Genetic inheritance: gene

The idea

A gene is a short section of DNA that codes for a particular protein, or for a functional RNA. Genes are found at fixed positions called loci on chromosomes. The sequence of bases in a gene determines the sequence of amino acids in a protein, which in turn affects the protein's shape and function. Because humans have two copies of most chromosomes, they usually have two copies of each gene, one inherited from each parent. Different versions of the same gene are called alleles, and alleles may be dominant or recessive. A change in the DNA base sequence is a mutation, which can produce a new allele and may alter the protein produced. Understanding genes explains inherited characteristics and genetic disorders.

What earns the marks

• A gene is a section of DNA that codes for a protein or functional RNA.
• Genes are located at specific positions called loci on chromosomes.
• The base sequence of a gene determines the amino acid sequence and therefore the protein's structure and function.
• Different versions of the same gene are called alleles, and organisms usually inherit two alleles, one from each parent.
• A mutation is a change in the DNA base sequence that can create a new allele and may affect the protein produced.
• Genes control inherited characteristics, and some alleles are dominant while others are recessive.

Common mistakes and how to put them right

• Confusing a gene with a chromosome.

  Put it right: A gene is a section of DNA, while a chromosome is a structure that carries many genes.

• Saying that a gene codes for a characteristic directly.

  Put it right: A gene codes for a protein, and the protein contributes to the characteristic.

• Writing that alleles are different genes.

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

Exam tips

• Define a gene in terms of DNA, base sequence and protein product rather than as a vague 'unit of inheritance'.
• Use the terms allele, locus, dominant and recessive accurately when explaining inheritance.
• When describing a mutation, state that it is a change in the base sequence and link it to a possible change in the protein.