Week 12 — Lecture Outline · Patterns of Inheritance

Course: Introduction to Biology — General Biology I (BIOL 101) · Silver Oak University (fictional sample) · Prof. Castellano
Objective covered: Objective 6 — Apply the principles of inheritance — extending Mendel to incomplete dominance, codominance, multiple alleles, sex linkage, and pedigrees — to predict genotypes, phenotypes, and probabilities.
SLOs touched: A (reason quantitatively about genetic crosses; interpret pedigree data) · B (connect allele combinations to phenotypes)
Meeting pattern: 2 sessions × 75 min = 150 min. Segment minutes below total ~150; scale to your own pattern.

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Week at a Glance

The week's big question	"When inheritance breaks Mendel's simple rules — blending, co-expression, sex linkage — how do we still predict the odds for the next child?"
By the end of the week, students can…	(1) distinguish incomplete dominance (a blend) from codominance (both alleles expressed) and work a cross for each; (2) handle multiple alleles with the ABO blood-type system and compute offspring probabilities; (3) predict sex-linked (X-linked recessive) outcomes and explain why such traits are more common in males; (4) read a human pedigree to deduce dominant vs. recessive and autosomal vs. X-linked.
Key vocabulary	incomplete dominance, codominance, multiple alleles, ABO blood type (Iᴬ, Iᴮ, i), antigen, dominant/recessive (review), genotype/phenotype (review), sex chromosome (X, Y), sex-linked / X-linked recessive, carrier, hemizygous, pedigree, autosomal, proband, linked genes, recombination
Materials	slides (Deck 12), the week's readings + video links, one approved chatbot (Gemini / Claude / ChatGPT) for the AI-critique moment and the tutorial, a free virtual blood-type/pedigree problem set for the lab
Timing note	8 segments, ~150 min total. Session 1 = Segments 1–4 (~75). Session 2 = Segments 5–8 (~75).

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Segment 1 — Hook & the Promise (8 min) · Session 1 opens

Hook. Put one question on a slide and let the room commit out loud: "A mother has type A blood, a father has type B. Can their child be type O?" Take a quick vote. Most students say no — "neither parent has any O in them." Then drop the twist: not only can they have a type-O child, there's a clean 1-in-4 chance. "By the end of today you'll prove it with a Punnett square — and you'll see that 'O' was hiding in both parents the whole time."

The promise (write it on the board): "By Friday you'll predict the odds in inheritance even when Mendel's simple rules break — when alleles blend, when both show at once, and when a gene rides on the X chromosome — and you'll read a family's pedigree the way a genetic counselor does."

Why it matters line (memory hook): "Week 11 gave us the machinery — Punnett squares and probability. Week 12 points that machinery at the messy, real cases."

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Segment 2 — Incomplete Dominance vs. Codominance (the distinction students blur) (24 min)

Plain language first. Mendel's pea traits were completely dominant: one allele fully masked the other, so a heterozygote looked exactly like the dominant homozygote. Real inheritance isn't always that tidy. Two big exceptions:

• Incomplete dominance — the heterozygote is a BLEND. Cross a true-breeding red snapdragon (RR) with a true-breeding white one (WW) and the offspring are pink (RW). Neither allele fully wins; the phenotype is in between. (Use R and W for the two alleles so neither looks "dominant.")
• Codominance — BOTH alleles are fully expressed, side by side. In AB blood type, a person makes both the A antigen and the B antigen — not a blend, but both at once. (Classic visual: a roan or speckled animal showing red hairs and white hairs, not pink.)

Memory hook (put it on a slide):

"Incomplete BLENDS (red + white = pink). Co-dominance, BOTH show (A and B, side by side)."

One fully worked example (do it on the board — every step).

Incomplete dominance: RW × RW (two pink snapdragons).
- Each parent's gametes: R (½) or W (½).
- Fill the 2×2 Punnett square:

	R	W
R	RR	RW
W	RW	WW

• Genotypes: 1 RR : 2 RW : 1 WW.
• Phenotypes (blend rule): RR = red, RW = pink, WW = white → 1 red : 2 pink : 1 white.
• So P(pink) = 2/4 = 1/2. Notice the phenotype ratio (1:2:1) now matches the genotype ratio — because every genotype looks different. (Verified.)

