Bio2 Activity
Probabilities, Pedigrees and Recombination
1. Match the Terms with their Definitions and Examples
___heterozygous
a. Blue-eyed blonde mates with brown-eyed brunette
___homozygous
b. BB or bb
___monohybrid cross
c. not on sex chromosomes
___autosomal
d. blue or brown eyes
___genotype
e. Bb
___phenotype
f. locus on a chromosome that codes for a given polypeptide*
___gene
g. Blonde mates with brunette
___allele
h. BB, Bb, or bb
___dihybrid cross
i. Males have only one for each gene on the
X chromosome
2. How many different kinds of gametes can individuals with each of the following
genotypes produce? List all possibilities next to the genotypes.
a. AA
b. aa
c. Aa
d. AaBB
e. AaBb
f. AaBbCC
Can you figure out an equation for this type of problem?
g. AaBbCc
h. AaBbCcDdEeFf
3. What is the probability that two individuals of the genotype AaBb and aaBb will have any aabb
offspring?
4. An organism that has the genotype AaBbCc is crossed with an organism that has the
genotype AABbCc. Assume all genes are on separate sets of chromosomes (that is,
they are not linked).
What is the probability that any of the offspring will have the genotype
AABBCC? (Hint: To get the answer, consider the six possible types of autosomal
crosses. Determine the individual probabilities of getting AA offspring from the
monohybrid cross. Then do the same to determine the probabilities of getting BB
offspring and CC offspring. Multiply these probabilities together.)
5. A heterozygous, brown-eyed human female who is a carrier of color blindness
marries a blue-eyed male who is not color-blind. Color blindness is a sex-linked
trait. Assume that eye color is an autosomal trait and that brown is dominant over
blue. What is the probability that any of the offspring produced have the following
traits?
a. Brown eyes
b. Blue eyes
c. Color blindness
d. Color-blind males (out of all offspring)
e. Brown-eyed, color-blind males (out of all offspring)
f. Blue-eyed, color-blind females
g. What is the probability that any of the males will be color-blind?
h. Why do males show sex-linked traits more often than females?
6. You are studying a new form of Hemophilia and are curious about the mode of inheritance.
You begin taking extensive history of a family in which this disorder was recently identified. The
propositus in this family is a 40 year old man named Mike with three children (one boy and two
girls) and none of them have the disorder. Mike’s second daughter just had a baby boy.
Determine the chances that this child has this disorder once you determine the mode of
inheritance from the history described below.
On Mike’s father’s side of the family there is no history of the disorder in his father, his
grandparents nor his two uncles.
On Mike’s mother’s side, his mother does not have the disorder nor do her two sisters and one
brother. However, her father has the disorder and her father’s grandfather had the disorder. In
addition, Mike tells you that his grandfather had two brothers and neither of them had the
disorder.
From the information above, diagram a pedigree and determine the mode of inheritance.
Then determine the chance that Mike’s new grandson has the disorder.
7. You are working as a genetic counselor and a patient comes in to ask about the chances that
she might be a carrier for a genetic disorder. The disorder is called Hereditary Geniospasm and it
causes tremor of the lower lip and chin when a person is under stress or is concentrating. You
begin to interview her and take a family history in order to determine the mode by which this
disease might be inherited.
She begins by telling you about her mother’s side of the family. The patient says she had a great
grandmother who suffered from this disorder. Her great grandparents then had four children (two
boys and two girls) and one of the boys had the disorder and this boy was her grandfather. Her
grandfather then married and had three children, two boys and a girl (this girl is the patient’s
mother), and of these three children one of the boys had the disorder. The patient has three
siblings (two sisters and a brother) and both sisters have the disorder.
On the patient’s father’s side of the family, all she can tell you is that her father had the disorder
her grandmother and great grandmother also had the disorder. Her father had two sisters that did
not have the disorder.
Diagram the pedigree, determine the mode of inheritance and assign genotypes for everyone in
the pedigree that can be determined using D for the dominant allele and d for the recessive allele
for this disorder. Is your patient a carrier for this disorder? How do you know?
8. The urofacial syndrome has two symptoms -- a smile that appears to be a frown, and
blockage of the urinary system, which can be fatal if untreated. Based on the pedigree
below, how does this disorder appear to be inherited? On this pedigree, assign genotypes
to everyone that can be determined?
I
1
2
3
4
II
1
2
3
4
5
III
1
2
3
4
5
6
7
IV
1
2
3
4
5
9. This pedigree represents a family with congenital contractural arachnodactyly, resulting
from a defect in the connective tissue protein fibrillin-2 that produces underdeveloped
and stiff muscles, outer ear deformities, stiff and long fingers, and sometimes heart
abnormalities. What is the most likely mode of inheritance? Why is no one affected in the
first generation?
I
1
2
II
1
2
3
4
5
III
1
2
3
4
5
6
7
8
9
10
11
12
IV
1
2
3
Recombination:
10.
What is do we mean by the term “linked genes”?
11. If two genes are unlinked, what is the probability that they will be inherited
together?
12. Determine the sequence of genes along a chromosome based on the following
recombination frequencies: A - B, 8 map units; A - C, 28 map units; A - D, 25 map
units; B - C, 20 map units; B - D, 33 map units.
13. At what stage of the cell cycle does recombination occur?