Edexcel A Level (IAL) Biology -2.15-2.16 Patterns of Inheritance & Sex Linkage- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -2.15-2.16 Patterns of Inheritance & Sex Linkage- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -2.15-2.16 Patterns of Inheritance & Sex Linkage- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 2.15 (i) understand what is meant by the terms gene, allele, genotype, phenotype, recessive, dominant, codominance, homozygote and heterozygote
(ii) understand patterns of inheritance, including the interpretation of genetic pedigree diagrams, in the context of monohybrid inheritance
(iii) understand sex linkage on the X chromosome, including red-green colour blindness in humans - 2.16 understand how the expression of a gene mutation in people with cystic fibrosis impairs the functioning of the gaseous exchange, digestive and reproductive systems
Genetic Inheritance & Key Terms
🌱 Introduction
Genetics is the study of how characteristics are passed from parents to offspring through genes.
The variations we see in individuals arise due to different forms of genes (alleles) and how they are inherited.
🧩 (i) Key Genetic Terms
| Term | Simple Meaning | Example |
|---|---|---|
| Gene | A short section of DNA that codes for a specific protein or trait. | Gene for eye colour |
| Allele | Different forms of the same gene. | Brown-eye allele, blue-eye allele |
| Genotype | The genetic makeup of an organism (the alleles it carries). | BB, Bb, or bb |
| Phenotype | The physical expression of the genotype. | Brown eyes |
| Dominant | Allele that always shows in the phenotype, even if only one copy is present. | B (brown eyes) |
| Recessive | Allele that shows only when two copies are present. | b (blue eyes) |
| Codominance | When both alleles are expressed equally in the phenotype. | Blood group AB (both A and B alleles expressed) |
| Homozygote | Both alleles are the same. | BB or bb |
| Heterozygote | Two different alleles for a gene. | Bb |
🧬 Example – Eye Colour
- Genotype BB or Bb → Brown eyes (dominant)
- Genotype bb → Blue eyes (recessive)
🧩 (ii) Monohybrid Inheritance
🧠 Definition
Monohybrid inheritance involves the study of one gene controlling a single characteristic.
It shows how alleles segregate and combine during fertilisation.
⚙️ How It Works
- Each parent carries two alleles for a gene (one on each homologous chromosome).
- During gamete formation (meiosis), these alleles separate — each gamete gets only one allele.
- When gametes fuse at fertilisation, the offspring inherits one allele from each parent.
💡 Punnett Square Example
Cross: Parent 1 = Bb (Brown eyes), Parent 2 = Bb (Brown eyes)
| Parent alleles ↓ × → | B | b |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Genotypic ratio: 1 BB : 2 Bb : 1 bb
Phenotypic ratio: 3 Brown : 1 Blue
🧬 Genetic Pedigree Diagrams
- Pedigrees show inheritance patterns of traits through generations.
- Square → Male, Circle → Female.
- Shaded symbol → Affected individual.
- If a trait skips generations, it’s likely recessive.
- If it appears in every generation, it’s likely dominant.
🧩 (iii) Sex Linkage (X-linked Inheritance)
Basic Idea
- Sex-linked genes are carried on the sex chromosomes (X or Y).
- Most sex-linked traits are on the X chromosome because it’s larger and carries more genes.
- Males have XY, females have XX.
Why Males Are More Affected
- Males have only one X chromosome, so any recessive allele on X will be expressed (no second X to mask it).
- Females must have two copies of the recessive allele to show the trait.
🎯 Example: Red-Green Colour Blindness
Controlled by a recessive allele (Xᶜ) on the X chromosome.
Normal vision allele = Xᴺ, Colour blind allele = Xᶜ
| Parent | Genotype | Phenotype |
|---|---|---|
| Male | XᴺY | Normal |
| Female carrier | XᴺXᶜ | Normal (carrier) |
| Colour-blind male | XᶜY | Colour blind |
| Colour-blind female | XᶜXᶜ | Colour blind |
Cross Example (Carrier Female × Normal Male):
| Female ↓ × → | Xᴺ | Y |
|---|---|---|
| Xᴺ | XᴺXᴺ (normal female) | XᴺY (normal male) |
| Xᶜ | XᴺXᶜ (carrier female) | XᶜY (colour-blind male) |
Result: 50% males are colour-blind, 50% females are carriers.
