Lewis/Life - Teaching Techniques

Chapter 13 - Teaching Techniques

A. General Tips
If you would like to give a word of encouragement to struggling students, tell them that Gregor Mendel flunked his exams to become a natural science teacher. And natural science isn't the only subject he flunked! Mendel's life is fascinating; one or more of your students might be interested in a little historical research.
This chapter is filled with interesting and informative tables. I suggest you study these tables carefully and use them liberally throughout your presentation of this material. The terms from Table 13.1 are specifically marked in the Key Terms section of this chapter.
Note, in Table 13.2, the reference to magenta urine after eating beets. This phenomenon has caused many parents to rush a child to the hospital emergency room!
You might wish to copy and distribute to your students the Overview of Chapter Objectives flowchart found at the beginning of this Instructor's Manual Chapter.
B. Possible Approaches to the Chapter
Again, this chapter is good for a non-quiz. If you are really interested in drawing out the misconceptions your students have, ask them if dominant traits are more common than recessive traits; what a mutation is; if a light-skinned black couple could have a dark-skinned child; is it a problem if a woman has type AB blood, her husband has type B blood, and their child has type O blood.
I suggest you take the major terms you wish to emphasize in this section and write them on the board. Keep the paired terms together, terms like homozygous and heterozygous, dominant and recessive, and genotype and phenotype. Tell the students you will be defining all the terms in context as you go through the material.
C. Hints Related to Specific Chapter Topics
- II. As shown in Figure 13.3, Mendel worked with seven different "pea characteristics." Interestingly, each of these seven characteristics is coded on a different chromosome pair. (And peas have 14 chromosomes -- 7 pairs!) Scientists have speculated as to whether Mendel was just plain lucky in choosing these seven traits, or whether he had experimented with more traits and found the mathematical relationships with these characteristics. No one seems to know the answer. Either way it makes interesting speculation.
  Another point from this section: Keep the numbers 1/3 and 1/4 (in the first paragraph) in mind if any of your students have trouble with the To Review question # 18.
- II.A. Certain misconceptions about terminology will arise as you cover this section. Here are some of the major misconceptions I have personally encountered:
  "An allele is an abnormal form of a gene." "The homozygous condition is necessarily better than the heterozygous condition." [If you encounter this one, discuss heterozygote superiority!] "Dominant genes are better, stronger, or more common than recessive genes." "Recessive genes are weaker or less common than dominant genes." "The wild-type is the 'weird' expression or version of the gene." "Mutations are necessarily bad (or deleterious)."
  Many geneticists (and educators!) today claim that "dominant" and "recessive" are poor descriptors of the situations they describe because of the implications of "stronger/weaker" and "more common/less common." If identified today, different terminology probably would be chosen. But, we have to work with what we've got. Stress that "dominant" and "recessive" have to do strictly with phenotypic expression when the gene is present in the individual. Occurrence or manifestation in the population as a whole is irrelevant. For instance, type O blood is recessive to types A and B yet type O is the most common blood type. The gene for Huntington Disease (also known as Huntington's Chorea) is dominant, yet it is a lethal gene.
- II.B. If you covered meiosis in the last chapter by tracing a single gene, you can pick it up again here.
  Some students understand genotypic and phenotypic ratios almost instinctively. Some do not. When you draw Punnett squares, be consistent. For instance, in the Lewis diagrams, the female gametes are identified across the top of the square and the male gametes are identified along the left side. Use this same pattern throughout.
  For some reason, the test cross becomes a difficult idea for some students. I suggest you draw Punnett squares on the board to show exactly what is happening with the test cross.
- III. Stress the first statement in this section, that Mendelian laws apply to all diploid species. So often students think only in terms of animals, even when they have just finished working with Mendel's seven plant characteristics.
  Very few traits are not in some way polygenetic. You can always state that the laws of genetics apply but that we simplify things to demonstrate these laws by assuming that certain phenotypic traits are the result of single gene inheritance.
- III.A. When discussing the autosome, I suggest you ask the students to recall the definition of autosome. Many will have forgotten.
  In this section is the phrase "can skip a generation." Explain that skipping a generation has to do with recessive inheritance and ask if anyone can figure out why this term exists. Note too the difference between can and does skip a generation.
- III.B. To study the pedigree, suggest that the students construct either a real or a hypothetical pedigree of a familial trait. I suggest you not insist on their using a real trait because biological relationships in families are not always what they seem.
- IV. Using hand drawn Punnett squares Mendel's second law can be demonstrated very nicely for two traits. For more than two traits, computers can be used.
  I suggest you use Figures 13.14 and 13.15. As an alternative to using these figures, draw this procedure out on the board as you lecture. If you have pre-drawn overlays for the overhead projector, cover up the parts you are not discussing so students will concentrate on the sequence of events rather than on the final idea.
- V.A. Lethal alleles in pregnancy were also referred to in the last chapter.
- V.B. As a point of interest, plants generally have more alleles for given loci than do animals.
- V.C. Some geneticists and educators believe we should teach that all alleles are expressed. The three examples given here demonstrate that point. All the alleles seem to be expressed whether as blended traits (piebald cats and pink snapdragons) or as codominant blood types.
- V.D. One of the classic examples of epistasis is found in the question of whether or not two blue-eyed people can have a brown-eyed child. Divorces, murders, and public humiliations have occurred over this question. Two genetically blue-eyed people cannot have a brown-eyed child. But, are all people phenotypically blue-eyed really genotypically blue-eyed also? Apparently the answer is, "No. About 8 of every 1000 blue-eyed people have at least one gene for brown eyes." If that is true and an epistatic condition were masking that "brown eye gene", then it would be possible to pass on the "brown eye gene" without passing on the gene responsible for the epistatic condition. Thus, the phenotypically blue-eyed person -- who is genotypically brown-eyed -- could indeed have a brown eyed child.
- V.E. Sometimes blood type A causes a problem with expressivity and someone might hear the phrase, "low titer type A." This means the person really does have type A blood but it is not showing up very well. To complicate matters, blood type A comes in several subtypes.
- V.H. "Phenocopy" is a classic problem with many diseases.
- VI.A.1. Keep in mind with empiric risks that statistics are not absolutes; statistics are predictors for populations