GENETICS LESSON I: Genotype and Phenotype

Key Concepts:
Genotype/Phenotype
Dominant/ Recessive
Environment vs. Inheritance

Overview:
10 minute introduction followed by (3) 10 minute stations: a) looking at c. elegans under light microscopes b) looking at drosophila with hand lenses, and c) looking at our taste buds

Materials Needed:

• Introduction:
  Coins with R on one side and or r on other side
  
• Drosophila station:
  Flies
  Hand lenses
  Cards of the drosophila life cycle
  
• Nematode station:
  Microscopes
  Worms
  
• Tastebud station:
  PTC paper
  Sodium benzoate paper
  Cards showing taste buds and areas of the tongue
  Q-tips
  Small cup for food coloring
  Blue food coloring
  Flashlights

Introduction

Today’s class will begin with the discussion of the inheritance of traits. A trait is a characteristic of a person, such as blood type or eye color. Ask students if they have heard of genes (not the pants). Genes are a unit of DNA that has the information that determines our inherited characteristics. Where do we get our genes? Remind students that we receive one copy of each gene from each of our parents, therefore, we have two copies of every gene.

Simple Traits
Start by discussing the inheritance of simple traits, those that are determined by single genes. Examples of simple traits include: the ability to roll one's tongue, having attached earlobes, and the ability to bend ones thumb back (hitchhiker's thumb)(see appendix, page 18). Ask students "Who can role your tongue?" Count rollers vs non-rollers. If time, can do same for other traits.

One of the simplest traits to examine is the ability to roll ones tongue. It is believed that the ability to roll one's tongue is due to the presence of a certain muscle in the tongue. If you have that muscle, you will be able to roll your tongue. Give the example of the gene "R" that specifies the presence of the muscle in the tongue. If a person has one or two copies of the "big R" gene then he/she will be able to roll their tongue.(genotype RR and Rr both role tongue). If he/she does not have this gene and instead has only two copies of the recessive "little r" gene (rr) then he/she will not have the ability to roll their tongue. The big R form of the gene is said to be dominant to the little r form because individuals who are Rr do make the muscle and can roll their tongue. You can use this explanation to remind them of the difference between a genotype and a phenotype.

Complex Traits
Besides traits that are determined by single genes, some of our traits are much more complex, involving many genes or interactions between genes. For example, hair color is determined by at least four genes, each one coding for the production of melanin, a brown pigment. Because the effect of these genes is additive, hair color can range from blond (little melanin) to very dark brown (much melanin). Ask students "What else can determine a trait?" Some traits such as height are determined primarily by our genes (80%) but are strongly influenced by the environment.

Environmental Factors
Ask the students, “What environmental factors could influence our height?" If no answers, ask “Does anybody drink milk?” Ask why? (discuss diet) Explain how height is an example where both genes and the environment influence the phenotype. People now are generally taller than they were 200 years ago. Why do you think that is? Ever visited a home built 200 years ago? The ceilings are generally lower than they are today. Did people then have access to the abundance of food we find in the grocery stores today?

Activity: The tongue rolling trait follows the Mendelian pattern of inheritance. Let's illustrate. Ask for two volunteers to come to the front of the room and give them each a coin with the allelic forms R or r written on each side representing a hypothetical genotype for the tongue rolling gene - don't tell the class what the given genotypes are. The students should flip the coins and the results of each toss should be written on the board; the rest of the class should help decipher what the phenotypes of the "offspring" are and based on the phenotypes what the genotypes of the “parents” could be.

Workstations: Divide children into three groups, they will spend about 10 minutes at each workstation.

1. Nematodes
Dissection Microscopes should be set up for the students to look at wild-type worms as well as those with different phenotypes, such as dumpy and rollers. Each dish has agar on bottom with bacteria spread on top (circle). After the worms are plated they migrate towards the food. Show students how to adjust microscopes (eye piece, course and fine adjustment). After the students look at the worms they may be interested in some simple Nematode trivia.

Worm Phenotypes

• Egl: egg-laying deficiency. The worms cannot form vulvae, so the eggs hatch inside the adults and eat it from the inside out. You can find the characteristic “bag o’ worms” phenotype, which is the shape of an adult worm but filled with wriggling babies.
  
• Dpy: dumpy. These worms have been described as "cute." Actually, worms are encased in a wrapper material (cuticle), much like sausages in casings. Worm shape is determined by the shape of the casing they are stuffed into. The collagens that make up the cuticle can be mutated to generate shorter and fatter "casings," leading to Dpy worms.
  
