DNA and the genome: use in tracing human migration patterns from the past.

The idea

DNA carries variations that accumulate slowly over generations, so comparing DNA from different populations can reveal how humans moved around the world in the past. Mitochondrial DNA is inherited only from the mother, and the Y chromosome only from the father, so each traces a single line of ancestry. By comparing these sequences, scientists build family trees of populations and estimate when groups separated. For example, mitochondrial DNA evidence supports the idea that modern humans originated in Africa and then migrated to other continents. The more similar the DNA sequences, the more closely related the populations are and the more recently they shared a common ancestor. This evidence is combined with archaeology and linguistics to reconstruct migration routes.

What earns the marks

• State that differences in DNA sequences between populations can be compared to work out how closely related they are.
• Describe how mitochondrial DNA is inherited only from the mother and the Y chromosome only from the father, so each traces ancestry along one line.
• Explain that the more similar the DNA sequences, the more recently the populations shared a common ancestor, and the more different they are, the longer ago they separated.
• Apply this to a named example, such as evidence that modern humans originated in Africa and then migrated to other parts of the world.
• Recognise that DNA evidence is used alongside other evidence, such as archaeology and linguistics, to trace past human migration.

Common mistakes and how to put them right

• Thinking that mitochondrial DNA is inherited from both parents.

  Put it right: Mitochondrial DNA is inherited only from the mother, so it traces the maternal line.

• Assuming that DNA evidence alone proves a migration route.

  Put it right: DNA evidence is combined with other evidence such as fossils, tools and languages.

• Confusing similarity of DNA with similarity of appearance.

  Put it right: DNA sequence similarity shows relatedness, which may not match visible features.

Exam tips

• Use the phrase 'common ancestor' when explaining why two populations have similar DNA sequences.
• If a question gives data on DNA differences, compare the numbers directly and state which populations are most closely related and why.
• Link the method to a specific type of DNA, such as mitochondrial DNA or the Y chromosome, to show precise understanding.