. Cells that lose water go soft, and the leaf wilts. This explains the wilting, but it does not tell you how long it takes, which depends on how much salt you used."

Use that pattern for your cousin, with your own wording and evidence.

Show your arithmetic. A reader should be able to reproduce every number you report from the counts in the problems without guessing which values or steps you used.

Deliverables

Submit a single report containing:

• the four answers, each with the arithmetic shown and the brief interpretation the task names for that problem;
• your message to your cousin, explaining in everyday words why two children of the same parents usually differ, with one limit; and
• a one-line restatement of the model cell you used (its three pairs and the B and R labels), so your results are reproducible.

Keep it to roughly one page. Clear and correct beats long.

Choose how you show it. Present your report as written text, or as a recorded audio or video explanation of about three minutes with a transcript, or as a labelled sketch of the model cell through both divisions with a short note. Every form is graded by the same scoring criteria. Submit in any accessible file type or as a link; a recorded response may be submitted as audio or video with a transcript, or as text.

How you'll be graded

The full analytic rubric is attached to this assignment in Canvas.

Academic integrity and AI use

This version is designed to be completed without AI: do the arithmetic, the interpretations, and the writing yourself, without a chatbot or other AI tool. You may use a calculator or a spreadsheet for the arithmetic. Submitting AI-generated work as your own, or copying another student's work, is an integrity violation under the course policy.

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