Some characteristics are controlled by a single gene, such as: fur colour…

The idea

While most phenotypic features result from multiple genes interacting, certain traits are determined by just one gene. This simple inheritance model means the specific combination of alleles—alternative versions of the same gene—directly dictates the observed trait. For instance, in mice, inheriting a dominant allele for fur colour ensures that trait is expressed, even if a recessive allele is present. Similarly, red-green colour blindness in humans operates on a single-gene basis, where specific alleles dictate normal vision or the condition. Understanding these crosses allows students to predict offspring phenotypes using genetic diagrams, showing how dominant and recessive alleles interact to produce distinct physical traits.

What earns the marks

• A single gene controls specific characteristics, meaning the trait is determined by the alleles inherited for that one gene.
• Alleles are different versions of the same gene that give rise to variations in a characteristic.
• Fur colour in mice is an example of a single-gene trait, where a dominant allele will be expressed over a recessive one.
• Red-green colour blindness in humans is another example of a characteristic controlled by a single gene.
• The phenotype of a single-gene trait can be predicted by identifying the combination of alleles (the genotype) present in the organism.

Common mistakes and how to put them right

• Assuming red-green colour blindness requires complex sex-linked Punnett squares at this level.

  Put it right by treating it simply as a single-gene trait, as X/Y chromosome tracking for this condition exceeds the specification.

• Believing that a dominant allele is always the most common one found in a population.

  Put it right by defining dominance strictly as an allele that is always expressed when present, regardless of its frequency.

• Confusing the terms 'gene' and 'allele'.

  Put it right: Clearly distinguishing that a gene is a section of DNA coding for a trait, whereas an allele is a specific version of that gene (e.g., the gene for fur colour has different alleles for black or brown fur).

Exam tips

• When asked for examples of single-gene inheritance, explicitly state 'fur colour in mice' or 'red-green colour blindness in humans' to secure marks.
• Use clear, distinct upper and lower case letters (e.g., 'B' and 'b') when drawing genetic diagrams to represent dominant and recessive alleles for a single-gene trait.