Toothpick Fish Lab Background information

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Toothpick Fish Lab

Background information:

In this lab activity, a population genetics simulation, you will observe and record the genotypic and phenotypic make-up of a fish population, which change in response to environmental conditions and an event that changes these conditions. Events similar to the catastrophic event in this activity (vegetation dying because of pollution) could happen in real streams in the real world. This activity provides a good synthesis of basic genetic concepts with a focus on the environment and natural selection.

Materials (for each pair):

• 1 “gene pool” container (e.g. a petri dish)

• 8 green toothpicks

• 8 red toothpicks

• 8 yellow toothpicks

The colored toothpicks represent three different forms of a gene (green, red, and yellow) that control one fish trait: skin color. The table below tells you which forms (alleles) of the gene are dominant, which are recessive, and which are equal (or co-dominant).

The green gene (G) is

• dominant to all other color genes

The red gene (R) is

• recessive to green

• equal (“co-dominant”) to yellow *

The yellow gene (Y) is

• recessive to green

• equal (“co-dominant”) to red *

* Combining red and yellow genes results in a fish with orange skin color.

  1. Count your toothpicks to make sure you have 8 of each color for a total of 24 toothpicks.

  1. Figure out which gene combinations (genotypes) give rise to which fish colors (phenotypes) and fill in the answers on the table below. The genotypes have been provided for you.

Fish Color (phenotype)

Gene combinations (genotype)





Based on the answers you gave in the table above, answer the questions below. (You may use Punnett Squares if you wish.)

a. Can two red fish mate and have green offspring? Why or why not? ________________________________________________________________________________________________________________________________________________________________
b. Can two orange fish mate and have red offspring? Why or why not? ________________________________________________________________________________________________________________________________________________________________
c. Can two green fish mate and have orange offspring? Why or why not? ________________________________________________________________________________________________________________________________________________________________

  1. Make a first generation of fish. To do this, pull out genes (toothpicks) in pairs without looking and set them aside carefully so that they stay in pairs. This simulates the way offspring are formed by sperm from the male fish combining randomly with eggs from the female fish.

  1. Once you have drawn your twelve pairs, record the results for the 1st generation in Table A on page 5. An example fish in the first generation is given in Table A in the shaded boxes (do not include this fish in your calculations).

  1. Count the numbers of each color of fish offspring and record the numbers in Table B on page 5 where it says first generation.

  1. STOP: IMPORTANT! The stream where the fish live is very green and lush with lots of vegetation and algae covering the streambed and banks. The green fish are very well camouflaged from predators in this environment and the red and orange fish fairly well also. However, none of the yellow fish survive or reproduce because predators can easily spot them in the green algae environment. If you have any yellow fish (fish in which both toothpicks are yellow), set those toothpicks aside (they will no longer be used in this lab).

  1. Put all the genes you have left back in the gene pool (remember, you have set aside any yellow fish).

  1. Draw a second generation of fish, again without looking. Record your gene pairs and colors in Table A.

  1. Total up the fish of each color and record the numbers in the second generation row in Table B.

  1. Set aside anymore yellow fish and then return surviving fish to the gene pool.

  1. The well-camouflaged fish live longer and have more offspring, so their numbers are increasing. Draw toothpicks to make a third generation of fish. Record your data in Table A.

  1. Now write in the total numbers of each color in the third generation row of Table B.

  1. Once again, set aside genes from yellow fish and return survivors to the gene pool.

  1. Discuss and answer the following three questions with your partner before determining the fourth generation.

  1. Have all the yellow genes disappeared? _______________________________________

  1. Has the population size increased or decreased? Would you expect this to occur in the

wild? Explain your answer. ________________________________________________________________________________________________________________________________________________

  1. How does the population color in the third generation compare to the population color in the earlier generations? ________________________________________________________________________________________________________________________________________________

  1. Draw more pairs of genes to make a fourth generation of fish. Record the data in Tables A and B. Do not remove yellow fish this time!

  1. STOP! An environmental disaster occurs. Factory waste harmful to algae is dumped into the stream, killing much of the algae very rapidly. The remaining rocks and sand are good camouflage for the yellow, red, and orange fish. Now the green fish are easily spotted by predators and can’t survive or reproduce.

  1. Because green fish don’t survive, set them aside.

  1. Now record the surviving offspring (all but the green) in the last row of Table B (fourth generation survivors’ row).

  1. Contribute your final data (from 4th generation survivors) to the class tally on the board. Your teacher will total the data for the entire class.

After examining the data for the entire class, discuss and answer the following questions with your partner.

  1. Has the population size changed compared to earlier generations? How? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________

  1. Have any genes disappeared entirely? If so, which ones? ______________________________________________________________________________________________________________________________________________________

  1. Yellow genes are recessive to green; green genes are dominant to both red and yellow.

Which color of genes disappeared faster when the environment was hostile to them? Why? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________


  1. Fish grown in a hatchery (fish farm) often have less genetic variation than wild fish populations.

How might this affect the ability of hatchery fish to adapt to an environmental disaster such as pollution? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________

  1. If the fish from a particular stream have become genetically adapted to their home stream over many generations, what might happen if their fertilized eggs are used to “restock” a different stream that has a different environment? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________

  1. Real populations change much more slowly than these toothpick fish. Why? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Toothpick Fish Data
Table A. Gene pairs and resulting fish color in Generations 1-4

1st Generation

2nd Generation

3rd Generation

4th Generation































Table B. Offspring color for Toothpick Generations

Lots of green seaweed growing everywhere

Seaweed all dies leaving only rocks and sand

1st Generation

2nd Generation

3rd Generation

4th Generation

4th (Survivors)

Class Totals





For Teacher

Table C. Fish surviving the pollution disaster: class pooled data

Fish Color


Red (RR)

Orange (RY)

Yellow (YY)


Fill in table on overhead, one line of data per group. Total results in bottom line.

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