CIE iGCSE Co-ordinated Sciences-B16.3 Monohybrid inheritance- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-Link – Study Notes
CIE iGCSE Co-ordinated Sciences-Link – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
Describe inheritance as the transmission of genetic information from generation to generation
Describe genotype as the genetic make-up of an organism and in terms of the alleles present
Describe phenotype as the observable features of an organism
Describe homozygous as having two identical alleles of a particular gene
State that two identical homozygous individuals that breed together will be pure-breeding
Describe heterozygous as having two different alleles of a particular gene
State that a heterozygous individual will not be pure-breeding
Describe a dominant allele as an allele that is expressed if it is present in the genotype
Describe a recessive allele as an allele that is only expressed when there is no dominant allele of the gene present in the genotype
Interpret pedigree diagrams for the inheritance of a given characteristic
Use genetic diagrams to predict the results of monohybrid crosses and calculate phenotypic ratios, limited to 1 : 1 and 3 : 1 ratios
Use Punnett squares in crosses which result in more than one genotype to work out and show the possible different genotypes
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Inheritance
📌 Definition
Inheritance is the transmission of genetic information from one generation to the next.
🌱 Key Points
- Genetic information is carried by genes on chromosomes.
- Determines traits in offspring (e.g., eye colour, blood group).
- Occurs in all living organisms.
- Can involve sexual reproduction (variation) or asexual reproduction (identical traits).
📊 Summary Table
| Feature | Description |
|---|---|
| Definition | Transmission of genetic information across generations |
| Carrier | Genes on chromosomes |
| Outcome | Offspring inherit traits from parents |
| Type | Sexual (variation) or asexual (identical) |
⚡ Quick Recap
Inheritance = passing genes to offspring
Memory tip: “Parents pass on traits via genes → children inherit them.”
Genotype
📌 Definition
Genotype = the genetic make-up of an organism. Describes the alleles present for a particular gene.
🌱 Key Points
- Determines what genes an organism carries.
- Can be homozygous (same alleles, e.g., AA or aa) or heterozygous (different alleles, e.g., Aa).
- Genotype influences the phenotype (observable traits).
📊 Summary Table
| Feature | Description | Example |
|---|---|---|
| Genotype | Genetic make-up of organism | AA, Aa, or aa |
| Homozygous | Two identical alleles | AA or aa |
| Heterozygous | Two different alleles | Aa |
| Role | Determines inherited traits | Eye colour, blood group |
⚡ Quick Recap
Genotype = which alleles you have
Memory tip: “Genotype is the genetic code inside; phenotype is what you see outside.”
Phenotype
📌 Definition
Phenotype = the observable features or characteristics of an organism.
🌱 Key Points
- Determined by the organism’s genotype and sometimes environmental factors.
- Examples of phenotypic traits: Eye colour, hair colour, height, blood group, presence of freckles.
- Different genotypes can produce the same phenotype (e.g., AA and Aa both giving brown eyes).
📊 Summary Table
| Feature | Description | Example |
|---|---|---|
| Phenotype | Observable traits of an organism | Brown eyes, tall height |
| Determined by | Genotype ± environment | Genes + nutrition |
| Note | Same phenotype can arise from different genotypes | AA or Aa → brown eyes |
⚡ Quick Recap
Phenotype = what you see
Memory tip: “Genotype is the blueprint, phenotype is the building you see.”
Homozygous
📌 Definition
Homozygous = having two identical alleles of a particular gene.
🌱 Key Points
- Can be homozygous dominant (e.g., AA) or homozygous recessive (e.g., aa).
- Both alleles code for the same version of a trait.
- Often affects the phenotype depending on whether the allele is dominant or recessive.
📊 Summary Table
| Feature | Description | Example |
|---|---|---|
| Homozygous | Two identical alleles for a gene | AA (dominant), aa (recessive) |
| Effect on trait | Determines phenotype if allele is dominant | AA → brown eyes; aa → blue eyes |
| Types | Dominant or recessive | AA / aa |
⚡ Quick Recap
Homozygous = identical alleles
Memory tip: “Same letters = homozygous.”
Pure Breeding
📌 Key Statement
Two identical homozygous individuals that breed together are said to be pure breeding.
🌱 Key Points
- Homozygous = both alleles for a gene are identical (AA or aa).
- Pure breeding parents always produce offspring with the same traits.
- Ensures trait consistency across generations.
📊 Summary Table
| Feature | Description | Example |
|---|---|---|
| Parents | Identical homozygous | AA × AA or aa × aa |
| Offspring | Genetically identical | All AA or all aa |
| Term | Pure breeding | True-breeding line |
⚡ Quick Recap
Homozygous × Homozygous = Pure breeding
Memory tip: “Same letters, same traits, pure line.”
Heterozygous
📌 Definition
Heterozygous = having two different alleles of a particular gene.
🌱 Key Points
- Example: Aa → one dominant allele (A) and one recessive allele (a).
- The dominant allele usually determines the phenotype, while the recessive allele may be masked.
- Heterozygous individuals are not pure breeding.
📊 Summary Table
| Feature | Description | Example |
|---|---|---|
| Heterozygous | Two different alleles for a gene | Aa |
| Effect on trait | Dominant allele expressed in phenotype | Aa → shows dominant trait |
| Breeding | Offspring may show variation | Aa × Aa → offspring can be AA, Aa, or aa |
⚡ Quick Recap
Heterozygous = different alleles
Memory tip: “Different letters = heterozygous, can produce variation.”
Heterozygous and Pure Breeding
📌 Key Statement
A heterozygous individual (two different alleles, e.g., Aa) will not be pure-breeding.
🌱 Key Points
- Heterozygous individuals carry one dominant and one recessive allele.
- When they breed, offspring may inherit different combinations of alleles (AA, Aa, or aa).
- Trait is not guaranteed to be the same in the next generation.
📊 Summary Table
| Feature | Description | Example |
|---|---|---|
| Genotype | Heterozygous | Aa |
| Pure breeding? | No | Offspring can be AA, Aa, or aa |
| Trait expression | Dominant allele shows in phenotype | Aa → shows dominant trait |
⚡ Quick Recap
Heterozygous ≠ pure breeding
Memory tip: “Different alleles → offspring may vary → not pure.”
Dominant Allele
📌 Definition
A dominant allele is an allele that is expressed in the phenotype even if only one copy is present in the genotype.
🌱 Key Points
- Represented by a capital letter (e.g., A).
- Homozygous dominant (AA) → trait expressed.
- Heterozygous (Aa) → trait still expressed (dominant masks recessive).
- Only a recessive allele can be hidden in the presence of a dominant allele.
- Determines the observable characteristics when present.
📊 Summary Table
| Feature | Description | Example |
|---|---|---|
| Symbol | Capital letter | A |
| Expression | Expressed if present | AA or Aa → shows dominant trait |
| Interaction with recessive | Masks recessive allele | Aa → shows dominant, a is hidden |
| Genotype types | Homozygous dominant / Heterozygous | AA / Aa |
⚡ Quick Recap
Dominant allele = always expressed if present
Memory tip: “Capital letter dominates → shows in the trait.”
Recessive Allele
📌 Definition
A recessive allele is an allele that is only expressed in the phenotype when no dominant allele is present in the genotype.
🌱 Key Points
- Represented by a lowercase letter (e.g., a).
- Homozygous recessive (aa) → trait is expressed.
- Heterozygous (Aa) → trait not expressed, masked by the dominant allele.
- Determines observable traits only in the absence of a dominant allele.
