1. Note to instructors: This worksheet represents a way that I have taught this material, which incorporates figures created by others. I have cited these figures’ sources, but I have not formally obtained permission to use the figures in this way. As far as I’m concerned, you’re welcome to modify this worksheet or use it as is; if you do so, please continue to cite the sources of these figures – and be aware that the figures’ inclusion here may or may not be permissible under “fair use” doctrine. --Greg Crowther, Everett Community College

2. Why is hair straight or curly?
Background Hair, like nails and the stratum corneum of skin, consists mostly of the protein keratin. Keratin, like all proteins, is composed of amino acids together. One of the 20 naturally occurring amino acids is cysteine. It is unique among the amino acids in that it contains a sulfhydryl (-SH) group at the very end of its “R group” (see below). This enables pairs of cysteines to form disulfide bridges within a protein (see below).
Images: magazine.lorealprofessionnel.co.uk, quora.com, slideshare.net/abahnassi/protein-structure

3. Figure: P. Strnad et al. (2011),
Q1. Proteins can be analyzed to determine their amino acid composition. If each amino acid was equally common in proteins (which isn’t actually true), what percentage of the total amino acids would be cysteine? Q2. Within the human body, there are many forms of keratin, some found in hair and some not. One of the hair keratins is K81. It is 505 amino acids long. 34 of these amino acids are cysteines. What percentage of the amino acids in this protein are cysteines? (You may use a calculator.)
Q3. The figure at right shows the frequency of cysteine in various forms of keratin. According to these data, which type(s) of keratin is/are likely to contain the most disulfide bridges? Why?
Figure: P. Strnad et al. (2011),
J Cell Sci 124:

4. It turns out that disulfide bridges help hold keratin in a curled or kinked position, as shown in the cartoon below.
Q4. Explain why either intense heat or chemicals can be used to straighten curly hair.
Image: youtube.com/watch?v=oCZva_6abFM

5. At least 1 parent with curly hair
So why is some people’s hair much curlier than others’? Let’s first consider the general hypothesis that this is a genetically controlled trait.
# of students
At least 1 parent with curly hair
Straight hair
Wavy hair
Curly hair
Q5. Do these data suggest that the curliness of hair is genetically controlled?

6. If hair curliness IS genetically controlled, let’s consider the following specific hypothesis: Hypothesis: People with curly hair inherit keratin genes which contain more cysteines, so their keratin can form more disulfide bridges. To evaluate this hypothesis, we will use the fact that, on average, Africans tend to have curlier hair than Asians, with Caucasians somewhere in between. Q6. Do the data in the table support this hypothesis?
Data:

7. Take two strips of paper.
Now we will consider an alternative hypothesis: the shape of the follicles determine the cross-sectional shape of the hair, which determines the curliness of the hair.
We will explore this hypothesis by making models or round hair and flat hair out of paper.
Take two strips of paper.
Make one strip into a round cylinder, using tape.
Keep the other strip as is to represent extremely flat hair.
Figure: bellaroshe.com/knowing-your-hair-101-disulfide-bonds-and-follicle-shape-part-3/

8. Q7. Which hair model – the round one or the flat one – is easier to bend into a wavy pattern? Q8. Which hair model – the round one or the flat one – is easier to twist into a corkscrew shape? Q9. Which 3D configuration – a hair that is straight, or a hair that bends or twists back on itself – would bring the most cysteines near each other, so that they could form disulfide bridges? Q10. Based on the above, explain why a more oval or flattened cross-sectional shape leads to curly hair with lots of disulfide bridges. Q11. Based on all of the above, speculate as to what is being controlled by the gene or genes for hair shape. (Hint: is it the amino acid sequence of keratin, or is it something else?)