Return to Biology IndexCONSTRUCTING A DICHOTOMOUS KEYMARGARET BANKHEAD ROBERT A. BLACK MAGNET SCHOOL 9101 SOUTH EUCLID AVE. CHICAGO, IL 60649 1-312-375-2040Objectives:To construct and use dichotomous keys.Materials:Shoes (Use one shoe from each of 10 students). Worksheets: "Using A Dichotomous Key" "Imaginary Animals"Strategies:Procedure: 1.Arrange chairs in a circle. 2.Instructor takes off one shoe and place it on the floor in the middle of the circle.Dialogue3."There's one of my shoes, so let's have a shoe from about ten of you". 4.Tell the students that they are to divide the shoes into two piles. Tell them the piles don't have to have equal numbers of shoes but, that they all have to agree on some obvious characteristic that will distinguish the shoes in one pile from the shoes in the other pile.After agreement is reached,tell the students a record of the agreement will be kept on the board. 5.Draw two horizontal lines some distance apart on the chalkboard. Label the lines with the agreed upon characteristics. (Fig.1). __________ __________ FIG.1 6.Return to the pile of shoes. Tell the students one pile will be pushed aside for the moment but, now they must again divide the pile of shoes into two distinct piles. After agreement is reached add this information to the chalkboard sketch. (Fig.2). _____________ _______________/ \_____________ FIG.2 7. Continue the procedure of dividing the shoes into two distinct piles and adding the information to the sketch until there is only one shoe with the identifying characteristic, at which point the shoe is identified and the owners name is added to the sketch (Fig.3). ______________ _____________/ _____________/ \______________ \_____________ FIG. 3 8. Divide the second pile of shoes in the same manner as the first pile until all the shoes have been identified. 9. Push all the shoes back together in one pile[Adding perhaps a shoe from the distance past or one with characteristics unlike those used in the original construction]. Discuss the meaning of the term dichotomous explaining that the word means "two forks". 10. Tell the students that dichotomous keys usually appear in a more compact form and that the diagram can be easily converted by adding numbers to each characteristic used. Label the diagram in numerical sequence following the same order the characteristics were agreed upon. 11. Have the students redeem their shoe by taking it from the pile and placing it on the correct branches of the key which will lead to its' correct identification. After all the shoes have been redeemed the shoe added earlier should remain. Ask a student to follow the key until the shoe is identified. Students should discover that a key works only for identification of those items used in its' original construction. 12. Pass out the two worksheets. Complete the " Using A Dichotomous Key" sheet in class. Assign the " Imaginary Animals" sheet for homework.