Genes 3.1

Genes provide the instructions to build proteins and more besides. The human genome has been decoded by the human genome project and now biologists can search databases to find the location of specific genes. Our understanding of how gene mutations can lead to genetic diseases like sickle cell disease illustrates how genes control our features. The ability to sequence whole genomes allows us to compare the genes in diffrent species and learn more about how they operate.

Key words

Learn and test your biological vocabulary for 3.1 Genes using these flashards 

Clean sweep - quick revision through the whole topic

These slides summarise the essential understanding and skills in this topic. 
They contain short explanations in text and images - good revision for all students.

Read the slides and look up any words or details you find difficult to understand.

Skills and applications

Video explainer of how a gene mutation causes a disease is planned here.

Revision summary list for topic 3.1 Genes

  • Definition of a gene is, "a heritable factor that consists of a length of DNA and influences a specific characteristic."
  • A gene locus is, "the specific position of a gene on a chromosome."
  • Alleles are, "The various specific forms of a gene which differ from each other by one or only a few bases".
  • New alleles are formed by mutation.
  • A genome is, "the whole of the genetic information of an organism."
  • The entire base sequence of human genes was sequenced in the Human Genome Project.
  • The Genbank® database can be used to search for DNA base sequences.

Skills & applications

  • Application to explain: The causes of sickle cell anemia, including a base substitution mutation, subsequent change to the mRNA transcribed from it and a change to the sequence of amino acids in a polypeptide of hemoglobin.
  • Recall of one specific base substitution that causes glutamic acid to be substituted by valine as the sixth amino acid in the hemoglobin polypeptide is required. (Deletions, insertions and frame shift mutations are not needed.)
  • Application: Comparison of the number of genes in humans with other species.
    At least one plant and one bacterium should be included in the comparison and at least one species with more genes and one with fewer genes than a human.
    (note: "genome size" is total amount of DNA, not the number of genes in a species)
  • Skill: Use of a database to determine differences in the base sequence of a gene in two species.

Revision mindmaps

These diagram summaries cover the main details of topic 3.1 Genes.
Study them and draw your own list or concept map, from memory if you can.

Review questions

A quiz containing multiple choice questions covering the understanding and skills outlined in the image above:


A gene is a heritable factor. What is it made from?

A gene is a heritable factor.

It is made from DNA and controlls one specific characteristic.


Which of the statements best describes "Alleles"?

The different forms of a gene are called alleles.


What is thought to be the origin of most new alleles?

New alleles often arise by mutations. In the case of sickle cell disease it is a point substitution mutation.


The Human Genome Project sequenced the entire base sequence of human genes in 2003?

How is the information stored, and why is this very useful to biologists today?

The function of all the genes are still not known, but the base sequence of one complete human genome was sequenced. This sequence is so juge that it is too big to fit into a text book, but a computer database can hold all the data which biologists can search.


In the graph below the size of a genome is plotted against the number of known protein coding genes for a whole range of species found in a genome database.

Humans have approximately 109 base pairs in their genome.
Which of the following statements is supported by this data?

The comparison of the number of genes in humans with other species is one of the skills required.
At least one plant and one bacterium should be included in the comparison and at least one species with more genes and one with fewer genes than a human.
(note: "genome size" is total amount of DNA, not the number of genes in a species)


The diagram below shows part of the hemoglobin DNA strand and its corresponding polypeptide.
Below this there is a mutated DNA strand which causes sickle cell disease in some people.

Which of the following answers best describes this type of mutation.

The base A is replaced with the base T in this diagram, showing a base substitution.


The diagram below shows the change which differentiates the HbA allele from the HbS allele in sickle cell disease and in the amino acid sequence of the protein.

How does the DNA mutation change the protein?

The protein structure (1° structure) changes because one amino acid is now different.
In this case the 6th amino acid is changed from Glu (glutamic acid) to Val (valine)
This is caused by the mRNA transcribed from the DNA having a different codon.

Total Score:

Revision fun activity

Everyone needs a bit of fun while they revise. Try this Genes card match game

Can you reach the leaderboard?

The idea here is for something fun but still revision.