📊 Summary Table
| Feature | Description | Example |
|---|---|---|
| Symbol | Lowercase letter | a |
| Expression | Only expressed if no dominant allele | aa → shows recessive trait |
| Interaction with dominant | Hidden if dominant allele present | Aa → dominant trait expressed, a hidden |
| Genotype types | Homozygous recessive | aa |
⚡ Quick Recap
Recessive allele = expressed only without dominant
Memory tip: “Lowercase allele hides if capital allele is around.”
Pedigree Diagrams
📌 Definition
Pedigree diagrams are charts showing the inheritance of a characteristic through generations of a family.
🌱 Key Points
- Symbols Used
- Circle → female
- Square → male
- Shaded → individual shows the trait
- Unshaded → individual does not show the trait
- Half-shaded → sometimes used for carriers of a recessive trait
- Reading the Pedigree
- Look at which generations the trait appears in.
- Determine whether the trait is:
- Dominant → appears in every generation
- Recessive → may skip generations
- Patterns to Identify
- Autosomal → trait appears equally in males and females
- Sex-linked → trait often appears more in one sex (commonly males for X-linked traits)
- Uses
- Predict inheritance of traits
- Identify carriers of recessive alleles
- Determine mode of inheritance (dominant/recessive, autosomal/sex-linked)
📊 Quick Guide Table
| Feature | Interpretation |
|---|---|
| Shaded symbol | Individual expresses the trait |
| Unshaded symbol | Individual does not express the trait |
| Circle | Female |
| Square | Male |
| Carrier (half-shaded) | Heterozygous for recessive trait |
| Trait in every generation | Likely dominant |
| Trait skipping generations | Likely recessive |
⚡ Quick Recap
Pedigree = family tree of traits
Look for dominant vs recessive and autosomal vs sex-linked patterns
Memory tip: “Shaded = has trait, circle/square = female/male, skip = recessive, every generation = dominant.”
Monohybrid Crosses
📌 Definition
A monohybrid cross is a genetic cross involving one gene with two alleles. Used to predict offspring genotypes and phenotypes.
🌱 Key Points
- Alleles
- Dominant allele → expressed if present (e.g., A)
- Recessive allele → expressed only if no dominant allele present (e.g., a)
- Steps for a Genetic Diagram
- Identify parent genotypes.
- Determine gametes each parent can produce.
- Fill in a Punnett square.
- Read offspring genotypes and phenotypes.
- Calculate phenotypic ratio.
- Common Ratios
- 1 : 1 ratio → heterozygous × homozygous recessive (Aa × aa)
- 3 : 1 ratio → heterozygous × heterozygous (Aa × Aa)
📊 Example Table
| Cross | Genotypes of offspring | Phenotypes | Phenotypic ratio |
|---|---|---|---|
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 dominant : 1 recessive | 3 : 1 |
| Aa × aa | 1 Aa : 1 aa | 1 dominant : 1 recessive | 1 : 1 |
⚡ Quick Recap
Monohybrid cross → one gene, two alleles
1 : 1 ratio → heterozygous × homozygous recessive
3 : 1 ratio → heterozygous × heterozygous
Memory tip: “Punnett square tells you the offspring and ratio.”
Punnett Squares for Multiple Genotypes
📌 Definition
A Punnett square is a diagram used to predict the genotypes of offspring from a genetic cross. Useful when a cross can produce more than one genotype.
🌱 Key Steps
- Identify parent genotypes
Example: Aa × Aa - Determine possible gametes
Parent 1 (Aa) → A or a
Parent 2 (Aa) → A or a - Set up Punnett square
Write gametes of one parent along top, the other along side - Fill in offspring genotypes
Combine each gamete from one parent with each from the other - Determine ratios
Count the number of each genotype
Count the phenotypes if needed
📊 Example: Aa × Aa
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Genotypes of offspring: 1 AA : 2 Aa : 1 aa
Phenotypes (dominant : recessive): 3 : 1
⚡ Quick Recap
Punnett squares → show all possible genotypes
Memory tip: “Cross gametes in a square → count genotypes and phenotypes.”
Can handle multiple genotypes easily.