3.1 Essential ideas

3.1.1 Genes

Figure 3.1.1a – Relationship of chromosomes to genesFigure 3.1.1a – Relationship of chromosomes to genes

  • A gene is a heritable factor located at a specific position, or locus, on a chromosome.
  • A gene consists of a length of DNA. The sequence of bases in the DNA influences a specific characteristic.
  • Each gene codes for one polypeptide during translation (see 2.1.7).
  • Most characteristics are influenced by more than one gene at different loci.
  • The number of genes varies between species, but not between members of the same species. For example, all normal and healthy humans have the same number of genes.
  • The table below compares total amount of DNA, the number of chromosomes, and the number of genes in different organisms. 



Estimated size of genome (DNA base pairs)

Number of chromosomes

Estimated number of genes



(Homo sapiens)

3 billion


23 000

(Oryza sativa)

4.2 million


41 000



Yeast (eukaryote)
(Saccharomyces cerevisiae)

12 million



Bacterium (prokaryote)
(Escherichia coli)

4.6 million




  • There are usually two or more forms of a gene. The various forms of a gene are called alleles.
  • Alleles differ from each other by one or only a few base pairs.
  • Alleles that differ by only one base pair are more accurately named single-nucleotide polymorphisms, or SNPs (pronounced ‘snips’).

Figure 3.1.1b – Three SNP alleles resulting in three variations of a traitFigure 3.1.1b – Three SNP alleles resulting in three variations of a trait

  • These small differences result in variations in the amino acid sequence of the resulting polypeptide, and eventually to differences in the overall characteristic.

New alleles are formed by mutation

  • Mutations are random changes to the DNA sequence. They can be caused by errors during DNA replication, or by external agents, called mutagens.
  • Most mutations are either lethal or neutral, meaning they have no effect on the characteristic.
  • Some mutations form new alleles. In order for the mutation to be passed on to the next generation, it has to be present in the gametes.
  • If the mutation is beneficial, it will be maintained in a population by natural selection.
  • There are many different kinds of mutation; one important type is called a ‘base substitution’, in which one of the four bases – A, T, C or G – is substituted for another.

Figure 3.1.1c – Base substitution in hemoglobinFigure 3.1.1c – Base substitution in hemoglobin

  • A single base substitution (dark blue in Figure 3.1.1c) in the human hemoglobin gene causes glutamic acid to be substituted by valine as the sixth amino acid in the resulting polypeptide.

A genome includes extra-chromosomal DNA

  • In addition to the DNA in chromosomes, there is also DNA in mitochondria, as well as in the chloroplasts of plants.
  • The word ‘genome’ refers to the whole of the genetic information of an organism, including extra-nuclear DNA.
  • In the case of prokaryotes, which have no nucleus, the genome consists of the main chromosome ring (nucleoid) and any plasmids that are present.

The Human Genome Project

  • The original aims of the Human Genome Project were to sequence the entire human genome and to create maps of human chromosomes.

Figure 3.1.1d – Map of the human X chromosome showing loci (left) and associated genes (right)Figure 3.1.1d – Map of the human X chromosome showing loci (left) and associated genes (right)

  • The genomes of other species useful for medical research were also sequenced (e.g. mouse and fruitfly).
  • The project began in the 1990s and involved cooperation between public institutions around the world, including Japan, France, Germany, the UK, USA and China.
  • Due to improvements in gene sequencing technology, the original aims were reached earlier than expected.
  • The entire base sequence of human genes was sequenced in the Human Genome Project.

Figure 3.1.1e – DNAFigure 3.1.1e – DNA

Essential idea

Every living organism inherits a blueprint for life from its parents.

Key questions

  • Define genome, gene, allele and mutation.
  • Outline one example of a base pair substitution.
  • Discuss the relevance of the Human Genome Project to themes in the Nature of Science.

Figure 3.1.1f – UV lightFigure 3.1.1f – UV light
UV light is a mutagen that causes changes in the DNA sequence.

Concept help

  • SNPs are found in genes, but they are also found in non-coding sequences of DNA. For example, there are approximately 10 million SNPs in the human genome, but only 25 000 genes!
  • There are many loci on chromosomes that do not contain genes.

Figure 3.1.1g - DNA sequencing machineFigure 3.1.1g - DNA sequencing machine
DNA sequencing machine: developments in research follow improvement in technology.

Course links

Figure 3.1.1h – Rice in handFigure 3.1.1h – Rice in hand

Food for thought

Do more ‘complex’ organisms have larger genomes? More genes? Discuss your ideas with a classmate.

Further reading

Details about the Human Genome Project goals, accomplishments and methods can be found at the National Library of Medicine website: