AP Biology 5.4 Non-Mendelian Genetics Study Notes - New Syllabus Effective 2025
AP Biology 5.4 Non-Mendelian Genetics Study Notes – New syllabus
AP Biology 5.4 Non-Mendelian Genetics Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain deviations from Mendel’s model of the inheritance of traits.
Key Concepts:
- Non-Mendelian Genetics
5.4.A – Deviations from Mendel’s Model of Inheritance
📌 What Mendel Proposed:
Gregor Mendel discovered basic rules of inheritance using pea plants:
- Law of Segregation: Each parent gives one allele
- Law of Independent Assortment: Genes for different traits are inherited independently
- Dominant & Recessive patterns predict phenotypic ratios like 3:1 or 9:3:3:1
🚨 But in real life? It’s not always that simple.
There are exceptions and extensions to Mendel’s rules. These are called non-Mendelian inheritance patterns.
🔍 Deviations from Mendel’s Model – Overview
| Type | What’s Happening? |
|---|---|
| Gene Linkage | Genes located close together on the same chromosome tend to be inherited together |
| Codominance | Both alleles are fully expressed — neither is masked |
| Incomplete Dominance | Alleles blend in heterozygotes (red + white = pink) |
| Sex-linked Inheritance | Genes on X or Y chromosomes follow different rules |
| Pleiotropy | One gene affects multiple traits |
| Non-nuclear Inheritance | Traits passed through mitochondrial or chloroplast DNA (not nuclear DNA!) |
| Epistasis | One gene can block or mask the expression of another gene |
| Polygenic Traits | Traits controlled by many genes (e.g., skin color, height) |
| Environmental Effects | The environment can influence how genes are expressed |
📊 How Do We Know It’s Non-Mendelian?
When we perform genetic crosses and the observed phenotypic ratios differ from Mendel’s predictions, it’s a clue that:
- Traits may involve linked genes
- Multiple genes or non-nuclear DNA might be involved
- Some traits don’t segregate independently
We can analyze this through quantitative data, Punnett squares, and pedigrees.
🧪 Examples of Deviations:
- Blood type AB (codominance): Both A and B are fully expressed
- Pink snapdragons (incomplete dominance): Red + white = pink
- Color blindness (X-linked trait): Mostly in males
- Mitochondrial disorders: Passed only from mother to child
- Sickle-cell gene: One gene affects blood shape, anemia, malaria resistance (pleiotropy)
🧠 Quick Reminder: Mendel’s Laws Still Apply… Sometimes
- These exceptions don’t disprove Mendel’s laws — they just show his model doesn’t explain all inheritance patterns.
- Mendel studied simple, single-gene traits. Most real-life traits are way more complex.
5.4.A.1 – When Traits Don’t Follow Mendel’s Ratios
📊 I. Quantitative Analysis: When Reality ≠ Mendel’s Prediction
- Sometimes, actual phenotypes seen in offspring don’t match Mendel’s expected ratios.
- These differences can be measured using statistics to figure out what’s going on.
- Often, these patterns show more complex inheritance.
🔗 II. Linked Genes (Genetic Linkage) → Genes that travel together!
- Linked genes are located on the same chromosome.
- During meiosis, genes that are close together tend to be inherited together instead of assorting independently (violates Mendel’s law of independent assortment).
- The closer the genes are, the less likely they’ll be separated by crossing over.
📏 Genetic Mapping:
- Scientists measure how often two genes are inherited together.
- That frequency gives us the distance between genes on a chromosome!
🧪 Map Units (centimorgans)
- 1 map unit = 1% chance of recombination between genes
- Used to build gene maps!
🟥 III. Codominance (Both Traits Show Fully!) → 🧬 Sharing the stage equally!
- In codominance, both alleles are fully expressed in a heterozygote.
- The heterozygous phenotype shows BOTH traits side by side — no blending!
🧪 Example:
- Blood type AB → expresses both A and B antigens
- Genotype: IAIB
📌 Codominance ≠ incomplete dominance!
🎨 IV. Incomplete Dominance (Blending Inheritance) → 🧪 Neither allele is strong enough to win!
- In incomplete dominance, neither allele is fully dominant.
- The heterozygote shows a blended phenotype — halfway between the two.
🧪 Example:
- Red (RR) + White (rr) = Pink (Rr)
📌 Phenotype ratio in F2 = 1 Red : 2 Pink : 1 White
🧠 Summary Table
| Pattern | Description | Example |
|---|---|---|
| Linked Genes | Genes on same chromosome stay together | Eye color + wing shape (flies) |
| Codominance | Both alleles show fully | Blood type AB |
| Incomplete Dominance | Traits blend in heterozygote | Pink snapdragons |
5.4.A.2 – Inheritance of Sex-Linked Traits
🧬 What Are Sex-Linked Traits?
- Sex-linked traits are traits controlled by genes on the sex chromosomes (X or Y).
- Humans have 23 pairs of chromosomes, where 22 pairs are autosomes, and 1 pair is sex chromosomes:
- XX = female
- XY = male
❗ Why Are X-Linked Traits More Common?
- The X chromosome is much larger and carries more genes than the Y.
