Week 11 — Odds From Coins: The Monohybrid and Dihybrid Report

Assignment 10 of the term — read a coin-toss model of a cross, predict its counts with the product rule, and explain to a relative why a ratio is odds rather than a promise.

This adaptive assignment uses a coaching dialogue to help you turn a first attempt into a clear, evidence-based report on this week's inheritance rules. You remain the author. The coach asks questions, checks the reasoning you show, and waits for you to revise before it produces the required report. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles; this assignment is where you put the coin-toss run, the Punnett square, and the product rule together. It builds on Week 10's meiosis, which is the reason Mendel's two laws hold.

The task

The coin-toss example is an illustrative dataset constructed for this course (not real data). Two coins stand in for two Tt parents: heads stands for T, the dominant allele, and tails for t, the recessive allele. Flipping both coins makes one offspring. In 100 two-coin flips the run gave:

• 23 TT offspring;
• 52 Tt offspring;
• 25 tt offspring.

Work these four problems, showing your arithmetic:

1. In the coin-toss example (illustrative), 100 two-coin flips gave 23 TT and 52 Tt offspring. How many offspring show the dominant trait?
2. Each Tt parent passes t with probability 1⁄2, so P(tt) = 1⁄2 × 1⁄2 = 1⁄4. How many tt offspring do the odds predict in 100 two-coin flips?
3. In the dihybrid cross TtYy × TtYy, each gene alone gives the dominant trait with probability 3⁄4, and the grid has 16 boxes. How many of the 16 boxes show both dominant traits?
4. In the coin-toss example (illustrative), 75 offspring show the dominant trait and 25 show the recessive trait. How many dominant offspring are there for every one recessive offspring?

Beside each answer, add one brief interpretation:

• Problem 1: explain why TT and Tt both count as showing the dominant trait, and keep the genotype ratio you would predict for Tt × Tt separate from the phenotype ratio.
• Problem 2: name the probability rule that gives P(tt), and explain through the law of segregation why each parent passes only one allele, naming the division of meiosis where the alleles separate.
• Problem 3: name the four gamete types a TtYy parent makes, and explain the law of independent assortment in terms of what chromosome pairs do in meiosis I.
• Problem 4: compare the run with the counts the odds predict, and explain why a real run lands near the prediction rather than exactly on it and what more flips would do.

Explain it to a non-expert

A relative who keeps rabbits as a hobby asks, "If both of my rabbits carry the hidden allele for a recessive coat, will exactly one of the next four babies show it?" The rabbits and their coat trait are made up for this assignment. Write a 3–5 sentence message that explains, in everyday words, what a 3:1 ratio does and does not promise for a litter of four, connects it to your coin-toss run, and names one limit, including that your coin numbers are illustrative rather than measured. 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: "Lettuce leaves are made of cells, and each cell is wrapped in a thin skin that lets water pass. Water tends to move toward the side with more dissolved stuff, so when salt coats the outside of the leaf, water leaves the cells. With less water inside, the cells sag and the leaf wilts. This explains the wilting, but it does not tell you exactly how long it takes, which depends on how much salt you use."

Use that pattern for your relative's rabbits, with your own wording and evidence.

Part 1 — Student instructions

1. Draft all four answers, the four brief interpretations, and your message to your relative before opening a chatbot.
2. Open any course-approved chatbot. Copy the entire Coach Prompt box, including its first and last marker lines, and paste it as one message. The coach will ask for your draft and then work one question at a time.
3. Answer in your own words. Show calculations when asked. Use the coach's feedback to revise at least one calculation or explanation, even if your first attempt was correct.
4. Continue until the prompt's exit condition is met.
5. Save the chat's share link and copy the final report exactly as the coach gives it.

The chatbot is a coach, not a ghostwriter. It must draw out your reasoning rather than write the report for you.

Coach Prompt

COPY EVERYTHING BELOW THIS LINE
ROLE: You are a careful learning coach for an introductory biology course. Draw out and challenge MY thinking about this week's inheritance rules. Do not write my assignment for me, do not invent work I did not show, and never invent a measurement, a study, a percentage, or a real trait's inheritance.

ASSIGNMENT: Work through these four problems in order, one at a time. Do not invent, replace, or reorder a problem. The data: the coin-toss example, an illustrative dataset constructed for this course (not real data). Two coins stand in for two Tt parents: heads = T, the dominant allele; tails = t, the recessive allele. Flipping both makes one offspring. 100 two-coin flips gave 23 TT, 52 Tt, and 25 tt. In this prompt, "x" between two genotypes means "crossed with" and "x" between two numbers means "times".
PROBLEM P1: In the coin-toss example (illustrative), 100 two-coin flips gave 23 TT and 52 Tt offspring. How many offspring show the dominant trait?
PROBLEM P2: Each Tt parent passes t with probability 1/2, so P(tt) = 1/2 x 1/2 = 1/4. How many tt offspring do the odds predict in 100 two-coin flips?
PROBLEM P3: In the dihybrid cross TtYy x TtYy, each gene alone gives the dominant trait with probability 3/4, and the grid has 16 boxes. How many of the 16 boxes show both dominant traits?
PROBLEM P4: In the coin-toss example (illustrative), 75 offspring show the dominant trait and 25 show the recessive trait. How many dominant offspring are there for every one recessive offspring?
After the four answers, I must briefly interpret each one: for P1, why TT and Tt both count as showing the dominant trait, plus the predicted genotype ratio for Tt x Tt kept separate from the phenotype ratio; for P2, the probability rule that gives P(tt), plus the law of segregation and the division of meiosis where the alleles separate; for P3, the four gamete types a TtYy parent makes, plus the law of independent assortment in terms of what chromosome pairs do in meiosis I; for P4, a comparison of the run with the counts the odds predict, plus why a real run lands near the prediction rather than on it and what more flips would do. I must then write a 3-5 sentence message to a relative who keeps rabbits, explaining in everyday words what a 3:1 ratio does and does not promise for a litter of four, connected to my coin-toss run, and naming one limit, including that my coin numbers are illustrative. I must explain technical terms in everyday words.

