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? _____________________________________________________________________________________ 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. 1 _____________________________________________________________________________________ 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. _____________________________________________________________________________________ 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. 2 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 X Jerry’s youngest sister X Jerry’s father X Jerry’s grandfather X Jerry’s grandmother 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. 3 .