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.
Pedigree Practice
More Pedigree Practice
The pedigree to the right shows the passing
on of straight thumbs and Hitchhiker’s
Thumb in a family. Shaded shapes mean the
person has a straight thumb. Dominant or
Recessive?
1. What is the genotype of IV-1?
I
1
II
1
IV
3. What is the genotype of III-1?
3
2
1
III
2. What is the genotype IV-3?
2
1
2
2
4
3
4
3
4. What is the genotype III-2?
5. What is the genotype II-3?
6. Is it possible for individual IV-2 to be a
carrier?
Why? _______
The pedigree to the right shows the passing
on of colorblindness. Fill in the numbers for
each generation (generation IV is done for
you).
7. What do the half shaded circles mean?
____
___
8. What is the ONLY sex carriers of
colorblindness can be?
9. Which individuals are colorblind?
10. What is the genotype of person II-2?
IV
_______
1
2
3
4
5
6
7
8
11. What is the genotype of person I-1?
12. What is the genotype of person III-3?
13. If person IV-1 marries a female who is not colorblind and is not a carrier, what are the
chances of their male offspring being colorblind?
offspring?
What about their female