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PEDIGREE PRACTICE
Essential Question: How can pedigrees be used to study the inheritance of human traits?
Studying inheritance in humans is more difficult than studying inheritance in fruit flies or pea plants. For
obvious reasons, geneticists studying humans cannot set up breeding experiments to study the resulting
offspring! Clearly, other approaches must be used when studying human genetics. Family trees (also called
"pedigrees") provide a useful way to gather evidence about inherited traits in humans.
One approach to studying genes in humans is to construct a pedigree from data on a single trait within a
family. These data can then be used for genetic analyses. Researchers use pedigrees to look for inheritance
patterns as traits are passed from one generation to the next. These patterns can provide clues to the way the
trait is passed from parents to their offspring.
The genotypes of most of the individuals in a pedigree can be determined by examining the pattern in which
the trait is inherited from one generation to the next. By applying Mendel's principles, you can figure out
whether the trait being examined is dominant or recessive. For some individuals in a pedigree, however, there
may not be enough information to determine the full genotype.
In humans, albinism is an autosomal recessive trait. The disorder causes a lack of pigment in the skin and hair, making an
albino appear very pale with white hair and pale blue eyes. This disorder also occurs in animals, a common albino found
in a laboratory is the white rat. The pedigrees below trace the inheritance of the allele that causes albinism.
11. Oscar Grouch and Vanessa Grouch had five children in this order. Calvin, the oldest, married Stephanie. Margaret,
the youngest, married Eric. Margaret and Eric had three children: Nancy, Ingrid, & Bob. Nancy married Larry, Ingrid
married Paul. Calvin and Stephanie also had three children (Douglas and Thomas). Douglas married Xantha, and Thomas
married Claire.
Oscar and Vanessa had 10 great-grandchildren (3 great-granddaughters and 7 great-grandsons). Larry and Nancy had
three children, Mark, Wayne, and Kim. Kim has two cousins, Henry and Greg. Kim has an aunt Ingrid, and an uncle Frank.
Douglas and Xantha had a son and a daughter. Thomas and Claire had three children (John, Ben, and Jennifer). Jennifer’s
cousins are named Zachary and Yianna. John has a brother-in-law, Ulysses. John also has an uncle named Richard.
Richard has two unmarried aunts named Anne and Diane and an unmarried uncle named Fred.
Create a pedigree showing the relationships between all the members of the Grouch family.
12. The Grouch family has several members who are color-blind. This is a recessive sex-linked trait. Oscar is color-blind,
and his wife Vanessa is a carrier. Their son Calvin is color-blind, their great-grandson Zach is color-blind, and their other
great grandchildren Wayne & Henry are also color-blind. Shade in the boxes of those who are color-blind, and work
backwards to determine who must be carriers. Identify carriers with a dot.
13. The great-great maternal grandmother of a boy was a carrier for color blindness, an X-linked disorder. His great uncle
on his mother's side was color blind but his great uncle's father was unaffected. The boy's mother has 2 brothers (1 color
blind, 1 unaffected) and 1 sister (unaffected). The boy's grandmother on his mother's side had 1 brother who was color
blind and 3 sisters. Two of these sisters were unaffected and one was a carrier. The boy's great grandmother on his
mother's side had 4 sisters. The boy has one unaffected sister and he is color blind.
What is the probability of the boy's sons being color blind if he marries a non-carrier?
14. A man and woman marry. They have five children: first 2 girls, then 3 boys. The mother is a carrier of hemophilia, an
X-linked disorder. She passes the gene on to her two youngest boys who died in childhood. The second daughter is also
a carrier. Both daughters marry men without hemophilia and each have 3 children (2 boys and then a girl). The carrier
daughter’s oldest son has hemophilia. The non-carrier daughter's oldest son marries a woman who is a carrier, and they
have twin daughters.
What is the percent chance that each daughter will also be a carrier?
15. The pedigree below traces the inheritance of alkaptonuria, a biochemical disorder. Affected individuals, indicated
here by the filled-in circles and squares, are unable to break down a substance called alkapton, which colors the urine
and stains body tissues.
Does alkaptonuria appear to be caused by a dominant or recessive allele? ____________________________
a. Fill in the genotypes of the following individuals (use A and a):
George ______ Arlene______ Carla ______ Michael ______
Tom ______
b. Fill in the possible genotypes for Christopher: _______ or ______
Another inheritance pattern that occurs in humans involves recessive alleles that are sex-linked. As you learned earlier,
sex-linked alleles are those located on one sex chromosome but not on the other. Remember that, in humans, females
are XX and males are XY. Most human sex-linked alleles are located on the X chromosome. Since the Y chromosome is
always inherited from the father, males only receive X-linked alleles from their mothers. A male therefore needs only
one copy of a sex-linked recessive allele to exhibit the recessive trait. In contrast, a female must inherit two such
recessive alleles—one from each parent—to exhibit the trait.
recessive