Week 12 — Readings & Resources · 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 — incomplete dominance, codominance, multiple alleles, sex linkage, and pedigrees — to predict genotypes, phenotypes, and probabilities.

How to use this page

Everything here is a link to an external resource — open it in your browser, the same way you'd open a YouTube link. Nothing needs to be downloaded.

This week's load is deliberately light: 2 short videos + 2 short readings, grouped by the ideas from the lecture, plus optional free references. Watch or read one item per group and you're ready for the quiz; do all of them and you'll be very comfortable. Total time is roughly 35–45 minutes if you do everything, far less if you pick one per group.

Order that matches the lecture: ① incomplete dominance, codominance & multiple alleles (ABO) → ② sex linkage (X-linked) → ③ reading pedigrees → ④ the big-picture extensions of Mendel.

A habit to start now: these patterns are exactly where a chatbot gets sloppy. Before you trust any genetics claim — in these resources or from an AI — ask the questions from class: Is it a blend or both-at-once? Can a male even be a carrier here? Does the Punnett square actually give that fraction?

① Incomplete Dominance, Codominance & Multiple Alleles (ABO)

Maps to Lecture Segments 2–3. The heterozygote can be a blend (incomplete dominance → pink) or show both alleles at once (codominance → AB blood). The ABO system adds a third allele (i is recessive → type O).

Video — "Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!" (Amoeba Sisters)
🔗 https://www.youtube.com/watch?v=YJHGfbW55l0
Why it earns the click: a friendly, funny tour of exactly the distinction we drew on the board — incomplete dominance as a blend versus codominance as both alleles showing at once — using the same snapdragon and blood-type style examples. The non-Mendelian patterns from Segment 2, in under ten minutes.
⏱ ~8 min

Reading — "Genes and Blood Type" (Learn.Genetics, University of Utah)
🔗 https://learn.genetics.utah.edu/content/basics/blood/
Why it's assigned: the clearest plain-language walkthrough of the ABO multiple-allele system — how the Iᴬ, Iᴮ, and i alleles make the A, B, AB, and O types, why Iᴬ and Iᴮ are codominant, and why type O has no antigen. This is the science behind the Type A × Type B cross we worked in class.
⏱ ~8 min

② Sex Linkage — Why X-Linked Traits Are More Common in Men

Maps to Lecture Segment 5. A male has only one X (hemizygous), so a single X-linked recessive allele makes him affected — there is no male carrier. That's why colorblindness and hemophilia are far more common in men.

Reading — "Sex Linkage" (Learn.Genetics, University of Utah)
🔗 https://learn.genetics.utah.edu/content/pigeons/sexlinkage/
Why it's assigned: explains, in plain terms, why males have no back-up copy of an X-linked gene and gives the exact numbers we cited in class — red-green colorblindness is about 1 in 12 boys versus 1 in 250 girls. It also names other X-linked disorders (Duchenne muscular dystrophy, hemophilia). Segment 5 in one short page.
⏱ ~7 min

③ Reading Pedigrees

Maps to Lecture Segments 6–7. A pedigree is a family tree for one trait. The two decisions: dominant or recessive? (two unaffected parents + an affected child ⇒ recessive) and autosomal or X-linked? (much more common in males ⇒ X-linked).

Video — "Pedigrees" (Amoeba Sisters)
🔗 https://www.youtube.com/watch?v=Gd09V2AkZv4
Why it earns the click: walks through pedigree symbols (square/circle, shaded/unshaded) and then tracks both an autosomal recessive trait and an X-linked recessive trait through a family — exactly the reading logic from Segment 7. It even explains the half-shaded "carrier" symbol.
⏱ ~9 min

④ Extensions of Mendel — the Big Picture (and Linked Genes)

Maps to Lecture Segments 2–6. All of this week's patterns are extensions of Mendel's laws — including linked genes, which sit close together on a chromosome and break the neat 9:3:3:1 unless crossing over separates them.

Reading — "12.3 Laws of Inheritance" (OpenStax Biology 2e)
🔗 https://openstax.org/books/biology-2e/pages/12-3-laws-of-inheritance
Why it's assigned: the cleanest free-to-read-online treatment of how real inheritance extends past Mendel — segregation and independent assortment, the product rule, linked genes and recombination, and epistasis. Read the linkage section to make sense of the Segment 6 footnote. (Free to read online; no account needed.)
⏱ ~12 min

Optional one-stop references (free online)

• Learn.Genetics — "What are Dominant and Recessive?" (University of Utah). A myth-busting page that explains why "dominant" does NOT mean "more common" and uses the sickle-cell allele to show how the same allele can look recessive, dominant, or co-dominant depending on the trait you measure. Great for the "type O is dominant" misconception.
  🔗 https://learn.genetics.utah.edu/content/basics/patterns/
• OpenStax Biology 2e — Chapter 12 introduction. A free, short on-ramp to the whole genetics chapter if you want the Mendelian context before the extensions.
  🔗 https://openstax.org/books/biology-2e/pages/12-introduction

Pick-one quick path (≈17 min total)

In a hurry? Do exactly these two and you'll be ready for the quiz:
1. Watch "Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!" (group ①), and skim "Genes and Blood Type" for the ABO cross.
2. Watch "Pedigrees" (group ③), and read "Sex Linkage" for the colorblindness logic (group ②).

Heads-up (links rot): these point to outside sites that occasionally move or rename pages. If a link ever fails, tell Prof. Castellano and use the OpenStax or Learn.Genetics references above in the meantime.