Were the babies switched? The Genetics of Blood Types i

Transcription

1 Were the babes swtched? The Genetcs of Blood Types Two couples had babes on the same day n the same hosptal. Dense and Earnest had a grl, Tonja. Danelle and Mchael had twns, a boy, Mchael, Jr., and a grl, Mchelle. Danelle was convnced that there had been a mx-up and she had the wrong baby grl, snce Mchelle had lght skn, whle Mchael Jr. and Tonja looked more lke twns snce they both had dark skn. Danelle nssted on blood type tests for both famles to check whether there had been a mx-up. To nterpret the results of these tests, you wll need to understand the genetcs of blood types. Genetcs of Blood Types The ABO blood type system s the major blood type classfcaton system that determnes whch type of blood can safely be used for a transfuson. The four blood types n the ABO system refer to dfferent versons of carbohydrate molecules whch are present on the surface of red blood cells. People wth: Have: Type A blood Type B blood Type A carbohydrate molecules on ther red blood cells Type B carbohydrate molecules on ther red blood cells Type AB blood Type O blood Both type A and type B carbohydrate molecules on ther red blood cells Nether type A nor type B carbohydrate molecules on ther red blood cells These dfferent blood types result from dfferent alleles of a gene n the DNA that gve the drectons for makng dfferent versons of a proten enzyme that puts dfferent types of carbohydrate molecules on the surface of red blood cells. Allele I A I B Gves the drectons for makng a verson of the enzyme that: puts type A carbohydrate molecules on the surface of red blood cells puts type B carbohydrate molecules on the surface of red blood cells s nactve; doesn't put ether type of carbohydrate molecule on the surface of red blood cells 1. Each person has two copes of ths gene, one nherted from hs/her mother and the other nherted from hs/her father. Complete the followng table to relate genotypes to blood types. Genotype Ths person's cells make: Blood Type I A I A the verson of the enzyme that puts type A carbohydrate molecules on the surface of red blood cells. the nactve proten that doesn t put ether type A or type B carbohydrate molecules on the surface of red blood cells. I A both the verson of the enzyme that puts type A carbohydrate molecules A on the surface of the nactve proten 2. In a person wth the I A genotype, whch allele s domnant, I A or? Explan your reasonng. 1

2 3. For the genotypes lsted below, whch type(s) of enzyme would ths person's cells make? What blood type would the person have? Genotype Wll ths person's cells make the verson of the enzyme needed to put ths carbohydrate on the surface of hs/her red blood cells? I B I B Type A yes no; Type B yes no I B Type A yes no; Type B yes no Blood Type I A I B Type A yes no; Type B yes no AB Codomnance refers to nhertance n whch two alleles of a gene each have a dfferent observable effect on the phenotype of a heterozygous ndvdual. Thus, n codomnance, nether allele s recessve both alleles are domnant. 4. Whch of the genotypes lsted above results n a blood type that provdes clear evdence of codomnance? Explan your reasonng. Ths expanded verson of a Punnett square wll show how meoss and fertlzaton result n the nhertance of a gene. 5a. The mother has type blood and the father has type blood. 5b. Complete ths chart to show the genetc makeup of each zygote produced by fertlzaton. A baby develops from a zygote by repeated rounds of mtoss, so each cell n a baby s body has the same genetc makeup as the zygote. Ths s the genotype of the chld. 5c. Draw a Punnett square n the usual format for ths same mother and father. 5d. Wrte n the blood type for each chld. Note that meoss and fertlzaton can produce a chld who has a dfferent blood type than ether parent. 2

3 Understandng Blood Type Tests The type A and type B carbohydrate molecules are called antgens because they can stmulate the body to produce an mmune response, ncludng antbodes. Each specfc type of antbody bnds to a specfc antgen. For example, ant-b antbodes bnd to type B antgens, but not to type A antgens. Normally, your body does not make antbodes aganst any antgens that are part of your body. For example, a person wth type A blood does not make ant-a antbodes aganst the type A antgen on hs/her red blood cells does make ant-b antbodes aganst the type B antgen whch s not present on hs/her red blood cells. 6. Fll n the blanks n ths chart. If you have type A blood, If you have type B blood, type A antgens on type B antgens on If you have type AB blood, both type and type antgens on nether ant-a nor ant-b antbodes n your blood. If you have type O blood, nether type of antgen on the surface of your red blood cells and both and If you are gven a blood transfuson that does not match your blood type, antbodes n your blood can react wth the antgens on the donated red blood cells. Ths reacton can cause the donated red blood cells to burst and/or clump together and block blood vessels. A transfuson reacton can be fatal. To prevent ths from happenng, doctors test whether donated blood s compatble wth a person s blood before they gve a blood transfuson. 7. Explan how a type B blood transfuson could be fatal for a person wth type A blood. Begn wth the antbody-antgen reacton n a person wth type A blood who has been gven a type B blood transfuson. (Hnt: See the fgure at the top of the page.) 3

4 To test blood types, you wll frst mx a blood sample wth a soluton that contans ant-a antbodes. If the blood sample has type A antgens, they wll react wth the ant-a antbodes, and ths wll result n clumpng. Then, to test whether ths blood has type B antgens, you wll mx a second sample of ths blood wth a soluton that contans ant-b antbodes. 8. To prepare to nterpret the blood type tests, fll n the followng chart. Wll ths blood type clump f mxed wth Blood type ant-a antbody? ant-b antbody? A B AB O Procedure Your group wll need a blood-typng tray or other testng surface for each person lsted below. For each person, you wll use two separate blood samples to test for the A antgen and test for the B antgen. Place two drops of the person s blood n each of the testng locatons on the testng surface. Place two drops of ant-a antbody soluton on the approprate blood sample and place two drops of ant-b antbody soluton on the other blood sample. Mx each blood sample wth the antbody soluton wth a clean toothpck and. Dscard each toothpck after you have used t. If your testng surfaces are transparent, place them on a whte background so you can more easly see whether there was a clumpng reacton. For each person, record the results of both tests n the table below. Wrte n the blood type and possble genotypes of each person. Results Mchael (father of twns) Danelle (mother of twns) Earnest (father of daughter) Dense (mother of daughter) Mchael Jr. (boy twn) Baby grl 1 (grl twn, accordng to hosptal) Baby grl 2 (daughter of Earnest and Dense, accordng to hosptal) Reacts wth ant-a antbody (Yes or No) Reacts wth ant-b antbody (Yes or No) Blood type (A, B, AB, O) Possble genotype or genotypes 4

5 Interpretaton Now you can use the results of your blood tests to evaluate whether Mchael and Danelle's baby grl was swtched wth Earnest and Dense's baby grl. 9a. Draw Punnett squares that show the possble genotypes for Mchael and Danelle and ther chldren. Wrte n the blood type for each genotype to show the possble blood types for Mchael and Danelle s chldren. 9b. Draw Punnett squares that show the possble genotypes for Earnest and Dense and ther chldren. Wrte n the blood type for each genotype to show the possble blood types for Earnest and Dense s chldren. 9c. Who are the parents of each baby grl? How do you know? 9d. Were the babes swtched? By Drs. Jennfer Doherty and Ingrd Waldron, Dept Bology, Unv Pennsylvana, Teachers are encouraged to copy ths Student Handout for classroom use. A Word fle of the Student Handout and Teacher Preparaton Notes wth background nformaton and teachng suggestons are avalable at 5