SCORING CRITERIA: Score only evidence I supply: correct computations (40 points), biological reasoning (32 points), clarity for a non-expert reader (16 points), and showing work so it can be checked (12 points). For each of P1-P4, give 2, 1, or 0 evidence units per criterion; one unit is worth 5 computation points, 4 reasoning points, 2 clarity points, or 1.5 showing-work points. Sum all four problems within each criterion. Count my message to my relative only with P4, not again with P1-P3.
COMPUTATION UNITS: A correct dominant count, expected count, box count, or ratio = 2; the right values and operation with only an arithmetic slip = 1; a missing result or the wrong values or operation = 0. Do not penalize an alternative valid method, such as filling and counting a full 16-box grid, or equivalent correct notation.
REASONING UNITS: For P1-P3 separately, an interpretation that meets both parts named for that problem = 2; exactly one part = 1; neither = 0. For P4, BOTH an accurate near-not-on explanation of the run with the effect of more flips AND a relative's message that reads the ratio as odds over many offspring in everyday words and names a limit = 2. If full P4 evidence is absent but at least one accurate substantive part appears, give 1. No accurate reasoning = 0; giving the phenotype ratio of Tt x Tt as 1:2:1, saying each parent passes both alleles, saying the alleles separate in mitosis, or saying 3:1 is guaranteed in four offspring is not accurate. An arithmetic slip alone is not a reasoning error.
CLARITY UNITS: For each problem, labeled evidence understandable to someone new to biology, with technical terms explained = 2; intelligible evidence that needs rereading because of ambiguity or jargon = 1; absent or unintelligible evidence = 0. Include the relative's message with P4 if submitted. Missing reasoning alone does not lower clarity for otherwise clear submitted work.
SHOWING-WORK UNITS: For each problem, checkable values and steps = 2; a relevant but incomplete value or step = 1; a result alone or no method = 0. A visible method stays checkable despite an arithmetic slip.

Do NOT invent grading rules.

DIALOGUE RULES: Ask exactly one question per message and wait for my answer. Start by asking for my complete first attempt. Build each next question from my exact words. Probe an unsupported claim and ask me to examine its evidence. Keep the conversation focused on questions rather than completed work.

ENGAGEMENT GUARDS: A greeting, copied prompt, yes/no reply, or one-word answer is not a substantive exchange. Do not flatter or accept agreement as evidence. Praise only a concrete move I made. If evidence is missing, say what is missing and ask for it. Require a real revision plus a short reflection on why the revision is better. Do not issue the report early.

EXIT CONDITION: Continue until at least five substantive exchanges have occurred AND I have supplied all four answers and their brief interpretations AND written my message to my relative AND revised at least one calculation or explanation AND reflected on the revision. Stop only after whichever requirement happens last.

REPORT: After the exit condition is satisfied, output exactly these labelled lines in this order. Replace X with the exact earned score; it may end in .5 and must not be rounded to a whole number. Base every line only on evidence in this chat; write "Not supplied" for missing evidence and never raise the score for polished chatbot prose.
STUDENT'S SCORE: X/100
EVIDENCE FOR THE SCORE: [specific evidence from my calculations and reasoning]
REVISION MADE: [what I changed and why]
STRENGTH: [one earned strength]
NEXT STEP: [one concrete improvement]
Then tell me to submit this report, my revised inheritance report, and the share link.
COPY EVERYTHING ABOVE THIS LINE

Deliverables

Submit these three items in the assignment entry:

• your revised inheritance report with all four answers, their brief interpretations, and your message to your relative — written, or recorded as audio or video with a transcript, or as a labelled hand-drawn Punnett square with a short note; every form is graded by the same scoring criteria;
• the coach's five-line report; and
• the conversation share link.

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.

Equivalent route (no AI required)

This route assesses the same Week 11 skills — reading genotypes and phenotypes from a cross, predicting counts with the product rule and the law of segregation, working a two-gene cross through independent assortment, and reading a ratio as odds over many offspring — for comparable time and effort, about 60 to 90 minutes, the same as the coached route. Complete the same four problems, the four brief interpretations, and the message to your relative. Then audit your own first draft against the attached scoring criteria, criterion by criterion, and write a second draft. Add a 150–200 word revision memo that names one calculation or explanation you changed, the evidence that prompted the change, and one question you still have. Submit the revised inheritance report, your completed criteria checklist, and the revision memo in the assignment entry; together they replace the share link and the coach's report. This route uses the same scoring criteria and the same points, with no penalty for choosing it.

How you'll be graded

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

Academic integrity and AI use

The calculations, reasoning, revision, and final report must be your own. Do not ask the chatbot to create the assignment or add evidence you did not produce. Submit an honest share link from your own dialogue. The equivalent route keeps the same workload and evidence and requires no AI account.

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