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Homologous Shoes?

By Thomas Carroll



ABSTRACT

This activity, originally developed by Harry Wong, is designed to enable students to understand the difficult concept of Homology. This "concept demonstration" provides students with a concrete example (pairs of shoes in a classroom "cell") of what Homology means. It is hoped that students will use this simple demonstration to help them visualize the situation that exists in actual chromosomes. In order to be affective, students must be familiar with DNA Structure, the gene concept and the central dogma.

ACTIVITY TYPE:

Concept Demonstration

TARGET AUDIENCE:

First year Biology up through A. P. Biology

INFORMATION FOR THE TEACHER

I have found that students of all levels have a particularly hard time understanding the concept of Homology: the fact that in every diploid cell an organism has two copies of every chromosome (excluding the sex chromosomes) and that these copies are "homologous". They are very good at memorizing the phrase "Homologous Pairs", but they find it extremely difficult to visualize what they are and understand Homology's significance. Unfortunately, understanding this seemingly simple concept is critical to the understanding of many other important basic concepts of Biology.

For example, it is very difficult to really understand the significance of Mitosis and Meiosis without understanding Homology. This concept is especially important for the ideas of Segregation, Independent Assortment, and Crossover. And if the student does not fully understand Meiosis, he or she will not really understand Heredity. Homology, it turns out, is a "Foundation Concept" that many other concepts are constructed upon. And if you strive, as I do, for thorough understanding, and application of concepts from your students then you must deal with this concept in an effective way early in the semester.

I like to use metaphor in my teaching, especially metaphors that have something to do with my students lives. One day while struggling to explain the concept of Homology to a freshmen class I blurted out "chromosomes are like pairs of shoes". This outburst led to the "concept demonstration" that I do every year to help illustrate the concept of homology.

INSTRUCTIONS FOR THE STUDENT

FIRST: I start by introducing the basic concepts of Homology and Homologous Pairs. Then, I tell everyone that chromosomes are like pairs of shoes and to take off their shoes and hold them over their heads. This, of course, is accompanied by much chaos and laughter because taking your shoes off in class and holding them over your head is, shall we say, out of the ordinary! Teachers, you must do this too. Now what you have is a diploid "nucleus"(your classroom) populated with shoe "chromosomes".

SECOND: I ask them to observe the shoes being held up around the room: What do they notice (besides smell)? How are pairs of shoes similar to pairs of chromosomes?

THIRD: Eventually someone observes the variety of shoes in the room: no two pairs of shoes are the same. I relate this to the unique pairs of chromosomes in cells and that the pairs are unique because they consist of different genes.

FOURTH: I then ask students to compare within the pairs: What do they notice? Well, the members of a pair are similar but not identical! You have left and right shoes! And you have "left and right" (maternal and paternal) chromosomes within the pair. You can even have the students alternately hold up the "left"(maternal) and "right" (paternal) "chromosomes" and ask them to discuss the differences between Haploid and Diploid and relate this to gametes and zygotes.

FIFTH: Summarize the observations: Each pair of shoes is different from every other pair, and each pair consists of similar but not identical members.

SIXTH: Connect these observations with the chromosomes in a diploid cell. Chromosome pairs are different from every other pair. They are different because each pair consists of different genes. Each pair consists of two similar but not identical pieces of DNA. They are similar in that the sequence of genes is the same on each piece but they are different because each piece is not made of exactly the same nucleotide sequence. (My students already have a firm grasp of the related concepts of DNA Structure, The Gene and Protein Synthesis from the previous unit.) Now every time you mention homologous pairs to your students they will remember this wacky demo and, hopefully have a concrete idea of how homologous chromosomes are similar but not identical!


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