Pedigrees How do scientists trace hereditary diseases through a family history?

Transcription

1 Why? Pedigrees How do scientists trace hereditary diseases through a family history? Imagine you want to learn about an inherited genetic trait present in your family. How would you find out the chances of passing the trait to your children? To do this, genetic counselors often prepare a pedigree, a chart that shows how a trait is inherited over several generations in a family. Model 1 Pedigree of Sickle Cell in Four Generations 1. What does the circle shape represent? 2. What does the square shape represent? 3. The pedigree represents the occurrence of sickle cell disease in a family. How are individuals with sickle cell represented in the pedigree? 4. Determine how parents are shown in a pedigree. (Include a picture and a description) 5. Determine how children are shown in a pedigree. (Include a picture and a description)

2 6. How many siblings does the patient have? 7. If you are the patient, what is the relationship of the people in Generation I to you? 8. Describe the phenotype of the patient. 9. Considering that the sickle cell allele is recessive, give a probable genotype for the patient. 10. Based on your answers for 8 and 9, determine the phenotype and genotype of the patient s parents. 11. Draw a pedigree below for the following information. Be sure to include the generation numbers! A girl with a recessive genetic disease has two sisters and one brother. Her brother has the same disease, but her sisters do not. Her mother has the disease, but her father does not. 12. How could a pedigree be used by a genetic counselor?

3 Read This! Scientists can use pedigrees to determine if a disease is caused by a dominant or recessive allele. Pedigrees are helpful if family members want to know if they are carriers or if their children might get a genetic disease. Carriers are individuals who are heterozygous for a genetic disease, but do not show symptoms of the disorder. Carriers can pass the allele for the disease to their offspring. Model 2 Pedigree Case Studies Case 1 Case 2 Case 3 [ DOMINANT/RECESSIVE/UNKNOWN] [ DOMINANT/RECESSIVE/UNKNOWN] [ DOMINANT/RECESSIVE/UNKNOWN] 13. Why are some people known as carriers? 14. For Case # 1, use a guess-and-check method to determine whether the disease is dominant, recessive, or unknown. Under each circle or square, write the genotypes for each person. Use A for dominant allele and a for recessive allele. You may use A- if you cannot determine the whole genotype. Explain the reasoning for your choice for Case 1 below. 15. For Case # 2, use a guess-and-check method to determine whether the disease is dominant, recessive, or unknown. Write the genotypes for each person, as you did for Case #1. Explain the reasoning for your choice for Case 2 below. 16. For Case # 3, use a guess-and-check method to determine whether the disease is dominant, recessive, or unknown. Write the genotypes for each person, as you did for Case #1. Explain the reasoning for your choice for Case 3 below.

4 Model 3 Pedigree of Hemophilia in a Royal Family 17. Identify the gender and phenotypes of Beatrice and Henry s children. 18. Beatrice s daughter, Eugenie, married Alfonso XIII of Spain. Identify the gender and phenotype of their children. 19. Based on the model, is hemophilia a dominant or recessive condition? Explain your reasoning.

5 20. Using model 3, complete the following table: Females Males # with hemophilia Total # of individuals % with hemophilia 21. Using the information in model 3 and your answers to prior questions, explain similarities and differences between model 3 and model 1. Similarities Differences Read This! The nuclei of human cells contain 22 pairs of autosomes and a pair of sex chromosomes. In females the two sex chromosomes are XX and in males the sex chromosomes are XY. Unlike the autosomes, the X and Y chromosomes are non-homologous with the X likely containing around genes that code for proteins compared to the Y chromosomes with only genes. As a result, most sex-linked alleles are encoded on the X chromosome. Pedigree charts can be analyzed to determine if the allele causing a condition is located on an autosome or the sex chromosome. 22. Using the information in Read This above and model 3, is the hemophilia allele located on an autosome or sex chromosome? Explain your reasoning.

6 Extension Questions Imagine you are a genetic counselor and a couple has asked you to determine the probability of their child developing a genetic disease, given their family history below. Husband s family history: The husband has the disease. The family history of the husband is unknown. Wife s family history: The wife does not have the disease. The wife has a sister without the disease and a brother with the disease. Both of the wife s parents have the disease. The wife s father is an only child; his father has the disease, but his mother does not. The wife s mother has two brothers without the disease; her mother had the disease, but her father did not. 23. Translate the family history into a pedigree using standard notations and symbols. Hint: Work as a group to do a rough draft on scratch paper first! 24. Circle one: This disease is [ DOMINANT / RECESSIVE ]. 25. Circle one: This disease is [ AUTOSOMAL /SEX-LINKED ]. 26. Explain how you know the disease allele is dominant or recessive. Use specific relationships (between family members) to support your claim. 27. Explain how you know the disease allele is autosomal or sex-linked. Use specific relationships (between family members) to support your claim. 28. Add genotypes to each family member on your pedigree for #23. Use A for dominant allele and a for recessive allele. You may use A- if you cannot determine the whole genotype.

7 29. If the husband is heterozygous for the disease-causing allele, what is the probability that their child will develop the disease? Include a Punnett Square and a genotypic and phenotypic ratio in your answer. 30. If the husband is homozygous for the disease-causing allele, what is the probability that their child will develop the disease? Include a Punnett Square and a genotypic and phenotypic ratio in your answer. 31. Based on your analysis of the husband and wife s family histories, what predications can you give the couple about the probability of their future offspring in relation to the husband s genetic disease?