Scene II: Will Alan ever play for the Lakers? After this lesson, the students will be able to: Explain x-linked inheritance using Factor VIII deficiency (hemophilia) as an example; Describe the treatment for hemophilia; Explain the importance of early diagnosis and carrier testing of hemophilia; and Describe some societal issues associated with hemophilia and its treatment.
Alan’s Situation Alan suffers a painful knee injury while shooting baskets in his parents’ driveway. Examination at the emergency room indicates that Alan has a bad sprain, in addition to bleeding within the knee joint. Further testing reveals a deficiency of Factor VIII, and a diagnosis of classical hemophilia is made. Stacy is found to have reduced Factor VIII levels, consistent with her being a hemophilia carrier.
Following referral for genetic counseling, Paul and Stacy understand that the gene for Factor VIII is on the X-chromosome. Since Stacy has two X-chromosomes, only half are unable to make usable Factor VIII. The other half make all of the clotting factor that she needs. But Alan happened to inherit from her the X-chromosome unable to make Factor VIII. Because his other sex chromosome is a Y, he has no “back up” gene for Factor VIII. Until his basketball accident, the deficiency went unnoticed. Alan’s Situation (contd.)
Blood clotting disorder usually affecting males 1 in 5,000 Caused by abnormal Factor VIII protein Factor VIII necessary for blood clotting Clotting takes longer Inheritance pattern: Sex-linked disorder (found on X chromosome) What is Hemophilia?
Sex determined by genes found on the X and Y chromosome Females: egg produced with an X chromosome Males: ½ sperm cells contain X; and the other ½ contain Y There is a 50/50 chance of child being a boy/girl Sex Determination X X X Y X X X X X Y X Y
Defined: Recessive inherited conditions linked to the sex chromosomes Females: XX chromosomes Males: XY chromosomes More common in males because only a single X chromosome needs to be damaged (both must be damaged in females) Ex: Colorblindness, Hemophilia, Muscular dystrophy Sex-Linked Disorders
Treatments • Factor VIII Replacement: • Injections of Factor VIII protein • Causes blood to clot when injured • Joint Replacement • Knee, hip, ankle replacement if arthritis is severe • Regular exercise for strong muscles to protect the weak joints
Alan’s Situation Alan is starting on Factor VIII home infusion. His parents are taught how to inject Alan with Factor VIII that has been removed from pooled, donated blood, purified and packaged for ready use. The Factor VIII concentration can be kept at home and prepared for injection only when Alan feels that bleeding has occurred. The concentration can also be taken with them on trips.
Factor VIII Infusions • Objective: replace missing clotting factor • When used: • Used at time of bleeding to prevent tissue damage • Prior to and during surgery • Cost: over $100,000/year (beyond lifetime cost of many insurance plans) • Prior to 1984: HIV prevalent in the Hemophilia community (70-90%)
Alan’s Situation After the increased risk of hepatitis is discussed, immunizations against Hepatitis B and D are scheduled. Stacy is concerned about hemophilia and AIDS. She is assured that while AIDS was a threat to hemophiliacs receiving pooled Factor VIII years ago, more strict donor guidelines and heat-treating of pooled blood reduces the chances of AIDS being a problem for hemophiliacs. Stacy and Paul are reassured. Yet Paul adds, “But let’s see if our insurance will pay for the recombinant Factor VIII.”
Alan’s Situation Stacy recalls that about a year ago, a boy in Frank’s school was discovered to be HIV positive, presumably from contaminated blood with which he was transfused during heart surgery. Several parents demanded that the boy be removed from school, threatening to remove their own children if their demands were not met. Stacy remembered following the issue in the newspapers. She remembered that the boy and his family moved out of state before the school board or disgruntled parents took any action. At the time, she took no particular position on the issue. What must it have been like for the boy and his family?
Expanding the Chance Family “Stacy, since hemophilia is X-linked and you’re a carrier, it’s important that we contact other family members of yours to discuss their risk of being carriers. Do you have any sisters?” inquires the genetic counselor. “Yes, one. And a brother, David.” “Your sister might also be a carrier. You should bring her up to date on this and ask her to make an appointment with us.” Stacy refuses. “And if you tell her anything about Alan’s hemophilia, I’ll sue you and this place for everything you’re worth.”
What is the probability of getting: Daughter with hemophilia? 0% Carrier child? 25% Child with hemophilia? 25% Two healthy children? ¾ x ¾ = 9/16 (56%) Practice: Jen is a healthy carrier of hemophilia and Adam has no history in his family. Healthy girl Healthy/ carrier girl Healthy boy Hemophilia boy Key XD = healthy Xd = disorder (hemophilia)
What is the probability of getting a: Healthy son? 25% Healthy daughter? 25% Child with hemophilia? 50% Carrier child? 25% Practice: Kelly is a healthy carrier of hemophilia, but Jim suffers from the disease. Healthy/ carrier girl Hemophilia girl Hemophilia boy Healthy boy Key XD = healthy Xd = disorder (hemophilia)