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Name _____________________________ Period ___________________ Date __________________
CHAPTER INVESTIGATION
Codominance
Codominant alleles are both expressed in a person's phenotype. A heterozygote
will have the traits associated with both alleles. In this lab, you will explore
codominance by analyzing the results of tests for sickle cell disease within a
family.
BACKGROUND
Sickle cell disease is caused by a change in the gene for hemoglobin, which is the
oxygen-carrying protein in red blood cells. Individuals who are homozygous for
the sickle cell trait often cannot endure exercise. Individuals who are heterozygous
for the trait can have sickle cell attacks under extreme conditions. Normal
individuals (HbS HbS) have only normal hemoglobin. Homozygous sickle cell
individuals (Hbs Hbs) have only sickle cell hemoglobin. Heterozygous individuals
(HbS Hbs) have both normal hemoglobin and sickle cell hemoglobin.
Jerry Smith collapsed while running a race for his track team. A doctor said that
he had a sickle cell attack. Genetic tests were run on several family members. The
test results are shown below. An X indicates that form of hemoglobin in red blood
cells.
PROBLEM
How can you determine the genotypes of people in a family?
TABLE 1. FAMILY PHENOTYPES
Subject
Normal Hemoglobin
Sickle Cell Hemoglobin
Jerry Smith
X
X
Jerry's brother
X
Jerry's younger sister
X
Jerry's youngest sister
X
Jerry's father
X
Jerry's grandfather
X
Jerry's grandmother
X
X
X
MATERIALS
paper
pencil
© Houghton Mifflin Harcourt Publishing Company
Holt McDougal Biology
Chapter Investigation
1
Extending Mendelian Genetics
Section 2: Complex Patterns of Inheritance
Name _____________________________ Period ___________________ Date __________________
Codominance continued
PROCESS SKILLS
Inferring
Predicting
PROCEDURE
1. Use the background information and the genetic test results to answer
questions 1–4.
2. Use the background information and a Punnett square to help you answer
question 5.
ANALYZE AND CONCLUDE
1. Analyze Are any of Jerry's sibling homozygous for the sickle cell trait? Are
any of Jerry's siblings heterozygous for sickle cell disease?
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2. Analyze What genotype is Jerry's father?
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3. Analyze What genotypes are Jerry's grandparents?
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4. Infer What is the genotype of Jerry's mother? Explain.
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© Houghton Mifflin Harcourt Publishing Company
Holt McDougal Biology
Chapter Investigation
2
Extending Mendelian Genetics
Section 2: Complex Patterns of Inheritance
Name _____________________________ Period ___________________ Date __________________
Codominance continued
5. Predict If Jerry marries a female who is heterozygous for the sickle cell trait,
what would be the possible genotypes and phenotypes of their children? Use a
Punnett square to find your answer. Fill in the top boxes with Jerry's genotype;
fill in the side boxes with Jerry's wife's genotype. Fill in the square with all
possible genotypes for their children.
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© Houghton Mifflin Harcourt Publishing Company
Holt McDougal Biology
Chapter Investigation
3
Extending Mendelian Genetics
Section 2: Complex Patterns of Inheritance
Name _____________________________ Period ___________________ Date __________________
OPTIONS FOR INQUIRY
Incomplete Dominance
When alleles are incompletely dominant, neither allele is completely dominant nor
completely recessive. A heterozygous individual has an intermediate phenotype.
In this lab, you will explore incomplete dominance by examining the human trait
of hair texture.
PROBLEM
What is the genotype of each family member?
MATERIALS
paper
pencil
PROCESS SKILLS
Predicting
PROCEDURE
Use the information below to answer the questions that follow.
TABLE 1. FAMILY PHENOTYPES
Individual
Hair Texture
Kathy's father
straight
Kathy's mother
curly
Kathy
wavy
Kathy's brother
wavy
© Houghton Mifflin Harcourt Publishing Company
Holt McDougal Biology
Chapter Investigation
4
Extending Mendelian Genetics
Section 2: Complex Patterns of Inheritance
Name _____________________________ Period ___________________ Date __________________
Incomplete Dominance continued
ANALYZE AND CONCLUDE
1. Conclude If IS is the allele for straight hair and IC is the allele for curly hair,
then what are the genotypes of each individual in Kathy's family? Explain your
answer using Punnett squares.
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2. Predict If Kathy married a man with straight hair and they had children, what
type of hair texture (straight, curly, wavy) might their children have? Explain
your answer using Punnett squares.
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3. Predict If Kathy married a man with curly hair and they had children, what
type of hair texture might their children have? Explain your answer using
Punnett squares.
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4. Infer If Kathy married a man with wavy hair and they had children, what type
of hair texture might their children have? Explain your answer using Punnett
squares.
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© Houghton Mifflin Harcourt Publishing Company
Holt McDougal Biology
Chapter Investigation
5
Extending Mendelian Genetics
Section 2: Complex Patterns of Inheritance