The clarification students always need: the cross mechanics are identical to Week 11 — you still segregate alleles and fill four boxes. What changes is only the phenotype rule for the heterozygote: blended (incomplete) vs. both-shown (codominant). Don't overthink the square; rethink the coloring-in.

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Segment 3 — Multiple Alleles & ABO Blood Type (the fully worked cross) (22 min)

Plain language first. A single person carries only two alleles of a gene — but a population can have more than two versions of that gene to choose from. The textbook case is ABO blood type, which has three alleles: Iᴬ, Iᴮ, and i.

The rules (one slide):
- Iᴬ makes the A antigen; Iᴮ makes the B antigen; i makes no antigen.
- Iᴬ and Iᴮ are codominant with each other (Iᴬ Iᴮ = type AB, both antigens).
- i is recessive to both (so ii = type O). "Type O isn't dominant — i is the recessive one that simply makes no antigen."

Genotype(s)	Blood type
Iᴬ Iᴬ or Iᴬ i	A
Iᴮ Iᴮ or Iᴮ i	B
Iᴬ Iᴮ	AB (codominant)
ii	O

One fully worked example (build it on the board — every step).

Type A (Iᴬ i) × Type B (Iᴮ i) — back to the opening puzzle.
- Mom's gametes: Iᴬ (½) or i (½). Dad's gametes: Iᴮ (½) or i (½).

	Iᴮ	i
Iᴬ	Iᴬ Iᴮ (AB)	Iᴬ i (A)
i	Iᴮ i (B)	i i (O)

• Four equally likely boxes → AB, A, B, O — each 1/4.
• So P(type O) = 1/4, P(type AB) = 1/4, P(A) = 1/4, P(B) = 1/4. (Verified.)
• The reveal: the "O" was hiding as the recessive i in each parent. Two parents who show A and B can produce a child who shows neither.

Tie back: this is just the multiple-allele version of a heterozygous × heterozygous cross. The product rule and the square are unchanged.

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Segment 4 — Misconceptions + Quick Interaction (21 min) · Session 1 closes (~75)

Name the misconceptions out loud, then cure each:

• ❌ "Incomplete dominance and codominance are the same thing."
  ✅ Cure: incomplete dominance = a blend (red + white = pink, a new in-between look). Codominance = both alleles fully expressed at once (AB blood shows A and B, not a blend). Pink vs. "both colors visible" is the tell.
• ❌ "Blood type O is dominant — it shows up so often."
  ✅ Cure: i is recessive; type O requires two copies (ii). It's common because the i allele is frequent in the population, not because it's dominant. (Frequency ≠ dominance.)
• ❌ "A man can be a carrier of red-green colorblindness."
  ✅ Cure (preview of Segment 6): a male has one X. For an X-linked recessive trait he is affected (XᵃY) or not (XᴬY) — never a silent carrier. Only females (two X's) can be unaffected carriers (XᴬXᵃ).
• ❌ "You can read a pedigree by just looking at who's shaded."
  ✅ Cure: you have to use the logic — two unaffected parents with an affected child means the trait is recessive; a trait appearing far more in males points to X-linked. Shading alone isn't the answer; the pattern is.

Interaction — Think-Pair-Share (rapid-fire, ~8 min):
Put four phenotype descriptions on a slide; for each, students decide incomplete dominance or codominance, solo (30 sec), compare with a neighbor (1 min), then vote. Suggested items: a pink snapdragon from red × white (incomplete) · an AB blood type (codominance) · a roan cow with distinct red and white hairs (codominance) · a "palomino"-style intermediate coat from a dark × light cross (incomplete). Have them name the tell — "is it a blend, or both at once?"

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Segment 5 — Sex Linkage: Why Colorblindness Runs from Mother to Son (24 min) · Session 2 opens

Hook back in: "Last session: alleles that blend or co-express. Today: what happens when a gene rides on a sex chromosome — and why that makes certain traits show up far more in men."

Plain language first — the sex chromosomes. Humans have 23 pairs of chromosomes. One pair is the sex chromosomes: females are XX, males are XY. The X carries many genes; the much smaller Y carries very few. So for a gene on the X:
- A female (XX) has two copies — she can be homozygous or heterozygous.
- A male (XY) has only one copy of any X gene (we say hemizygous). Whatever is on his single X shows — there's no second copy to mask it.

