Gene

In the syllabus and the exam

A gene is defined as a section of DNA on a chromosome that carries the code for one particular characteristic, and this chapter builds directly on earlier chapters covering DNA and chromosome structure, so candidates are expected to place gene correctly within that hierarchy: DNA is the chemical molecule, a chromosome is a structure made of a very long DNA molecule coiled around proteins, and a gene is one specific segment of that DNA molecule occupying a fixed position, called a locus, on the chromosome. Exam questions commonly test whether candidates can distinguish a gene from an allele: a gene is the general unit coding for a characteristic (such as 'the gene for flower colour'), while an allele is one specific version of that gene (such as the allele for purple flowers or the allele for white flowers); describing 'purple' itself as a gene, rather than as an allele of the flower-colour gene, is a very common and heavily penalised error in monohybrid inheritance answers. Candidates should also know that, in a diploid organism, each gene is normally present as a pair of alleles, one inherited from each parent, positioned at the same locus on each of the two homologous chromosomes. A gene's exact DNA sequence ultimately determines the sequence of amino acids in a protein, which is the underlying mechanism by which a gene produces an observable characteristic. Candidates should further note that the same gene can exist in more than two allelic forms across a whole population, even though any single individual can carry at most two of those alleles at one time.

Example

The gene for seed shape in pea plants exists as two alleles, one for round seeds and one for wrinkled seeds, occupying the same location on a pair of homologous chromosomes.

Related terms

AlleleDeoxyribonucleic acid (DNA)Genotype

Source:SRC-DSKP-EN