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:
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:
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:
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 |