Application: Analyzing Pedigrees
Today’s activity will examine the different inheritance patterns of human traits. These inheritance
patterns can be deduced from looking at the family history of that trait, displayed in a pedigree.
AUTOSOMAL DOMINANT CONDITION - ACHONDROPLASIA
Examine the pedigree shown below:
In pedigrees:
 males are depicted with a square
 females are depicted with a circle
 mating between two individuals is depicted by joining with a horizontal line
 offspring between two individuals is depicted with a vertical line
 shaded shapes indicate that the individual displays the phenotype in question
 unshaded shapes indicate that the individual does NOT display the phenotype
The above pedigree is for achondroplasia, a dominant trait. This trait is carried on an autosome
(a chromosome other than X or Y). Therefore, this trait is called autosomal dominant because
of the way it is inherited. Achondroplasia is a form of dwarfism in which the head and torso
develop normally, but the arms and legs are short. The homozygous dominant genotype for this
characteristic causes death of the embryo. About 1 in 25,000 people have achondroplasia.
Fill in the genotypes of the individuals in the above pedigree, using A for the dominant allele and
a for the recessive allele.
You will notice, for an autosomal dominant trait, in order for the children to have this trait, at least
one parent had to display this phenotype.
Question:
Two individuals with achondroplasia visit a genetic counselor. They want to have children, but
are concerned about passing on their condition to their children. Draw a Punnett square for this
couple and list the expected genotypes and phenotypes their children could have (give
probabilities).
AUTOSOMAL RECESSIVE CONDITION – CYSTIC FIBROSIS
Examine the pedigree shown below:
The above pedigree is for cystic fibrosis, a recessive trait. This trait is carried on an autosome (a
chromosome other than X or Y). Therefore, this trait is called autosomal recessive because of
the way it is inherited. In cystic fibrosis, there is an excessive amount of thick mucus that builds
up in t he lungs, pancreas and other organs. Untreated, this can kill a child by age 5. With
treatment, individuals can live to adulthood. Cystic fibrosis is the most lethal genetic disease in
the United States and is more prevalent in people of European ancestry.
Fill in the genotypes of the individuals in the above pedigree, using F for the dominant allele and f
for the recessive allele. If the specific genotype cannot be determined, list all possibilities.
You will notice, for an autosomal recessive trait, children displayed this trait even though their
parents did not.
Question:
In generation 1, the genotype of persons 1 & 4 is unknown. List the 2 possible genotypes. Which
genotype makes the actual observed phenotypes of the offspring (generation 2) more likely?
Draw both Punnett squares and give the probability.
Sex-linked recessive (X-linked) - HEMOPHILIA
Examine the pedigree below:
XX
XX
The above pedigree is for hemophilia, a sex-linked recessive trait (X-linked). This trait is carried
on a sex chromosome, the X chromosome. Therefore, this trait is called sex-linked (X-linked)
recessive because of the way it is inherited. Hemophiliacs bleed excessively when injured
because they have inherited an abnormal allele for a factor involved in blood clotting.
Fill in the genotypes of the remaining individuals in the above pedigree, using X for the dominant
allele and Xh for the recessive allele.
You will notice, for a sex-linked (X-linked) recessive trait:
 the disease is never passed from father to son (sons inherited a Y from their father)
 Males are much more likely to be affected than females. If affected males cannot
reproduce, only males will be affected (females would have to get a hemophilia gene
from BOTH parents to have the trait)
 All affected males in a family are related through their mothers.
 Trait or disease is typically passed from an affected grandfather, through his carrier
daughters, to half of his grandsons.
Questions:
1. In generation III in the above pedigree, the affected male (2) marries his cousin (5). What is
the probability that this couple will have a hemophiliac son? Show the Punnett square.
For the pedigrees below, determine whether the trait shown is inherited in an autosomal
dominant, autosomal recessive or X-linked recessive fashion. Then fill in the genotypes of
the individuals listed in the pedigree.