Explain why there are differences in the inherited characteristics as a…

The idea

Alleles are different versions of the same gene. An organism inherits two alleles for each gene, one from each parent, and the combination can differ between individuals. Differences in inherited characteristics arise because offspring receive different allele combinations: a dominant allele is expressed even when only one copy is present, while a recessive allele is only expressed when two copies are present. Some alleles are codominant, so both are expressed and a combined phenotype appears. Meiosis and random fertilisation shuffle alleles, so siblings differ. For the MCQ, connect the cause (different allele combinations, dominance relationships) to the effect (variation in inherited characteristics), and reject options that confuse alleles with environmental effects or with the gene itself.

What earns the marks

• Alleles are different versions of the same gene, and individuals inherit two alleles for each gene, one from each parent.
• Different combinations of alleles produce different inherited characteristics, so offspring vary.
• A dominant allele is expressed even if only one copy is present, masking a recessive allele.
• A recessive allele is only expressed when two copies are present (homozygous recessive).
• Codominant alleles are both expressed, producing a combined phenotype.
• Meiosis and random fertilisation produce new allele combinations, contributing to variation between offspring.

Common mistakes and how to put them right

• Confusing an allele with a gene.

  Put it right by stating a gene is a section of DNA and alleles are its different versions.

• Thinking a recessive allele is 'weaker' or lost.

  Put it right by explaining it is only expressed when two copies are present and can reappear.

• Assuming all variation is genetic.

  Put it right by noting the question asks about inherited differences caused by alleles, not environmental effects.

Exam tips

• Identify the dominance relationship (dominant, recessive or codominant) before choosing an option.
• Check that the option explains the cause (allele combination) and links it to the inherited characteristic.
• Eliminate options that describe environmental variation or mutation when the question is about allele combinations.