🧩 Summary Table
| Concept | Key Idea | Example |
|---|---|---|
| Gene | DNA segment coding for a protein | Gene for insulin |
| Allele | Alternative version of a gene | A, a |
| Dominant | Expressed in heterozygote | A in Aa |
| Recessive | Expressed only in homozygote | a in aa |
| Codominance | Both alleles visible | AB blood group |
| Monohybrid inheritance | One gene involved | Tall × short pea plants |
| Sex linkage | Gene on X chromosome | Red-green colour blindness |
📦 Quick Recap
| Term | One-Line Summary |
|---|---|
| Gene | Unit of inheritance on DNA |
| Allele | Alternate form of a gene |
| Genotype | Allele combination (BB, Bb, bb) |
| Phenotype | Physical expression of genotype |
| Dominant | Expressed with one copy |
| Recessive | Needs two copies to show |
| Codominance | Both alleles expressed equally |
| Homozygous | Both alleles same |
| Heterozygous | Alleles different |
| Sex linkage | Trait carried on X chromosome |
| Example | Red-green colour blindness (more common in males) |
Cystic Fibrosis – Gene Mutation and Its Effects on Body Systems
📌 Introduction
Cystic Fibrosis (CF) is a genetic disorder caused by a mutation in a single gene that affects membrane transport of chloride ions.
This leads to thick, sticky mucus building up in organs such as the lungs, digestive system, and reproductive system.
The condition is inherited in a recessive pattern – a person must have two copies of the faulty allele (homozygous recessive) to show symptoms.
🧩 1. The Genetic Cause
| Feature | Description |
|---|---|
| Gene affected | CFTR gene (Cystic Fibrosis Transmembrane Conductance Regulator) |
| Normal CFTR function | Codes for a channel protein that controls the movement of chloride ions across cell membranes. |
| Mutation | Commonly the ΔF508 mutation – deletion of 3 DNA bases → missing phenylalanine amino acid. |
| Effect of mutation | CFTR protein misfolds → either not transported to cell surface or doesn’t function properly. |
| Result | Less chloride leaves cells → water doesn’t follow by osmosis → mucus becomes thick and sticky. |
🌬️ 2. Effect on the Gaseous Exchange System (Lungs)
- Normal Function
Mucus traps dust and microorganisms in airways.
Cilia move mucus upward to be coughed out or swallowed. - In CF
Mucus becomes abnormally thick → cilia cannot move it effectively.
Mucus builds up, blocking airways.
Gas exchange becomes difficult → less oxygen reaching blood.
Creates anaerobic conditions → bacteria grow, causing lung infections. - Consequences
Repeated lung infections and inflammation.
Breathing difficulties and reduced oxygen levels.
Long-term lung damage.
🍽️ 3. Effect on the Digestive System
- Normal Function
Pancreatic ducts carry enzymes (lipase, amylase, protease) into the small intestine to digest food. - In CF
Thick mucus blocks pancreatic ducts → enzymes cannot reach intestine.
Food not digested properly → poor absorption of nutrients.
Energy deficiency, malnutrition, and poor growth result.
Cysts and fibrosis may form in the pancreas (hence “cystic fibrosis”). - Consequences
Frequent constipation and abdominal pain.
Fatty stools (steatorrhea) due to undigested fats.
Vitamin deficiencies (especially A, D, E, K).
💫 4. Effect on the Reproductive System
- In Males
Vas deferens (sperm duct) often blocked or missing due to mucus build-up.
Sperm cannot leave the testes → infertility. - In Females
Thick cervical mucus makes it hard for sperm to pass through the cervix.
Reduced chance of fertilization but not complete infertility.
⚙️ Summary Table – How CF Affects Body Systems
| System | Normal Function | Effect of CF | Outcome |
|---|---|---|---|
| Gaseous exchange | Mucus traps dust, cilia clear it | Thick mucus blocks airways, cilia can’t move it | Breathing difficulty, infections |
| Digestive | Enzymes secreted into intestine | Pancreatic ducts blocked | Malabsorption, malnutrition |
| Reproductive (Male) | Sperm duct transports sperm | Duct blocked or missing | Infertility |
| Reproductive (Female) | Cervical mucus allows sperm entry | Mucus too thick | Reduced fertility |
🧠 Quick Recap
Cause → Mutation in CFTR gene → faulty chloride channel
Effect → Less water in mucus → thick and sticky mucus
Lungs → Airways blocked → infections, low oxygen
Digestive system → Pancreatic ducts blocked → poor digestion, malnutrition
Reproductive system → Blocked sperm duct / thick cervical mucus → infertility or reduced fertility
Inheritance → Recessive (must have two faulty alleles)