AP Biology 5.3 Mendelian Genetics Study Notes - New Syllabus Effective 2025
AP Biology 5.3 Mendelian Genetics Study Notes – New syllabus
AP Biology 5.3 Mendelian Genetics Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain the inheritance of genes and traits as described by Mendel’s laws.
Key Concepts:
- Mendelian Genetics
5.3.A – Mendelian Inheritance: How Traits Are Passed On
📚 What is Mendelian Genetics?
- Genetics = Study of heredity (how traits are passed from parents to offspring).
- Gregor Mendel, a 19th-century monk, is the “Father of Genetics.” 🌱
- He studied pea plants and figured out how traits are inherited.
🔑 Mendel’s 3 Laws of Inheritance
Law of Dominance
- When two different alleles are present (like Aa), one may mask the other.
- The dominant allele is expressed (capital letter, e.g., “A”).
- The recessive allele is hidden unless both copies are recessive (aa).
Example: If purple flower (P) is dominant to white (p):
- PP or Pp = purple 🌸
- pp = white 🌼
Law of Segregation
- Each organism has 2 alleles per gene but passes only 1 allele to each gamete.
- During meiosis, the two alleles separate (segregate) randomly.
📊 In a monohybrid cross (Aa × Aa):
- Genotype ratio = 1 AA : 2 Aa : 1 aa
- Phenotype ratio = 3 dominant : 1 recessive
Law of Independent Assortment
- Genes for different traits separate independently when gametes are made.
- Applies when genes are on different chromosomes (not linked).
💡 In a dihybrid cross (AaBb × AaBb):
- Phenotype ratio = 9:3:3:1 (classic result!)
- Leads to lots of combinations = genetic variation!
🧬 Key Terms
| Term | Meaning |
|---|---|
| Gene | A section of DNA coding for a trait |
| Allele | A version of a gene (e.g., A or a) |
| Homozygous | Same alleles (AA or aa) |
| Heterozygous | Different alleles (Aa) |
| Genotype | The genetic makeup (e.g., Aa) |
| Phenotype | The physical trait shown (e.g., tall) |
How Traits Are Inherited
- Genes are passed from parents to offspring via gametes (sperm & egg).
- Each parent gives one allele for each gene.
- Traits are influenced by:
- Which alleles are inherited
- Whether they’re dominant or recessive
- How chromosomes segregate during meiosis
What Causes Genetic Variation?
- Random fertilization = any sperm can meet any egg
- Independent assortment = chromosomes line up randomly in meiosis I
- Crossing over = alleles get mixed on the same chromosome
✅ Key Takeaway:
Mendel’s laws explain why children inherit genes, not traits, and how those genes combine in predictable ways. Dominant and recessive alleles, allele segregation, and independent assortment all shape how traits are passed down.
5.3.A.1 – Mendel’s Laws & Genes on Different Chromosomes
🌱 What Did Mendel Discover?
Gregor Mendel, the “father of genetics,” discovered how traits are inherited by studying pea plants. His work led to two foundational laws of inheritance:
⚖️ 1. Law of Segregation
Each organism has two alleles for a gene but passes only one to each gamete.
📝 Random Chance in Offspring:
Since each parent passes only one allele to the offspring, the flower’s final color depends on which allele the gamete receives. If a heterozygous (Pp) parent produces gametes, half will carry P (purple) and half will carry p (white)—creating a 50/50 chance for each individual allele.
🌱 Visible Traits vs Hidden Genes:
Even if a flower is purple (Pp), it still carries the hidden white allele (p). This means if two purple (Pp) flowers are crossed, some offspring may end up white (pp), because each parent has a chance to pass the recessive allele.
🎲 2. Law of Independent Assortment
Genes for different traits are passed on independently of each other – IF they’re on different chromosomes.
💡 This means:
The allele a gamete gets for flower color doesn’t affect the allele it gets for seed shape.
🔍 When Do These Laws Apply Perfectly?
Mendel’s laws apply without exception when:
- Genes are located on different chromosomes
- OR genes are far apart on the same chromosome (so they can be separated by crossing over)
🧠 These genes behave independently, especially during Metaphase I of meiosis, when homologous chromosomes line up and separate randomly.
🚫 When the Laws Don’t Fully Apply
Mendel didn’t know about linked genes—genes that are:
- Found close together on the same chromosome
- Tend to be inherited together (they don’t assort independently)
🔁 Linked genes can sometimes be separated if crossing over happens during Prophase I of meiosis.
Why This Matters:
These laws help explain:
- How traits are inherited
- Why siblings look different
- How genetic variation happens naturally
🧠 Easy Memory Tip:
🧩 “Segregation = Separation”
Just remember: During gamete formation, alleles go their separate ways!
🧬 Summary Table
| Law | What It Means | When It Applies |
|---|---|---|
| Law of Segregation | Each gamete gets only one allele from each pair | Always (during Anaphase I of meiosis) |
| Law of Independent Assortment | Alleles for different genes assort independently | Only for genes on different chromosomes or far apart on the same one |
5.3.A.2 – Mendelian Inheritance & Probability Rules
I. How Fertilization Affects Inheritance 🧬
- Fertilization = fusion of two haploid gametes (n) → forms a diploid (2n) zygote
- Restores chromosome number & mixes maternal and paternal alleles
- Because each gamete is genetically unique (due to meiosis), fertilization creates: New allele combinations → More genetic variation!
II. Rules of Probability in Genetics 🟢
Inheritance isn’t magic—it follows simple math!
| Situation | Rule | Formula |
|---|---|---|
| Mutually exclusive events | “Either A or B” | P(A or B) = P(A) + P(B) |
| Independent events | “A and B together” | P(A and B) = P(A) × P(B) |
III. Genetic Cross Types 🧪
🧪 1. Monohybrid Cross (1 gene, 2 alleles)
- Example: Tt × Tt
- Phenotype ratio: 3 Tall : 1 Short
- Genotype ratio: 1 TT : 2 Tt : 1 tt
🧪 2. Dihybrid Cross (2 genes, both heterozygous)
- Example: RrYy × RrYy
- Phenotype ratio: 9:3:3:1 (9 both dominant, 3 dom/rec, 3 rec/dom, 1 both rec)
- Easier to use probability method than drawing huge Punnett squares.
🧪 3. Test Cross
- Use to find whether an organism showing the dominant trait is:
- Homozygous dominant (RR)
- or Heterozygous (Rr)
- Cross it with homozygous recessive (rr):
- If all offspring show dominant → likely RR
- If some show recessive → definitely Rr
IV. Genotype vs Phenotype 🧬
| Term | Meaning | Example |
|---|---|---|
| Genotype | Genetic makeup (alleles) | AA, Aa, or aa |
| Phenotype | Observable traits (physical expression) | Tall plant, blue eyes, etc. |
Homozygous = same alleles (AA or aa)
Heterozygous = different alleles (Aa)
V. Patterns of Inheritance 📊
| Pattern | Key Features |
|---|---|
| Autosomal | Gene on non-sex chromosome (1–22 pairs), affects both sexes equally |
| Sex-linked (X-linked) | Gene on X chromosome; more common in males (XY) |
| Linked Genes | Genes on same chromosome; often inherited together unless crossing over occurs |
VI. Pedigrees & Predicting Traits
- Pedigree = family tree of traits
- Helps figure out:
- Dominant or recessive?
- Autosomal or sex-linked?
- Combine pedigree + Punnett square = predict genotypes & phenotypes
📌 KEY TAKEAWAY:
Mendel’s laws + probability = powerful tools to predict how traits are passed.
Punnett squares + rules of probability help analyze inheritance of single-gene traits.