Sheryl A. DeCaprio
Pedigree charts are a pictorial description of the expression of a certain genetic trait in a family. The reading and construction of pedigree charts will reinforce students skills in gathering and analyzing data as well as writing information in alternate forms. There are a few basic symbols and guidelines to follow when creating a pedigree chart for analysis. The symbols are as follows:
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. . . . . . . . . . . . . . . female
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. . . . . . . . . . . . . . . male
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. . . . . . . . . . . . . . . shaded symbol indicates that this individual expresses the trait being charted.
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. . . . . . .indicates a marriage
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. . . . . . . . . .indicates the three females are offspring of this pairing. The first female child is the oldest (left most ) and the order of birth moves from left to right.
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. . . . . . . . . .indicates identical twins
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. . . . . . indicates fraternal twins
The location of an individual is described by the tier of the tree on which he or she resides and the numbered placement from left to right. See Figure IV.
Figure IV
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Charted above is the pedigree chart of three generations of a family. Tier I represents the grandparents of tier III. Relationships can be found between the dominant trait and the number of individuals expressing this trait. In addition to finding ratios and percentages of the appearance of this trait, the genotype of each member can be deduced by studying the individuals and assuming the genotype of the grandparents.
Pedigree charts for recessive traits may also be examined. Sickle cell anemia disease is a recessive trait. An individual may carry the trait without ever expressing the disease ( a genotype of Ss, heterozygous carrier, a genotype of ss, homozygous diseased, a genotype of SS, homozygous, unaffected). Students can study a pedigree chart and determine which individuals may be carriers of the disease, which are affected, and which are unaffected. See Figure V.
Figure V
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Students can deduce, using their knowledge of the behavior of recessive traits, the genotype of the parents of the affected individual. Both must be sickle cell carriers i.e. heterozygous, in order for a diseased child to be produced. Students may speculate on whether individuals II1 and II2 are heterozygous or homozygous. Discussions on the practicality of having children when two people are known carriers of a disease will lead to some very interesting discussions on the responsibility of having children especially when a risk of disease is apparent. Students should be encouraged to think out all possible solutions and restrictions before making a decision about the analysis of a pedigree chart. (see sample lesson)
I have developed some lessons in which the information presented here is combined with the reinforcement of mathematics skills. Students will find the presentation of this material informative. They will be able to examine their own families and gather data about gene transmission, and practice analyzing the material they’ve gathered. It is anticipated that students will become scientifically curious, become open to the information presented and improve their mathematics skills. Most importantly, students will become aware of the role of mathematics in scientific study, completing a circle of learning for them and developing a base of knowledge on which they may nurture their learning curiosity.