Genes and Alleles

What a gene is

A gene is a specific segment of DNA located at a fixed position, called a locus, on a chromosome. Each gene carries the genetic instructions for a particular characteristic, such as flower colour in a plant or blood group in a human, by coding for a specific protein.

A human is estimated to have tens of thousands of genes spread across all of their chromosomes, each responsible for one characteristic or helping to control one bodily process.

What an allele is

An allele is one of two or more alternative forms of the same gene. Different alleles of a gene arise from small differences in their DNA sequence, and these differences lead to different versions of a characteristic, such as the dominant allele for tallness and the recessive allele for shortness in a pea plant.

An individual normally inherits one allele from each parent for every gene, and the combination of these two alleles determines which characteristic is expressed.

From DNA to chromosome

1. DNA, the molecule that stores genetic information as a sequence of bases.
2. Gene, a specific segment of DNA that codes for one characteristic.
3. Locus, the fixed position of a gene on a chromosome.
4. Chromosome, a structure made of tightly coiled DNA carrying a large number of genes.
5. Homologous chromosomes, a matching pair of chromosomes, one from each parent, each carrying an allele for the same gene at the same locus.

How this is examined

SPM questions commonly ask you to define gene and allele and explain the difference between them, to identify the relationship between DNA, gene, and chromosome from a diagram, or to explain why an organism has two alleles for each gene, one on each chromosome of a homologous pair.

Worked exam-style question

Question. In a hypothetical plant, flower colour is controlled by a single gene with two alleles: purple (dominant) and white (recessive). A plant grown from seed produces purple flowers, but its parent plant had white flowers.

(a) State what is meant by a gene, and by an allele. (b) State the genotype (using letters of your choice) that the purple-flowered plant most likely has, and explain your reasoning.

(c) Explain why this plant carries two alleles for flower colour rather than just one. (d) Name the structure on which the flower-colour gene is located, and state its fixed position on that structure.

Model answer. (a) A gene is a segment of DNA that codes for a particular characteristic; an allele is one of the different forms that gene can take. (b) Using P for the dominant purple allele and p for the recessive white allele, the plant is most likely heterozygous (Pp), because it shows the dominant purple characteristic yet must carry a recessive white allele inherited from its white-flowered parent.

(c) The plant carries two alleles because it received one chromosome from each parent in a homologous pair, and each of the two chromosomes carries one allele for the flower-colour gene at the same locus. (d) The gene is located on a chromosome, at a fixed position called the locus.

Practice question

Try this. A student states: "A gene and an allele are the same thing, because both are found on a chromosome." Explain why this statement is inaccurate, using the correct relationship between the two terms.

Exam tip

Key terms

These Chapter 26 terms build directly on this standard:

• DNA, the molecule that stores genetic information as a sequence of bases.
• Chromosome, a structure of tightly coiled DNA that carries a large number of genes.
• Dominant allele, an allele that is expressed in the phenotype even when only one copy is present.
• Recessive allele, an allele that is expressed only when two copies are present.
• Genotype, the combination of alleles an organism carries for a gene.

Source:SRC-DSKP-EN

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