- Most sex-linked traits are X-linked because there are few genes on the Y chromosome.
Males (XY) have only one X, so:
- Even a recessive X-linked allele will be expressed, because there’s no backup copy.
Females (XX) have two X chromosomes, so:
- They need two copies of the recessive allele to express the trait.
🧬 Examples of X-Linked Traits
| Trait | Type | Notes |
|---|---|---|
| Color blindness | Recessive | More common in males |
| Hemophilia | Recessive | Blood clotting disorder |
| Duchenne MD | Recessive | Muscle deterioration disease |
Carrier Females
- A carrier is a female who has one recessive allele for the trait but does not show symptoms.
- She can pass the trait to her offspring.
🧠 Genotype: XᴺXⁿ
- Trait is masked by the dominant allele.
🔎 Using Pedigrees for Sex-Linked Traits
Pedigrees help us predict inheritance across generations. Here’s what to watch for:
🧩 X-linked recessive clues:
- Trait skips generations
- More males affected than females
- Carrier moms → affected sons
🧪 Example:
If a carrier mother (XᴺXⁿ) and normal father (XᴺY) have kids:
- 50% sons: ½ XⁿY = affected
- 50% daughters: ½ XᴺXⁿ = carriers
⚙️ Barr Bodies: What Happens to the Extra X?
- In female cells, one X chromosome is randomly inactivated during development.
- This inactive X = Barr body — a condensed, silent chromosome.
🧪 Fun Fact: That’s why female cats can be tortoiseshell-colored — due to random X inactivation!
🧬 Summary Chart
| Term | Meaning |
|---|---|
| Sex-linked traits | Traits found on X or Y chromosomes |
| X-linked | Common, especially in males |
| Carrier | Female with 1 copy of recessive gene |
| Barr body | Inactivated X chromosome in female cells |
5.4.A.3 – Pleiotropy: One Gene, Many Effects
💡 What is Pleiotropy?
Pleiotropy is when one single gene affects multiple traits in an organism.
🧠 In other words:
One gene → many phenotypic outcomes
🔍 Why does this happen?
The protein made by that gene might:
- Be used in many different tissues (like collagen in skin, bones, eyes).
- Be part of a common pathway that controls several processes.
- Trigger a chain reaction in the body that leads to more than one outcome.
🧪 Real-Life Examples
| Disorder / Gene | Effects (Phenotypes) |
|---|---|
| Sickle Cell Disease 🩸 | – Abnormal red blood cells – Pain crises – Organ damage |
| Marfan Syndrome 🦴 | – Long limbs – Heart problems – Eye issues |
| Phenylketonuria (PKU) 🍽️ | – Mental retardation – Skin disorders – Behavior changes |
Each of these conditions is caused by a mutation in a single gene, but the symptoms show up in multiple body systems.
🎯 How Is This Different from Polygenic Traits?
| Pleiotropy | Polygenic Inheritance |
|---|---|
| One gene → multiple traits | Multiple genes → one trait |
| Ex: Sickle cell gene → many effects | Ex: Skin color → controlled by many genes |
📌 Why it’s important:
- Mendel’s Laws say traits are passed independently.
- But pleiotropy breaks that rule—traits linked by one gene don’t segregate independently because they come as a package from a single source.
5.4.A.4 – Non-Nuclear Inheritance: Traits Beyond the Nucleus
Not all traits are passed through the nucleus or follow Mendel’s laws. Some traits are inherited from other parts of the cell — like the mitochondria and chloroplasts!
🧬 What is Non-Nuclear Inheritance?
Traits passed down through DNA found outside the nucleus, specifically in:
- Mitochondria (found in all eukaryotes)
- Chloroplasts (found in plants and algae)
This inheritance pattern is non-Mendelian — it doesn’t follow the usual dominant/recessive segregation seen in nuclear genes.
i. How it works: Random Assortment
During cell division and gamete formation:
- Mitochondria and chloroplasts are randomly sorted into daughter cells and gametes.
- There’s no “meiosis” for these organelles, so the inheritance is more unpredictable.
- Because they aren’t linked to nuclear chromosomes, they don’t assort independently or segregate the way Mendelian genes do.
ii. Maternal Inheritance in Animals (Mitochondria)
In most animals:
- Only the egg contributes mitochondria to the zygote.
- The sperm’s mitochondria are usually destroyed or excluded after fertilization.
✅ So, mitochondrial traits come only from the mother. This is called maternal inheritance.
🔍 Example:
Diseases like Leber’s Hereditary Optic Neuropathy (LHON) are inherited maternally because they’re caused by mitochondrial DNA mutations.
iii. Maternal Inheritance in Plants (Mitochondria & Chloroplasts)
In plants:
- The egg (ovule) contributes both mitochondria and chloroplasts.
- Pollen (sperm) usually does not contribute these organelles.
✅ So traits controlled by chloroplast or mitochondrial DNA are also maternally inherited in plants.
🧪 Example:
Leaf color in some plants depends on chloroplast DNA, so it comes only from the maternal parent.
🚫 Why It’s Not Mendelian:
- No dominance or recessiveness involved
- No law of segregation
- No independent assortment
- Traits don’t follow the 3:1 or 9:3:3:1 ratio