A diagram that shows the occurrence of a particular phenotype in a family is called a pedigree chart. It indicates the pattern of inheritance of the alleles of the gene that code for the phenotype from one generation to the next. Exam questions using pedigree charts have proved tricky for students in the past with, on average, less than 20% of students being awarded full marks. In this Upgrade, we look at pedigree charts showing the inheritance of different phenotypes and indicate how you can use the information in these charts to answer exam questions.
Pedigree charts are usually used to show inheritance of human diseases. A pedigree chart represents a male by a square and a female by a circle. The symbols are either shaded (has the phenotype) or not (lacks the phenotype in question) — for example, whether an individual suffers from a genetic disease or not. Relationships in pedigree charts are shown by a series of lines (see Figure 1).
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