Edexcel iGCSE Biology-4.20 Key Terms in Genetics- Study Notes- New Syllabus
Edexcel iGCSE Biology-4.20 Key Terms in Genetics- Study Notes- New syllabus
Edexcel iGCSE Biology-4.20 Key Terms in Genetics- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
4.20 understand the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, and genotype
Key Terms in Genetics
📝 Introduction
- When studying inheritance, scientists use certain genetic terms to explain how characteristics are passed from parents to offspring.
- Every individual inherits two alleles for each gene, one from the mother and one from the father.
- The combination of these alleles determines which characteristic is expressed.
- These terms are commonly used in genetics questions and genetic crosses.
Dominant
Definition
- A dominant allele is an allele that is always expressed in the phenotype whenever it is present.
- It masks or hides the effect of a recessive allele.
Characteristics of a Dominant Allele
- Represented by a capital letter.
- Only one dominant allele is needed for the characteristic to appear.
- It is expressed in both homozygous dominant and heterozygous individuals.
Example
Suppose:
- B = Brown eyes (Dominant)
- b = Blue eyes (Recessive)
| Genotype | Phenotype |
|---|---|
| BB | Brown eyes |
| Bb | Brown eyes |
In Bb, the dominant allele B masks the recessive allele b.
Recessive
Definition
- A recessive allele is an allele whose effect is hidden when a dominant allele is present.
- It is expressed only when both alleles are recessive.
Characteristics of a Recessive Allele
- Represented by a small letter.
- Needs two copies to be expressed.
- Hidden whenever a dominant allele is present.
Example
Using eye colour:
| Genotype | Phenotype |
|---|---|
| bb | Blue eyes |
Only individuals with bb show the recessive characteristic.
Homozygous
Definition
- Homozygous means an individual has two identical alleles for a particular gene.
- The two alleles may both be dominant or both be recessive.
Types of Homozygous
Homozygous Dominant: Both alleles are dominant.
Examples:
- BB
- RR
- TT
The dominant characteristic is expressed.
Homozygous Recessive: Both alleles are recessive.
Examples:
- bb
- rr
- tt
The recessive characteristic is expressed.
Important Points
- Both alleles are the same.
- Can be either dominant or recessive.
- The phenotype depends on whether the alleles are dominant or recessive.
Heterozygous
Definition
- Heterozygous means an individual has two different alleles for a particular gene.
- One allele is dominant and the other is recessive.
Examples
- Bb
- Rr
- Tt
Important Points
- Alleles are different.
- The dominant allele determines the phenotype.
- The recessive allele is present but not expressed.
Example
| Genotype | Phenotype |
|---|---|
| Bb | Brown eyes |
Although b is present, it is hidden by the dominant allele B.
Genotype
Definition
- The genotype is the genetic makeup of an organism.
- It describes the combination of alleles inherited for a particular gene.
Examples
- BB
- Bb
- bb
- RR
- rr
These represent different genotypes.
Important Points
- Refers to the alleles an organism possesses.
- Cannot always be identified just by looking at the organism.
- Different genotypes may produce the same phenotype.
Example:
- BB and Bb both produce brown eyes.
Phenotype
Definition
- The phenotype is the observable characteristic of an organism.
- It is the feature that can be seen or measured.
Examples
- Brown eyes
- Blue eyes
- Red flowers
- White flowers
- Round seeds
- Wrinkled seeds
Important Points
- Determined mainly by the genotype.
- Can sometimes be influenced by environmental factors.
- Individuals with different genotypes may have the same phenotype if a dominant allele is present.
🔄 Relationship Between Genotype and Phenotype
The genotype determines which phenotype is produced.
Example
Suppose:
- B = Brown eyes (Dominant)
- b = Blue eyes (Recessive)
| Genotype | Phenotype | Reason |
|---|---|---|
| BB | Brown eyes | Two dominant alleles |
| Bb | Brown eyes | Dominant allele masks recessive allele |
| bb | Blue eyes | No dominant allele present |
🌸 Example Using Flower Colour
Suppose:
- R = Red flowers (Dominant)
- r = White flowers (Recessive)
| Genotype | Type | Phenotype |
|---|---|---|
| RR | Homozygous dominant | Red flowers |
| Rr | Heterozygous | Red flowers |
| rr | Homozygous recessive | White flowers |
📊 Summary of All Genetic Terms
| Term | Meaning | Example |
|---|---|---|
| Dominant | Allele that is expressed whenever present | B |
| Recessive | Allele expressed only if both alleles are recessive | b |
| Homozygous | Two identical alleles | BB or bb |
| Heterozygous | Two different alleles | Bb |
| Genotype | Allele combination of an organism | BB, Bb, bb |
| Phenotype | Observable characteristic | Brown eyes |
📊 Homozygous vs Heterozygous
| Feature | Homozygous | Heterozygous |
|---|---|---|
| Alleles | Same | Different |
| Examples | BB, bb | Bb |
| Dominant allele present? | May or may not be | Yes |
| Phenotype | Depends on alleles | Dominant characteristic shown |
📊 Dominant vs Recessive
| Feature | Dominant | Recessive |
|---|---|---|
| Letter | Capital | Small |
| Expressed when | At least one dominant allele is present | Only when both alleles are recessive |
| Example | B | b |
🌟 Quick Recap
✔ Dominant alleles always appear if present.
✔ Recessive alleles appear only when both alleles are recessive.
✔ Homozygous means both alleles are the same.
✔ Heterozygous means the alleles are different.
✔ Genotype refers to the allele combination.
✔ Phenotype refers to the observable characteristic.
✔ One allele is inherited from each parent.
✔ The combination of alleles determines the phenotype.