• Unc: uncoordinated. Mutations affecting myosin chains in the body wall muscles causes the worms to be uncoordinated (severely paralyzed).

A quick google search can give you information on other phenotypes. More information can be found at www.wormbook.org

Nematode trivia:

1. Nematodes live in the soil and are so abundant there that the only place on earth one can pick up a handful of soil without a nematode in it is in Antarctica.

2. Nematodes help fertilize the soil by eating bacteria produced from dying plants and animals and excreting the waste products back into the soil.

3. There are many different kinds of Nematodes, many of which are benign but some are parasitic such as the ringworm, heartworm, and tapeworm.

4. Nematodes are hermaphrodites meaning that one animal has both female and male parts; because the nematode can self-fertilize, it's offspring are genetically identical to itself. Sometimes male offspring arise when a one X chromosome is lost during gamete formation. They are smaller than the hermaphrodites and have a hook like structure on one end.

Note: Some of the students may have a fear of worms. Explain that the worms are under the plastic lid and that it is safe to touch the dish. If some of the students finish looking at the worms before the end of the rotation, have them count the worms, they can trade dishes with other students to see who can count the most worms.

2. Drosophila
The students should look at several different fly vials containing flies with different phenotypes. Ask about body parts-how many legs, etc.. Distinguish the different sexes of Drosophila. In males, the lower tip of the abdomen is quite dark and considerably lighter in females. The tip of the abdomen is more rounded in males than in females. Interestingly, the male is smaller than the female. Examine the stages of the Drosophila life cycle present within a vial: egg-larvae-pupae adult. In advanced pupae, you can start to see body parts. Introduce the term metamorphosis. Let the students determine which flies have white or red eyes or curly wings, etc.

Potential topics for generating conservation:

1. Why are these called fruit flies? Have the students ever seen them at home around rotting fruit?

2. The flies eat the fruit or the mold on the fruit and lay their eggs in it.

3. Ask students about other insects that have similar life cycles? (e.g. butterflies/moths)

4. Are males or females smaller? (answer:males)

Note: The smell of the Drosophila media may not be appreciated by all of the students. Be ready for some reluctance to touch the vials.

3. Heritable Traits and Taste Buds
(Place this workstation as close to a sink as possible)

Start by asking, "Does everyone taste food the same way"? If not, why? Examine this issue by having students see if they can taste anything when they place a piece of phenylthiocarbamide (PTC) soaked paper on their tongues. Hand out the taster paper but have the students wait until everyone has a piece before putting on their tongues. Because it will taste very bitter to some students, allow them to rinse their mouth with water at the sink. Ask students how the paper tasted. Most will taste the PTC paper but there will be some students who do not. Make the other students aware of the student who is unable to taste the PTC paper. Ask why?

People differ considerably in taste sensitivity. Phenylthiocarbamide (PTC) tastes bitter to about 7 out of 10 adults but less is known about children. A structure on the tongue containing taste buds is called a fungiform papillae. The number and size of the fungiform papillae affects the ability to taste the chemical on the paper. In adults, people who taste less, have larger and fewer papillae whereas strong tasters have many more smaller papillae.

Have ready a small bowl with blue food coloring, Q-tips, flashlights and mirrors. Take a clean Qtip and absorb a small amount of the food coloring. Roll it on the end of the students' tongue. After a minute, shine light on the tongue. Papillae don't absorb food coloring so they look pink on a blue background. Give them a flashlight and mirror to see for themselves. Make sure to keep control of the food coloring, otherwise, some children may start painting their whole tongue, etc. Allow students to rinse their mouth if they would like. Food coloring is not harmful if swallowed.

At the end of the session, give the students sodium benzoate tasting paper. Most students will be able to taste the paper (including non-PTC-tasters, which will make it more fun for them), but their reactions will differ. Some will think it is sweet, while others will say it is salty or bitter. Show the chart of the the tongue and discuss different tastes, i.e. sweet, salty, bitter, and sour.

Note : some children do not want to participate—that is just fine. No one should feel pressured to participate.

Also, try to have a non-taster run the taste bud station. The students are rather reluctant to taste the PROP strips if the leader do not taste it with them. However, if the leader running this station is a taster, the repeated exposure tends to become disgusting! Try to rotate the leader running this station. After painting the students’ tongues, discuss other heritable traits such as eye color, tongue rolling ability, hitchhikers’ thumb, etc.