Edexcel iGCSE Biology-4.19 Alleles- Study Notes- New Syllabus

Genes and Alleles – Inherited Characteristics

📝 Introduction

• Every living organism inherits characteristics from its parents through genes.
• A gene does not always exist in only one form. It can exist in different versions.
• These different versions of the same gene are called alleles.
• Different alleles can produce different forms of the same characteristic.
• The combination of alleles inherited from the parents determines many of an individual’s inherited characteristics.

🧬 What is a Gene?

• A gene is a section of DNA found on a chromosome.
• Each gene contains the instructions for making one specific protein.
• The protein produced by the gene helps determine a particular characteristic of an organism.

Key Points

• Genes are made of DNA.
• Genes are located on chromosomes.
• Each gene has a specific position (locus) on a chromosome.
• A gene usually controls one particular characteristic by coding for a specific protein.

Example

A gene may control:

• Eye colour
• Flower colour
• Blood group
• Seed shape
• Hair colour

What is an Allele?

• An allele is an alternative form of the same gene.
• Although alleles control the same characteristic, they may produce different versions of that characteristic.

Simple Definition: Alleles are different forms of the same gene that produce different inherited characteristics.

Why Does a Gene Have Different Alleles?

• A gene can exist in different forms because there are small differences in its DNA base sequence.
• These small differences may change the protein that is produced.
• As a result, different versions of a characteristic are seen.

Example

• A gene controls flower colour.
• Different alleles of this gene may produce:
  • Purple flowers
  • White flowers
• The characteristic remains flower colour, but the appearance is different.

How Are Alleles Inherited?

• Every body cell contains two copies of each chromosome.
• Therefore, every individual usually has two alleles for each gene.
• One allele is inherited from the mother.
• One allele is inherited from the father.
• These two alleles work together to determine the characteristic shown by the individual.

Example

• Eye colour gene
• One allele comes from the mother.
• One allele comes from the father.
• The combination of these alleles determines the person’s eye colour.

Alleles Control the Same Characteristic

• It is important to remember that:
• Alleles do not control different characteristics.
• They control the same characteristic but produce different forms of it.

Examples

How Alleles Produce Different Characteristics

• Genes work by producing proteins.
• Different alleles may produce:
  • Different forms of the same protein.
  • Different amounts of a protein.
  • A protein that works differently.
• Because proteins control body characteristics, different alleles can produce different inherited traits.

Simple Flow: Gene → Allele → Protein → Characteristic

Alleles Cause Variation

• Variation means differences between individuals of the same species.
• Different combinations of alleles cause people, animals and plants to have different inherited characteristics.

Examples of Variation

• Height
• Eye colour
• Hair colour
• Blood group
• Flower colour

Even brothers and sisters usually have different combinations of alleles, so they do not look exactly the same.

Importance of Alleles

• Alleles are important because they:
• Produce variation within a species.
• Explain why offspring are not identical to their parents.
• Increase genetic diversity.
• Help populations survive environmental changes.
• Provide the raw material for evolution through natural selection.

🔗 Relationship Between DNA, Genes and Alleles

• The genetic information is organised in levels.
• DNA makes up chromosomes.
• Chromosomes contain many genes.
• Each gene may exist in different forms called alleles.
• Alleles influence the proteins that are produced.
• Proteins determine inherited characteristics.

Simple Flow: DNA → Chromosome → Gene → Allele → Protein → Characteristic

📊 Gene vs Allele

📊 Examples of Characteristics and Alleles

🌟 Key Points to Remember

• A gene is a section of DNA that codes for a protein.
• A gene may exist in different forms called alleles.
• Alleles control the same characteristic but may produce different versions of that characteristic.
• Every individual usually inherits one allele from the mother and one allele from the father.
• Different combinations of alleles produce different inherited characteristics.
• Alleles are responsible for much of the variation seen within a species.

📌 Quick Recap
– Genes can exist in different forms = alleles.
– Allele combinations determine the phenotype.
– Dominant allele shows up if at least one copy is present.
– Recessive allele shows only if both copies are recessive.
– Homozygous = same alleles, Heterozygous = different alleles.
Mnemonic: “Alleles = Alternative, Dominant hides Recessive”