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from The Textbook Letter, March-April 1993

Reviewing a middle-school book in the Prentice Hall Science Series

Heredity: The Code of Life
1993. 112 pages. ISBN of the teacher's edition: 0-13-986514-4.

Sowing Confusion

Ann T. Bowling

I have looked at Heredity: The Code of Life from two perspectives, because I am not only a biologist but also a parent whose daughter will enter middle school in two years or so. Accordingly, I have asked myself whether I would be happy if our school district used this book for introducing my daughter to genetics, the exciting subject that has been my professional specialty for twenty-five years.

My answer, in a word, is no. Prentice Hall's book carries on the tradition of sowing confusion about genetic principles and about the application of those principles to real-life situations. The writers provide watered-down versions of old material (including the usual overworked examples) that evidently has been taken from books intended for older students. By so doing, the writers waste their opportunity to teach basic genetics and to convey the excitement that genetics can hold. I hope that someone soon will produce a book that meets the needs of middle-school students -- something that this one fails to do.

The book's flawed title sets the tone for the flawed genetics in the text to follow. Heredity is not a code. The expression "code of life" could conceivably be applied to the triplet code of DNA, and so we might be persuaded to accept Heredity: Understanding the Code of Life, but the title that Prentice Hall has chosen is puzzling and misleading.

There are other puzzles, too. In the teacher's edition, each "Chapter Overview" suggests to the teacher that evolution can be related to concepts in the chapter's text, but I have been unable to find any case in which the subject of evolution is presented directly to the student. What is the reason for this? And what phobia makes the writers refuse to use the words homozygote and heterozygote, both of which are absolutely necessary in any discussion of Mendelian principles? Why do the writers try to struggle along by substituting the inadequate, misleading words purebred and hybrid, which definitely are not synonyms for homozygote and heterozygote? And why do they describe the diagnosis of chromosomal disorders by amniocentesis, though they fail to tell why a parent would want to have the resulting information? Surely a pregnant woman would not undergo amniocentesis unless she wanted to be able to consider practical options, such as terminating her pregnancy if the fetus were seriously defective.

Chapter 1 ("What Is Genetics?") could have provided a stimulating explanation of Mendelian principles by using organisms and traits that are familiar and inherently interesting to students: coat color and pattern in domestic animals, for example, or blood groups and hereditary diseases in humans. Instead, it tells about Mendel and peas. Mendel's experiments could have been put into a sidebar or omitted altogether. They are much more appropriate in a high-school or college curriculum, where a historical view is more pertinent.

On page 16 the writers describe dominant traits as "stronger" and recessive ones as "weaker." This implies value judgments about traits, leading to serious misunderstanding of the concept of the adaptive value of genes. On page 20, the pictures of flowers (presumably meant to illustrate the idea of heterozygosity) must be confusing to students. So must the photos of rhinoceroses and puppies on page 21, because the photos are unrelated to the subject of the adjacent text (the law of segregation).

The multiple-choice questions at the end of this chapter (and the ones in the other chapters, too) test the retention of trivia, not the understanding of concepts. The true-false questions are slightly better, and the remaining questions are relatively good.

The opening of chapter 2 ("How Chromosomes Work") is awful. A computer model of DNA is likened to "a strange galaxy in outer space" as the atoms in DNA "sparkle against a black background." This abstraction emphasizes characteristics of computer models but says nothing about biological reality.

On page 38 the writers wrongly assign the symbols X and Y to the sex chromosomes of birds, though geneticists -- for important biological reasons -- call those chromosomes Z and W.

The passage about "Harmful Mutations" and "Helpful Mutations" (pages 40 and 41) is confusing at best. As an example of a "harmful" mutation, the writers cite sickle-cell anemia in humans. They present the sickle-cell gene as deleterious to the organism that bears it and they ignore its adaptive aspect (resistance to infectious malaria), even though the adaptive aspect is mentioned elsewhere in the book. As an example of a "helpful" mutation, these writers point to seedless navel oranges. But seedlessness -- which absolutely precludes reproduction -- is even more deleterious than sickle-cell anemia is. The writers tout seedlessness as "helpful" because seedless oranges are prized by humans! The chapter concludes with a relatively good section on molecular genetics.

Chapter 3 ("Human Genetics") starts with some material about sex, and we soon find an incorrect generalization: "The X chromosome is rod shaped and the Y chromosome is hook shaped" (page 59). The writers seem to be recalling fruit flies, not humans. On the next two pages they mention that skin color is controlled by multiple genes while ABO blood type is controlled by multiple alleles of one gene. I doubt that the difference in mechanisms can be understood by the student, because there are no diagrams to make things clear. The passage about blood type would have been a perfect place to introduce an important point about human genetics: In complex situations, the testing of hypotheses requires the analysis of tremendous amounts of family data. This point is not made, however.

Pages 70 and 71 are the sites of several disasters. The writers seem to imagine that the terms "twenty-first chromosome" and "chromosome 21" are interchangeable -- but they are not. The text fails to tell that chromosomal anomalies generally have drastic, deleterious effects on the organisms involved. And while it's laudable to try to discourage social discrimination against afflicted people, it's grossly inaccurate to say that "many people with Down syndrome lead normal, active lives and often make valuable contributions to society" (emphasis added).

