Back Home

2.8 perform an investigation to construct pedigrees or family trees, trace the inheritance of selected characteristics and discuss their current use

perform an investigation to construct pedigrees or family trees, trace the inheritance of selected characteristics and discuss their current use

Background

A pedigree is a family tree showing a line of descent. It can be used to trace the occurrence of inherited traits in parents and offspring through a number of generations.

By convention, circles represent females and squares, males. A line between a square and a circle represents a union and a line down indicates offspring from the union.Filled in symbols represent individuals displaying the phenotype being studied. For example:

pattern1

In pattern 1, the son and father are both affected. This is a reasonable indication that the characteristic is dominant. An affected offspring must have at least one affected parent if the phenotype is dominant. Other features of pedigrees of a dominant trait are:

  • Heterozygous individuals will be affected
  • Two affected parents can produce an unaffected child (both parents would be heterozygous)

pattern2

In pattern 2, the daughter is affected but neither parent is. This can only happen if the characteristic is recessive and the offspring are homozygous, e.g. bb. Both parent must be heterozygous, Bb. Other features of pedigrees of a recessive trait are:

  • Heterozygous parents will be unaffected
  • Two affected parents will always have an affected child.

Pedigrees are valuable tools in genetic counselling. It allows a pattern of inheritance to be traced throughout generations of a family. This can allow identification of the genetic disease and advice can be made available on the probability of a couple having an affected child. Cystic fibrosis is an example of a recessive genetic disease. Huntington’s chorea is an example of a dominant genetic disease.

Characteristic Dominant form Recessive form
ear lobes free attached
hair straight curly
dimples present absent
tooth gap no gap gap
mid digital hair hair present hair absent
widow’s peak present absent
eyelashes long short