Understanding Pedigrees: Grade 9 Genetics for IGCSE Biology 3.24

Sometimes genetics problems are based around a pedigree diagram.  These diagrams show the phenotypes of individuals over several generations and allow deductions to be made about certain individuals phenotypes.  Often pedigrees are used to show the inheritance of a particular disease in a family.


You can see that circles in the pedigree represent females, squares represent males.  If the symbol is filled in, then the person suffers from the disease.  Empty symbols represent people who do not have the disease.

Have a look at the pedigree above?  What does this tell you about the disease?

Well the first and most obvious thing is that this disease is caused by a recessive allele, h.

If you see two people who don’t have the disease producing one or more children who do, then this must be a genetic disease caused by a recessive allele.  In the top generation, parents 1 and 2 do not have the disease, but they have three children 2,3,4 one of whom has the disease.

What does this tell us about the genotype of parents 1 and 2 in generation I?  Well if neither have the disease and they have a child who does, both 1 and 2 in the top generation must be heterozygous – Hh

Anyone with the disease must be homozygous recessive hh.

Have a look at generation II in the diagram above?

The man, number 2, who is a sufferer and so genotype hh marries woman 1 who does not have the disease.  They produce 4 children, three with the disease and one without.  What must the genotype of the woman 1 be?  Well she must be heterozygous Hh.  How do we know?  What children would she produce if she were a homozygous HH woman?


A pedigree caused by a dominant allele would look very different.  Every sufferer would have at least one parent who also suffers from the disease.  Two sufferers producing some children who do not have the disease is indicative of a disease caused by a dominant allele.  If we use the symbol P for the dominant allele that causes the disease, and p for the recessive allele that is “normal”, can you work out the genotypes of all 12 people on the diagram above?

  1. Pp
  2. Pp
  3. pp
  4. Pp
  5. PP or Pp
  6. PP or Pp
  7. pp
  8. pp
  9. Pp
  10. pp
  11. pp
  12. pp

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