Lesson 9 — Pedigree Analysis


More than One Way to Construct Pedigrees

Pedigree charts can have various designs to show the same thing. Examine the two pedigree charts below that show the same information, but look at it from different directions.

Brown eyes are dominant to blue eyes. The pedigree chart on the left shows brown eyes as the autosomal dominant trait being expressed. The pedigree on the right shows blue eyes as the autosomal recessive trait being expressed. They are from the same family. They have the same individuals, but the symbols are shaded differently to reflect the trait being analyzed. They are both acceptable and demonstrate more than one way to write a pedigree. Keep this in mind when studying the examples.





Examples

  1. The pedigree below shows the inheritance of curved hitchhiker's thumb in a family. Determine the mode of inheritance that best accounts for the transmission of this trait. Write the genotypes of all family members.

    Solution

    The hitchhiker's thumb is an autosomal recessive condition. This can be deduced by looking at individual II-2 who has no affected parent. Therefore, individuals I-1 and I-2 must be heterozygous carriers for individual II-2 to receive two copies of the affected, recessive allele. Because individual II-1 does not have children, we cannot determine the genotype of this individual with certainty.

    Although individual II-3 married into the family without additional pedigree information, we can be certain that she is a heterozygous carrier because two of her children, III-3 and III-4, express the trait. II-3 must have passed the affected allele to them. Because individual II-2 is affected, he has two copies of the recessive allele and must pass the affected, recessive allele to all his children. Therefore, individuals III-1 and III-2 are heterozygous carriers.


     I-1  Hh
     I-2  Hh
     II-1  H_
     II-2  hh
     II-3  Hh
     III-1  Hh
     III-2  Hh
     III-3  hh
     III-4  hh

  2. The pedigree below shows the inheritance of tongue rolling in a family. Determine the mode of inheritance that best accounts for the transmission of this trait. Write the genotypes of all family members.

    Solution

    3. Tongue rolling is an autosomal dominant condition. This can be deduced by looking at the affected individuals who always have affected parents. We can determine that individual I-1 is heterozygous for the gene because she passed on the unaffected, recessive allele to her daughter, II-5. The same reasoning can be applied to individual II-2.

    Although individuals II-3 and II-4 do not have any children, they are heterozygous dominant individuals because their father, I-2, is an unaffected individual. I-2 must pass on one of the unaffected, recessive alleles and I-1 can pass the dominant or recessive allele to her children.

    All unaffected individuals are homozygous recessive.
     I-1   Rr
     I-2   rr
     II-1   rr
     II-2   Rr
     II-3   Rr
     II-4   Rr 
     II-5   rr
     II-6   rr
     III-1   Rr
     III-2   rr
     III-3   rr
     III-4   rr
     III-5   rr

  3. The pedigree below shows the inheritance of Duchenne muscular dystrophy. Determine the mode of inheritance that best accounts for the transmission of this trait. Write the genotypes of all family members.



    Solution

    Duchenne muscular dystrophy is an X-linked recessive disorder. This can be deduced because only males are affected. This means all unaffected males do not have the recessive X chromosome. Unaffected females can be homozygous dominant or heterozygous carriers. However, this pedigree indicates a heterozygous carrier of the disorder, III-2, with a dot inside the pedigree symbol.

    Although individuals I-1 and II-3 were not identified as carriers, we can deduce that they are actually carriers. Although they are not affected, they both had sons who had Duchenne muscular dystrophy disorder. They must be carriers to pass the affected X chromosomes to their sons.

    We cannot confirm the genotype of the individuals II-2 and III-4 because we do not have enough information in this pedigree.

      I-1   X DXd
     I-2   X DY
     II-1   XdY
     II-2   XD_
     II-3   X DXd
     II-4   X DY
     III-1   X dY
     III-2   X DXd
     III-3   X DY
     III-4   X D_



Practice Problems
Examine the following pedigree for the inheritance of cystic fibrosis, an autosomal recessive trait. Determine the genotype of each individuals represented in the pedigree.




The genotypes of the affected individuals must be noted first: aa.

Individual I1 and I2 must be heterozygous (Aa) to be unaffected but to have affected children.

Individuals II-5 and II-6 must be heterozygous (Aa) to be unaffected but to have an affected child.

The genotypes of the remaining unaffected individuals are unknown, but we know that they have at least one copy of the dominant allele. Therefore, their genotype is A_.