Pedigree analysis
Many traits in humans are controlled by genes. Some of these traits are common features like eye
color, straight or curly hair, baldness, attached vs. free ear lobes, the ability to taste certain
substances, and even whether you have dry or sticky earwax! Other genes may actually cause
disease. Sickle cell anemia, muscular dystrophy, cystic fibrosis are each caused by a specific allele
of a human gene, and can therefore be inherited from one generation to the next.
Human geneticists illustrate the inheritance of a gene within a family by using a pedigree chart. On
such a chart, males are symbolized by a square (□) and females are symbolized by a circle (○).
People who are affected by a disease are symbolized by a dark circle or square.
The pedigree chart below shows inheritance of the gene that causes albinism. A and B represent a
couple who had five children, including C and E. Only one of the children, E, was albino. E and her
husband had five children, including G.
In the pedigree below write the genotypes of the individuals who are labeled with letters, using (A) to
represent the dominant allele and (a) to represent the recessive allele. Start by indicating the
genotypes of E and F. Then use a Punnett Square to figure out what the genotypes for C and D must
be. Next, determine the genotypes of A and B. Finally, determine the genotype of G.
A
C
B
D
E
G
F
Many other genes are inherited in the same manner as this recessive allele which causes albinism.
These include the genes for some genetic diseases, such as cystic fibrosis, Tay Sachs disease and
phenylketonuria.
Blackett Family Members
The Blackett Family DNA Activity is largely a genetic study of the inheritance of alleles in an extended family.
Bob Blackett has tested DNA samples from himself and 13 other relatives. The first task of a human geneticist
is the creation of a family tree, or pedigree to help with the interpretation of genotypes. From the following
relationships, construct a pedigree for the Bob and his relatives.
Person Family Relationship
Bob
Our propositus
Anne
Wife
David
Son
Katie
Daughter
Fred
Father
Norma Mother
Karen
Sister
Steve
Husband of Karen
Tiffany Daughter of Karen and Steve
Melissa Daughter of Karen and Steve
Amanda Daughter of Karen and Steve
Louise
Sister of Fred; Bob's Aunt
Bud
Husband of Louise
Buddy
Son of Bud and Louise
Dick
Son of Bud and Louise
Marilyn Daughter of Bud and Louise
Janet
Daughter of Bud and Louise
Alice and Bob have a two year old son, Charles, who is showing mental retardation, short stature,
and cryptorchidism. Alice has two living, unaffected, brothers but her eldest brother died at age 9
and a second brother died aged 10 months. Both had similar problems to Charles. Alice's father,
David, who was symptomless, has a sister, Ethel, who has an unaffected boy and girl, and a
brother, Fred, who also has two unaffected children. Alice's mother, Gertrude, has two living sisters
and had a brother who had died in childhood and who, she remembers, had been mentally retarded.
Bob has two brothers, Henry and Ignatius, who are still unmarried. His parents, John and Kate, had
tragic lives, both were adopted and never knew their biological parents and both died as the result
of a road accident.