Adapted from “Life” Lewis 4^th edition Chapter 10

Chapter 13- How Inherited Traits Are Transmitted

I. Major objectives of the chapter

1- Traits are controlled by the presence or absence of certain proteins.

2- Traits may be dominant or recessive, and the causative genes are referred to in the same manner.

3- Meiosis contributes one set of genes from each parent, resulting in a new genetic combination.

4- Mendel’s experiments provided statistical rules for the transmission of genes from parent to offspring that are explained by meiosis.

5- Some traits are the result of a single gene. Many others are the cumulative effect of the interactions between two or more independent genes.

6- The probability of certain combinations of alleles can be predicted by deduction.

II. Major areas of difficulty for students

1- What is the relationship between DNA and genes and chromosomes and Mendel’s experiments?

Resolution- Thanks to “Jurassic Park”, students today bring to the classroom some prior knowledge of DNA. Although some of what they bring with them is incorrect, they do seem to know that DNA has something to do with our genes and how we appear and function. I have found that attempting to present Mendelian genetics without acknowledging that students know what DNA is tends to create more confusion in the minds of the students. Classical treatments of genetics usually attempts to treat traits and genes as vague, undefinable things. Mendel treated genes in this way, because he lacked the knowledge students now bring with them. As a result, I teach the students from the start that DNA is not mysterious, but information to build proteins. By making that connection at the beginning, and then interpreting Mendel’s experiments in light of that information, genetics tends to make more sense to the students.

2- What really is the difference between dominant and recessive?

Resolution- Dominant traits, in their simplest distinction, are those traits that result from the presence of a functional gene, and, therefore, the presence of a functional protein. Recessive traits, in nearly every case, are the traits that result from the absence of a protein or are “default traits” that show-up only when “unmasked” by the absence of a particular protein. Although there are exceptions, I find it vital to establish this as the rule. An aspect of this rule is that traits cannot be dominant in one individual and recessive in another, which is a very common conclusion students attempt to make. A trait MUST be linked to the genetic cause. Since genes are simply the information for making proteins, the presence of the corresponding protein confers the trait on someone. They can either make the protein or they cannot. The gene they possess is either intact and unaltered, or it is defective. Most defective genes result in the failure to manufacture a particular protein.

The single most common mistake students tend to make is to conclude that the most common trait in a family is dominant, because it is prevalent in that family. They will never be able to work genetics problems with this idea, since the designation of dominance is changed with each family and each person. I find it much more logical to stress that the characteristic of dominance has to do with the effect a protein has on a body, and if the protein is absent, so is the effect. This also lends itself very well to an accurate interpretation of what recessive means. Mutations are also more easily depicted as changes that prevent the correct protein from being made. The protein may, in fact be made in a different form, but it is most likely it will not be present at all.

Using this as the foundation, complex traits can be explained as the effects of increased amounts of a protein or of the effect of one protein upon another. I stress that genes don’t actually DO anything except provide information for building a protein. It is the protein that catalyzes some reaction or is deposited in hair or skin.

3- As a student, I find Punnet squares confusing.

Resolution- Emphasize that the Punnet square is just a device for keeping track of how the different ways that gametes may be combined to ensure that all possible combinations are represented. To accurately use a Punnet square, the possible genotypes of the gametes must first be determined. For humans, a gamete will only have one of each type of letter, whether it is capitalized or not.

4- What do all of the letters mean and how do I use them?

Resolution- Each letter represents an allele particular trait, and is usually the first letter of the description of the dominant trait. For instance, W could represent the dominant allele for having wooly hair. The lower case letter usually signifies the recessive version of the same allele. It is very important that letters never be used to designate just the trait or the phenotype. Letters are reserved for the alleles only. Traits are signified simply by a description of the appearance. Likewise, a common mistake for students is that they try to refer to traits and genes simultaneously. For instance, I have seen students refer to a trait as “homozygous”.

5- How do I make sense of pedigrees?

Resolution- Some instructors avoid addressing pedigrees entirely, but I find that this is an ideal topic for introducing students to logic. I provide a set of rules for my students. First, a pedigree is tracking only one trait. People either have that trait, and are so marked on the pedigree, or they do not, and no marking is used. It does not matter what else they have. Second, I stress that students must know that each person receives one copy of a gene from each parent. This sounds ridiculously simple, but students are always submitting papers that imply that two or more copies of a gene come from one parent. Third, I stress that they must understand what dominance means. (These last two are crucial. The student MUST comprehend the ramifications of these ideas.) Finally, I instruct students to look for a part of the pedigree that shows both parents with a phenotype that is different from at least one child. Then, the logic begins. Students must ask “IF the trait possessed by this child is dominant, what MUST be true concerning the genetics of the parents?” Since genes must come from those parents, if a trait is recessive, parents have the potential to transmit recessive alleles to offspring and give them a trait neither parent possesses.

The most common mistake students make with pedigrees is to attempt to interpret them to always show the dominant trait with filled-in boxes and circles. This is where I stress that this is a process of logic that eliminates what cannot be true. Another caution is that students will attempt to impose Mendelian ratios on pedigrees. Stress to the students that pedigrees represent real families, not probabilities.

6- What is the difference between proportion, fraction, frequency and percentage?

Resolution- I found myself early in my teaching career assuming students would know things that I take for granted. It took years for me to identify those terms and ideas that students were most likely NOT to understand. All three of these are in that category. I am constantly amazed at how many students have no real concept of what a percentage is, much less how to calculate one. Geneticists tend to freely move between each of these forms of representing these numbers. A proportion is a type of fraction that indicates how many of one type there are in a group of different types. A frequency is a percentage that has been converted to a decimal fraction of which 1 is the highest possible number. All of them are ways of comparing subsets of larger groups.

III. Teaching Suggestions

To help students see that probabilities may not be represented in real families, I ask the students to indicate how many siblings they have and what proportion is male versus female. After explaining that the sperm determines gender by whether it contains a Y or an X chromosome, I encourage students to make the conclusion that gender must be a 50:50 chance. I invariably find someone with a large family that strays far from this probability. I once had a student who came from a family of 18 siblings, two of which were female.

Most students quickly come to the conclusion that a coin toss is a 50:50 proposition. By making that connection to meiosis, I try to help them see that probabilities explain the likelihood of inheriting one or another of the two alleles each parent has. By using two coins, the 1:2:1 and 3:1 Mendelian ratios can be easily reproduced.

IV. Gee whiz

Mendel did his experiments on literally tens of thousands of plants in a fairly small plot of ground no bigger than most of our small classrooms. Each plant was manipulated by hand and careful notes were recorded for each experiment. Knowing nothing about DNA, Mendel was able to use these experiments to accurately determine the results of what would later come to be known as meiosis. He was also able to predict the existence and some of the characteristics of genes and the chromosomes that carry them.