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

The idea

A gene is a short section of DNA that codes for a characteristic. Some characteristics, such as fur colour in mice or red-green colour blindness in humans, are controlled by a single gene, so inheritance follows simple dominant and recessive patterns. Each gene can exist in different forms called alleles; for example, a mouse fur-colour gene may have a black allele and a brown allele. You represent alleles with letters: dominant as a capital, recessive as a lower case. A homozygous organism has two identical alleles, heterozygous has two different alleles. Red-green colour blindness is caused by a recessive allele on the X chromosome, so it is more common in males.

What earns the marks

• A gene is a section of DNA that codes for a protein and therefore controls a characteristic.
• Some characteristics are controlled by a single gene, giving simple inheritance patterns.
• Named examples include fur colour in mice and red-green colour blindness in humans.
• Alleles are different forms of the same gene, such as a black-fur allele and a brown-fur allele.
• Dominant alleles are represented by capital letters and recessive alleles by lower case letters.
• Homozygous means two identical alleles; heterozygous means two different alleles for that gene.
• Red-green colour blindness is a recessive X-linked condition, so males are more likely to be affected.

Common mistakes and how to put them right

• Saying alleles are different genes.

  Put it right: Alleles are different forms of the same gene at the same locus.

• Writing dominant alleles in lower case.

  Put it right: Use a capital letter for the dominant allele and the same lower case letter for the recessive allele.

• Claiming all characteristics are controlled by a single gene.

  Put it right: Many characteristics are polygenic or affected by the environment, and only some follow simple single-gene inheritance.

Exam tips

• Choose one letter per gene and keep capital and lower case versions consistent throughout a genetic cross.
• For colour blindness, remember it is carried on the X chromosome, so use X and Y notation when explaining why males are affected more often.
• Link each allele to its effect, for example B gives black fur and b gives brown fur, so phenotype predictions are clear.