The key consequence: for an X-linked recessive trait (red-green colorblindness, hemophilia), a male needs just one recessive allele to be affected, because he has no back-up X. A female needs two. That's why these traits are much more common in males — Learn.Genetics: red-green colorblindness is about 1 in 12 boys vs. 1 in 250 girls.

One fully worked example (build it on the board — every step). Use Xᴬ = normal, Xᵃ = colorblind allele.

Carrier mother (XᴬXᵃ) × unaffected father (XᴬY).
- Mom's gametes: Xᴬ (½) or Xᵃ (½). Dad's gametes: Xᴬ (½) or Y (½).

	Xᴬ (from dad)	Y (from dad)
Xᴬ (from mom)	XᴬXᴬ — daughter, normal	XᴬY — son, normal
Xᵃ (from mom)	XᴬXᵃ — daughter, carrier	XᵃY — son, colorblind

• Daughters (left column): XᴬXᴬ (normal) and XᴬXᵃ (carrier) → 1/2 carriers, 0 affected.
• Sons (right column): XᴬY (normal) and XᵃY (colorblind) → 1/2 of sons affected.
• All children: of the four boxes, exactly one (XᵃY) is affected → 1/4 of all children affected — and that affected child is always male. (Verified.)

Land the misconception again: a son gets his only X from his mother. So an X-linked recessive trait classically passes from a carrier mother to her sons — the father (who gives sons a Y, not an X) doesn't pass it to them at all. There is no male carrier: a son's single X either has the allele (affected) or doesn't (unaffected).

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Segment 6 — Linked Genes (a brief, honest footnote) + Pedigrees Setup (12 min)

Linked genes (keep it short — ~4 min). Mendel's law of independent assortment assumes genes sort into gametes independently. That's true for genes on different chromosomes. But genes that sit close together on the same chromosome tend to be inherited together — they're linked, and they break the neat 9:3:3:1. Crossing over (recombination, from meiosis in Week 10) can separate them; the farther apart two genes are, the more often crossing over splits them. Overview only: the takeaway is that physical position on a chromosome can override independent assortment. (Reference: OpenStax §12.3.)

Pedigrees — the plain-language setup (~8 min). A pedigree is a family tree for a single trait — the tool a genetic counselor uses with real families. The symbols (put them on a slide):
- Square = male, circle = female.
- Shaded = affected (has the trait); unshaded = unaffected.
- A horizontal line between a square and circle = a mating; vertical lines drop to their children; Roman numerals label generations (I, II, III) and numbers label individuals.
- A half-shaded symbol is sometimes used to mark a known carrier.

"You won't memorize a family — you'll reason from the pattern. Two clues do most of the work."

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Segment 7 — Reading a Pedigree: Two Decisions (the worked logic) (20 min)

Set it up: "Every pedigree question is really two yes/no decisions: (1) dominant or recessive? and (2) autosomal or X-linked? Here are the rules of thumb — and a worked case."

Decision 1 — dominant or recessive?
- If two unaffected parents have an affected child, the trait must be recessive (the parents each carried a hidden copy). "Affected child from two clear parents ⇒ recessive."
- If the trait appears in every generation and every affected child has an affected parent, it's likely dominant.

Decision 2 — autosomal or X-linked?
- If the trait is roughly equal in males and females, lean autosomal.
- If it's much more common in males, and especially if it passes from carrier mothers to sons (skipping the fathers), lean X-linked recessive.

One worked pedigree (describe it fully in text so anyone can follow — no figure needed):

Generation I: an unaffected father (I-1, unshaded square) and an unaffected mother (I-2, unshaded circle).
Generation II: their children include an affected son (II-1, shaded square) and two unaffected daughters (II-2, II-3, unshaded circles).
Reasoning: two unaffected parents produced an affected child → the trait is recessive (Decision 1). The only affected individual is male, and his unaffected mother must have carried the allele on one X → consistent with X-linked recessive, passed from a carrier mother to her son (Decision 2). A genetic counselor would tell mother I-2 she is an obligate carrier (XᴬXᵃ), and that each future son has a 1/2 chance of being affected — exactly the Segment 5 cross. (All consistent and verified.)

Misconception + cure:
- ❌ "The affected son got it from his father."
✅ Cure: for an X-linked trait, a son's X comes from his mother. The father gives sons a Y. So the carrier mother is the source — which is why it looks like the trait "skipped" the dad.

