Aims of the episode:

  • Describe the structure of DNA;
  • State what the process of DNA fingerprinting is used for;
  • Explain how DNA controls your characteristics;
  • Identify the uses of Punnett Squares.

What is DNA?

DNA is contained within the Nucleus of a cell and it makes up your Chromosomes. Its shape is described as a Double Helix – click here to see a video about the discovery of the Double Helix.

Each long strand of DNA can be broken down into different sections, called Genes. Each Gene is further made up of bases – scientists are still discovering what each Gene is for. There are Genes to control everything from your hair colour to whether you are susceptible to certain diseases or not.

For every Gene there are different forms of the Gene (Alleles) – for example, options for the hair colour Gene are the blonde hair Allele, brown hair Allele etc.

DNA Fingerprinting

DNA is pretty unique (but not as unique as your actual fingerprints). This means it can be used to identify someone who may have been at a crime scene. During the process of DNA fingerprinting the DNA strand is broken down into different sections – each section will move through a gel at different speeds, leaving a trail that looks like a bar code. Bar codes can be compared to find a match.

In some cases the bar codes may be similar, but not identical, which may suggest a link to a relative; you will share some aspects of your DNA with your blood relatives.


Some Alleles are considered dominant, whilst some are recessive. If an Allele is dominant, you will see its effect even if that Allele has only come from one parent. An example would be brown eyes, if one of your parents passes the brown eye Allele to you then you will have brown eyes.

Recessive Alleles need to be passed on from both parents in order for you to see the effect. The Allele for blue eyes is recessive, meaning both you mother and father need to pass on the blue eye Allele for you to have blue eyes.

Punnett Squares are a useful tool to predict the likelihood of an offspring having certain characteristics. An example of a Punnett Square is shown below. Click here to see a video explaining how to figure out the likelihood of certain characteristics being passed on.

S@S 014: DNA and Inheritance
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