Download Biology 105: Laboratory 9 – Genetics Using a Test Cross to

Biology 105: Laboratory 9 – Genetics
Using a Test Cross to Determine Genotype
Suppose you work for a company that sells plant seeds. You are studying a plant species in which the
dominant phenotype is pink flowers (FF or Ff). The recessive phenotype is white flowers (ff). Customers
have been requesting more plants with pink flowers. To meet this demand, you need to determine the
genotypes of some of the plants you are currently working with.
Problem: What is the genotype of each plant?
Procedure:
 Suppose you are presented with Plant A of the species you are studying, which has pink flowers. You
want to determine the genotype of the plant.
 You cross Plant A with Plant B of the same species, which has white flowers and a known genotype of
ff.
 The resulting cross yields 4 plants with pink flowers and 4 plants with white flowers. Use Punnett
squares to determine the genotype of Plant A.
Questions:
1.) What is the genotype of Plant A? Explain how you arrived at your answer.
2.) What are the possible genotypes and phenotypes of offspring if Plant A is crossed with a plant that
has a genotype of FF?
3.) What ratio of dominant to recessive phenotypes would exist if Plant A were crossed with a plant that
has a genotype of Ff?
4.) Is Plant A the best plant, in terms of genotype, that you can work with to produce as many of the
requested seeds as possible? Why or why not? Which genotype would be best to work with?
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Autosomal Recessive and Dominant Disorders
Cystic fibrosis is an example of an autosomal recessive disorder, which means that the disease is only
expressed if an individual is homozygous for the recessive allele. ACHOO syndrome is an example of an
autosomal dominant disorder, which means the disorder is expressed if an individual is heterozygous or
homozygous for the dominant allele.
Questions:
1.) Mary and John are both carriers for the recessive cystic fibrosis allele (Ff), but do not express the
disease. If they have children, what are the possible genotypes and phenotypes of their children? What
is the probability that they will have a child with cystic fibrosis? Explain your answer using a Punnett
square.
2.) Beth has ACHOO syndrome, but her father did not. If Beth has children with a man who does not
have ACHOO syndrome, what are the possible genotypes and phenotypes of their children? What is the
probability that they will have a child who has ACHOO syndrome? Explain your answer using a Punnett
square.
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Incomplete Dominance
When alleles are incompletely dominant, neither allele is completely dominant or completely recessive.
A heterozygous individual has an intermediate phenotype. In this problem, you will explore incomplete
dominance by examining the human trait of hair texture.
Problem: What is the genotype of each family member?
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
Questions:
1.) If HT is the allele for straight hair and Ht is the allele for curly hair, then what are the genotypes of
each individual in Kathy’s family? Explain your answer using Punnett squares.
2.) If Kathy has children with a man with straight hair, what type of hair texture (straight, curly, wavy)
might their children have? Explain your answer using Punnett squares.
3.) If Kathy has children with a man with curly hair, what type of hair texture might their children have?
Explain your answer using Punnett squares.
4.) If Kathy has children with a man with wavy hair, what type of hair texture might their children have?
Explain your answer using Punnett squares.
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Codominance
Codominant alleles are both expressed in a person’s phenotype. A heterozygote will have the traits
associated with both alleles. In this problem, 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 (HbNHbN) have only normal hemoglobin. Homozygous sickle cell
individuals (HbnHbn) have only sickle cell hemoglobin. Heterozygous individuals (HbNHbn) 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 is present in red blood cells.
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Problem: How can you determine the genotypes of people in a family?
TABLE 1. FAMILY PHENOTYPES
Subject
Normal Hemoglobin
Jerry Smith
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
Sickle Cell Hemoglobin
X
X
X
Procedure:
 Use the background information and the genetic test results to answer questions 1-4.
 Use the background information and a Punnett square to help you answer question 5.
Questions:
1.) Are any of Jerry’s siblings homozygous for the sickle cell trait? Are any of Jerry’s siblings heterozygous
for sickle cell disease?
2.) What genotype is Jerry’s father?
3.) What genotypes are Jerry’s grandparents?
4.) What is the genotype of Jerry’s mother? Explain.
5.) If Jerry marries a woman who is heterozygous for the sickle cell trait and they have children, 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 squares with all possible genotypes for their children.
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