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

Genes and Alleles – Inherited Characteristics

🌱 Introduction

A gene = a section of DNA that codes for a specific protein and controls a trait (e.g., eye color).
Alleles = different versions of the same gene causing variation in a trait.
Example: Gene for flower color → two alleles: R (red) and r (white).

📖 Key Points About Alleles 

• Homozygous → both alleles are the same (e.g., RR or rr).
• Heterozygous → alleles are different (e.g., Rr).
• Dominant allele → masks the effect of another allele (capital letter, e.g., R).
• Recessive allele → effect masked by dominant allele (lowercase, e.g., r).
• Alleles are at the same position (locus) on homologous chromosomes.

⚙️ How Alleles Affect Inherited Traits

• Combination of alleles determines the phenotype (observable trait).
• Examples:
  • Eye color: B (brown, dominant), b (blue, recessive)
    BB → brown eyes, Bb → brown eyes, bb → blue eyes
  • Seed shape in peas: R (round, dominant), r (wrinkled, recessive)
• Genotype = genetic makeup (allele combination, e.g., Bb)
• Phenotype = observable characteristic (e.g., brown eyes)

📊 Summary Table

📌 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”