Academic journal article The Science Teacher

Color Code: Using Hair Color to Make a Clear Connection between Genotype and Phenotype

Academic journal article The Science Teacher

Color Code: Using Hair Color to Make a Clear Connection between Genotype and Phenotype

Article excerpt

Students may wonder why they look the way they do, maybe seeing Dad's nose and Mom's eyes when they glance in the mirror. The answer lies in genetics, the branch of biology that deals with heredity and the variation of inherited traits. However, understanding how an organism's genetic code (i.e., genotype) affects its characteristics (i.e., phenotype) is more than a matter of idle curiosity: It's essential for understanding heredity and the processes of mutation and evolution. Yet, students often have trouble making the connection between genotype and phenotype. This article offers a simple way to illustrate that connection--through a discussion of human hair color.


Common Misconceptions

In creating a new generation, parents pass DNA to their offspring. The DNA carries genes that hold the information for the proteins and RNA molecules that will determine the offsprings' traits. We can't see the DNA, RNA, and protein molecules, but we can see some of the traits they produce. It is challenging to work backward, inferring invisible mechanisms from a handful of visible traits. Yet this is precisely what 19th-century Austrian scientist Gregor Mendel did.

Mendel, the founder of genetics, studied generations of pea plants. Peas are self-fertilizing, and Mendel's pea population was largely separate from other peas. Self-pollination leads to a loss of genetic diversity, leaving Mendel with a number of traits for which only two variations were left (e.g., green pea pods versus yellow pea pods). The variations were determined by alleles, which are different genetic variations of a particular gene (e.g., y [for the recessive yellow] and Y [for the dominant green]).

It is common to introduce genetics using Mendel's examples, but this can lead to misconceptions. Consider, for example, a simple genetic cross between two homozygotes (identical alleles): YY x yy = Yy. The offspring, the F1 generation, are all heterozygotes (different alleles). Crossing the F1s among themselves produces the [F.sub.2]s: Yy x Yy = YY, Yy, and yy, in a 1:2:1 ratio (YY, Yy, Yy, yy) of genotypes but a 3:1 ratio of phenotypes, 3 green peas to 1 yellow.

A similar series of genetic crosses cannot be performed with humans (as it would require sibling marriages), but the pattern of inheritance fits many human traits. Yet students sometimes generalize from particular examples and develop misconceptions. A particularly durable one is that all traits have two alleles: one dominant, the other recessive. This belief leads students to infer that alleles are stable and can't change, which, in turn, leads to thinking that genetics has no role in evolution--a conclusion that may have been shared by Mendel himself (Callender 1988). Students with this view imagine that evolutionary mutations are some kind of magical transformation of individuals from one species to another, entirely unrelated to changes in DNA sequence.

We can help students avoid these misconceptions by clarifying the definition of mutation and by expanding students' understanding of alleles and the most common mechanism of dominance.

From peas to hair

Peas may be a terrific organism for the study of genetics, but students naturally find a discussion of human genetics more relevant to their lives. But human genetics is messy and complicated. Yet, there is a way to take advantage of our tremendous genetic diversity and simultaneously address the genotype and phenotype connection: hair color.

Have your students look around the classroom. Does everyone look the same? Does everyone have the same hair color? There are many shades of brown hair, from light brown (blonde) to medium brown to dark brown to a brown that is so dark it looks black (Figure 1). Why is this?

People aren't peas or fruit flies, so students can't just "set up the crosses"--mix the parents together and collect the [F. …

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