How to Solve Problems Involving Pedigrees

1. Since the pedigrees presented are simple pedigrees, the following assumptions can be made:
   1. No new mutations occur in the families. This means that a dominant trait must occur in a parent if it appears in a child. This also means that if a recessive trait appears in a child but does not occur in either parent, both parents must be heterozygous carriers for the trait.
   2. All traits are 100% penetrant. This means that if the genotype for the trait occurs in an individual, the trail will appear.

    

2. If neither parent is affected,
   1. the trait cannot be dominant.
   2. the trait could be recessive and either parent or both could be heterozygous carriers.

    

3. If one parent is affected,
   1. the trait could be dominant and the affected parent could be heterozygous while the unaffected parent is not a carrier
   2. the trait could be recessive and the affected parent is homozygous while the unaffected parent could be a heterozygous carrier

    

4. If both parents are affected
   1. the trait could be dominant and both parents could be heterozygous carriers which means that some of the children could be unaffected
   2. the trait could be recessive meaning that both parents would have to be homozygous and all the children would have to be affected

    

5. Consideration of X-linked or Y-linked traits:
   1. X-linked recessive
      1. affected male children must have, at least, an heterozygous mother
      2. an affected female will transmit the trait to all male children
   2. X-linked dominant
      1. affected males transmit the trait to all daughters
      2. affected sons must have an affected mother
   3. Y-linkage
      1. male to male transmission only
      2. no affected females
6. Mitochondria associated traits
   1. affected females transmit the trait to all children
   2. affected males do not transmit the trait to any children