Objectives:
Apply genetic principles to predict the outcome of genetic crosses and statistically analyze the results. (SLO 4)
Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems
Develop a Punnett square to calculate the expected proportions of genotypes and phenotypes in a single trait cross cross
Construct charts and/or diagrams to illustrate biological data (SLO 14c)
Quantify and predict outcomes of genetic crosses and analyze results. (SLO 14d)
This exercise is adapted from HHMI’s: Biointeractive.orgLinks to an external site., The Making of the Fittest: Natural Selection in Humans
Hemoglobin is a protein found in red blood cells (RBCs) that transports oxygen throughout the body. The hemoglobin protein consists of four polypeptide chains: two alpha chains and two beta chains. Sickle cell disease (also called sickle cell anemia) is caused by a genetic mutation in the DNA sequence that codes for the beta chain of the hemoglobin protein. The gene is located on chromosome 11. The mutation causes an amino acid substitution, replacing glutamic acid with valine. Due to this change in amino acid sequence, the hemoglobin tends to precipitate (or clump together) within the RBC after releasing its oxygen. This clumping causes the RBC to assume an abnormal “sickled” shape.
Individuals who are homozygous for the normal hemoglobin allele (HbA) receive a normal hemoglobin allele from each parent and are designated AA. People who are homozygous for normal hemoglobin do not have any sickled RBCs. Individuals who receive one normal hemoglobin allele from one parent and one mutant hemoglobin, or sickle cell allele (HbS), from the other parent are heterozygous and are said to have sickle cell trait. Their genotype is Aa. Heterozygous individuals produce both normal and mutant hemoglobin proteins. These individuals do not have sickle cell disease, and most of their RBCs are normal. However, due to having one copy of the sickle cell allele, these individuals do manifest some sickling of their RBCs in low-oxygen environments. People with sickle cell disease are homozygous for the sickle cell allele (aa genotype); they have received one copy of the mutant hemoglobin allele from each parent. The resulting abnormal, sickle-shaped RBCs in these people block blood flow in blood vessels, causing pain, serious infections, and organ damage.
This exercise is an opportunity to practice identifying genotypes and phenotypes. Record your answers on the Word version of these questions. You will submit this as part of your lab submission. The Hemoglobin Beta gene is located on chromosome 11, an autosome.
Work through the problems on this document – Exercise 2: Mendelian Genetics and Sickle Cell AnemiaDownload Exercise 2: Mendelian Genetics and Sickle Cell Anemia
If two people with sickle cell trait have children, what is the chance that a child will have normal RBCs in both high- and low-oxygen environments? What is the chance that a child will have sickle cell disease? Write the possible genotypes in the Punnett square.
What is the chance that a child will have normal RBCs in high- and low-oxygen environments?
What is the chance that a child will have sickle cell disease?
What is the chance that a child will carry the HbS gene but not have sickle cell disease?
What are the chances that these parents will have three children who are homozygous for normal RBCs? (Show your work.)
What are the chances that these parents will have three children who have both normal and mutant hemoglobin beta chains? (Show your work.)
An individual who has sickle cell trait has children with an individual who does not have the HbS allele.
What are the genotypes of the parents?
In the Punnett square, show all the possible genotypes of their children. State the genotype and phenotype ratios of the offspring.
What are the chances that any one of this couple’s children will have sickle cell disease?
A woman with sickle cell disease has children with a man who has sickle cell trait. Answer the following questions.
What are the genotypes of the parents?
What is the genetic makeup of the gametes the mother can produce?
What is the genetic makeup of the gametes the father can produce?
In the Punnett square, show all the possible genotypes of their children. Then summarize the genotype and phenotype ratios of the possible offspring.
What are the chances that any one of this couple’s children will have sickle cell disease?
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