Pedigree charts

The genotypes of individuals can be identified by using a pedigree chart. This is a diagram that displays the phenotypes of individuals in several generations of a family.

For example, there are two alleles of a gene that produces a taste receptor on your tongue for a chemical called PTC. One allele codes for the receptor for PTC, which tastes very bitter.

The other allele does not code for the receptor, so PTC on your tongue has no taste at all. The diagram shows a pedigree chart for PTC tasting.

Gran (GG) Grandpa (RR) Aunt (GR) Father (GR) Mum (GR) Granddaughter (GG) Grandson (GR) Granddaughter 2 (RR). G PTC no taste. R PTC bitter. GR PTC tastes bitter.

The allele allowing PTC to be tasted can be represented by capital T, showing that it is the dominant allele. The allele that does not produce a receptor for PTC can be represented by a lower case t, indicating that it is recessive.

We know that the genotype of the grandfather and the granddaughter, who cannot taste PTC, must be tt as people who cannot taste PTC can only have this genotype.

As the granddaughter therefore received two copies of the t allele, her parents must be heterozygous (Tt) as they can taste PTC (T) but also passed on t to their daughter.

The aunt must also be heterozygous as she can taste PTC (T) but must have received t from the grandfather. The grandmother and grandson, who can taste PTC, may be TT or Tt – we know they have T but do not know which other allele they received.

A pedigree chart could also be used to investigate the occurrence of an inherited condition such as cystic fibrosis. The allele that causes this condition is recessive, c. The dominant allele C produces an individual who is free from the condition.

The symbols P, F1 and F2 can be used to describe different generations when talking about genetic inheritance. P refers to the parent, F1 refers to first generation offspring and F2 refers to second generation offspring. In the above example, the grandparents would be described as P, the father (and aunt) as F1 and the grandchildren as F2.