A note to the teacher gives a wrong answer for a "Find Out by Calculating" exercise on page 71: If 2,400 is divided by 800, the quotient is 3, not 4.

The final chapter ("Applied Genetics") repeats the same defects that the earlier chapters have shown. In particular, the passages about inbreeding and about purebred animals are difficult to follow. How can wild cheetahs be inbred if inbreeding is defined as a "selective-breeding technique"? Who has been breeding wild-bred cheetahs? More confusion comes from the implication that the ability to pass on desirable traits is confined to purebreds. And the "mule" in the bottom illustration on page 82 is not a mule. It seems to be a horse, but it may be a hinny. (A hinny is the offspring of a male horse and a female ass. A mule results from the mating of a male ass and a female horse.)

Heredity: The Code of Life does not fill the need for a text that can explain genetics to middle-school students. In reading it I have been surprised to find out how poor it is, since it looks so nice at first glance. I hope that we can expect a scientifically valid, exciting book in the future.

These Fumblers Are Wrong

William J. Bennetta

Heredity: The Code of Life, marketed as a book for middle-school students, is confused, silly and unacceptable. The writers evidently thought that they could cobble a middle-school genetics book by mentioning and "simplifying," into incomprehensible mush, all the topics that usually appear in the genetics unit of a high-school biology text. Moreover, they evidently tossed this book together without bothering to study genetics.

To suggest just how bad the book is, I point out that the writers fumble their way through 112 pages without comprehending the terms hybrid and hybridization. On page 20 they say that a hybrid is an "organism that has genes that are different for a trait," whatever that is supposed to mean. Later they guess that "Hybridization is the crossing of two genetically different but related species of organisms" (page 81). That nonsense deserves analysis:

Because all species are genetically distinct from each other, and because all species are also genealogically related to each other, the reference to "genetically different but related" species says exactly nothing. It is all the sillier because the book does not provide the evolutionary context that gives meaning to the idea of relatedness. So let us get rid of the writers' empty phrase and see what is left: "Hybridization is the crossing of two species of organisms." That is poppycock. If students believed it, they would get a grossly wrong impression about applications of genetics to agriculture, and they would not even understand the labels on the packets of seeds that are sold in shops. Consider what I saw a few days ago, when I looked at the seed display in a hardware store. The packets of tomato seeds showed such names as "Better Boy Hybrid" and "Burpee's Big Boy Hybrid." On packets of squash seeds I found "Zuchlong Hybrid" and "Dixie Hybrid." And on the packets of maize seeds I saw "Silver Bullet (Hybrid)" and "Aztec (Hybrid)," among other names.

Prentice Hall's writers may guess that all those names denote crosses between species, but students shouldn't be led to believe any such thing. Those hybrids, like nearly all others, are created by crossing two strains of the same species. Hybridization between species, whether in nature or in an agricultural setting, is so rare that it hardly merits a mention in an introductory book for middle-school students.

Two other cases furnish especially firm evidence that the writers don't know their subject. They guess that testosterone is "a protein," and they totally confuse the two meanings of the term inbreeding. Or maybe they don't even know that two different meanings exist. In any case, their text about inbreeding makes no sense.

The notes in the teacher's edition are often silly, and some of them seem so blatantly fatuous and self-serving that teachers will probably be insulted. On page 12 a "Multicultural Opportunity" note says: "Suggest that students look around the classroom and notice the ways in which each person is unique. Point out that although we may differ dramatically in appearance, as humans we are all more alike than we are different." No teacher worthy of the name will have anything to do with that pseudoscientific doubletalk.

In fact, the book offers the teacher ten "Multicultural Opportunity" items, whose chief purpose, I infer, is to enable the writers to use the buzz-word multicultural. Of the ten, eight fail to recount anything about any identified culture or about any cross-cultural comparison, and some cruelly promote the confusion of culture with race. Of the two that do have cultural aspects, one is valid and the other is absurd. The valid one, seen on page 70, tells of a connection between the use of milk as food (a cultural variable) and the prevalence of genes that govern the digestion of lactose. The absurd one, set in a context of genetic engineering, includes this:

Isn't it possible that a majority culture would use the power of genetic engineering to its own advantage? Students may enjoy reading Brave New World by Aldous Huxley, which shows a future scenario. Ask students who have read this book to share their opinions with the class.

Let me share my opinions. First, citing Brave New World is stupid. One striking aspect of the biological manipulations described in Huxley's "scenario" is that they are entirely somatic. They don't involve genetics, and invoking them in a genetic context can only confuse students. Second, this seems to be another case in which Prentice Hall's writers mention literature that they have not read or cannot understand. See "Read Any Good Books Lately?" in the January-February issue of TTL, page 12.


Ann T. Bowling, a specialist in the genetics of domestic animals, teaches in the School of Veterinary Medicine at the University of California at Davis.

William J. Bennetta is a professional editor, a fellow of the California Academy of Sciences, the president of The Textbook League, and the editor of The Textbook Letter. He writes frequently about the propagation of quackery, false "science" and false "history" in schoolbooks.

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