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Segment 8 — Technology Workflow + AI-Critique, Callback & Hand-off (19 min) · Session 2 closes (~75)

Technology workflow — the non-Mendelian cross habit, on demand:
1. Identify the pattern first: incomplete dominance (blend), codominance (both show), multiple alleles (ABO), or sex linkage (gene on X).
2. Write each parent's genotype with the right notation (RW; Iᴬi; XᴬXᵃ).
3. List each parent's gametes, fill the Punnett square, then count boxes to get the probability — exactly the Week-11 move.
4. For a pedigree: make the two decisions (dominant/recessive, then autosomal/X-linked) using the rules of thumb.

AI-critique moment (students verify, not consume):

Paste this to an approved chatbot: "In humans, a woman who is a carrier for red-green colorblindness (XᴬXᵃ) has children with a man who is NOT colorblind (XᴬY). What fraction of their sons will be colorblind, what fraction of their daughters, and can any of their sons be 'carriers'?"
Then check its work against today's cross. The right answers are 1/2 of sons affected, 0 of daughters affected (1/2 are carriers), and NO — a son cannot be a carrier (one X: affected or not). Chatbots frequently call a son a "carrier," mix up the son/daughter fractions, or confuse incomplete dominance with codominance elsewhere. Your job all semester: the tool drafts, you judge. This is exactly how the weekly Lecture Tutorial works — you catch the model, not trust it.

Callback + tease:
- Callback: "Everything today was Week 11's machinery — Punnett squares and probability — pointed at the messier, more realistic patterns: blends, co-expression, sex linkage, and family pedigrees."
- Tease next week: "We've been treating genes as abstract 'alleles.' Next week we open the molecule itself — DNA: the double helix, how it copies itself so faithfully, and the base-pairing rule that makes inheritance possible at all."

Hand-off (the week's graded work):
- Lecture Tutorial 12 (AI tutor, share-link submission) — incomplete vs. codominance, ABO blood types, sex linkage, and pedigrees.
- Quiz 12 and Discussion 12 ("Why Colorblindness Skips the Women / Counsel the Couple") and Assignment 12 (an incomplete-dominance cross, an ABO cross, a sex-linkage cross, and a pedigree).
- Lab 12 — "Blood-Type & Pedigree Detective" — a virtual problem set where you compute the odds and then audit the AI's reading of a pedigree.

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Instructor FAQ — Common Stumbles

Student says / does	Quick cure
Confuses incomplete dominance and codominance.	Incomplete = a blend (pink). Codominant = both alleles show at once (A and B).
"Blood type O is dominant."	i is recessive; type O = ii (two copies). It's common, not dominant.
"A man can be a carrier of colorblindness."	Males have one X: affected (XᵃY) or not (XᴬY). Only females can be carriers (XᴬXᵃ).
Says the affected son got the X-linked trait from his father.	A son's X comes from his mother; the father gives sons a Y. Source = carrier mother.
Gives "3 pink : 1 white" for RW × RW.	Heterozygote is pink, so the ratio is 1 red : 2 pink : 1 white, P(pink) = 1/2.
Thinks A × B parents can't make a type-O child.	Each can carry a hidden i: Iᴬi × Iᴮi → AB, A, B, O each 1/4, so P(O) = 1/4.
Reads a pedigree by shading alone.	Use the two decisions: unaffected parents + affected child ⇒ recessive; male-skewed ⇒ X-linked.
Thinks linked genes still give a 9:3:3:1.	Genes close together on one chromosome are inherited together (linked); crossing over can separate them.

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Scope flag

This outline stays within Objective 6 (extensions of Mendelian inheritance and pedigree analysis). It builds directly on Week 11 (Punnett squares, probability, the product rule) and Week 10 (meiosis, crossing over) — both referenced, not re-taught. Linked genes are introduced as a brief overview footnote, not a recombination-mapping unit. Molecular mechanisms (what a gene is at the DNA level) are Week 13, only teased here. Named, real items — the ABO system, X-linked colorblindness and hemophilia, Mendel's laws, recombination — are referenced factually; the instructor and institution remain fictional. Every probability used (1/2 pink, 1/4 type O, 1/2 of sons affected, 1/4 of all children affected) is pre-computed and independently re-verified.