The Mystery Of Sickle Cell: Modeling Evolution In Varying Populations

The Mystery Of Sickle Cell: Modeling Evolution In Varying Populations

Subject: Biology
Time: 2-3 class periods (45 minutes each)

High School Biology (Regular, AP, or IB)


Students will use models to simulate how it is possible that sickle cell disease can exist in widely varying frequencies across multiple human populations. Each group will investigate varying factors that would affect allele frequency change and use this information to make predictions about how medicine or technology will influence these frequencies in the future.

Learner Objectives

  • How do genes cause diseases?
  • What factors can create a high incidence of genetic disease linked to recessive genes in a population?
  • How might genetic disorders be affected if it were dominant versus recessive?
  • Are mutations always harmful?
  • When might mutations become advantageous?


You may want to review the concepts of genes, alleles, and chromosomes. Students should also have a basic understanding of genotype vs. phenotype as well as dominant vs. recessive genes. It is also possible to introduce these concepts within the framework of this lesson. 

Compatible With




chrome books



What's Next?


Next Generation Science Standards
  • Life Science
    • [HS-LS3] Heredity: Inheritance and Variation of Traits
    • [HS-LS4] Biological Evolution: Unity and Diversity

Computational Thinking in STEM
  • Modeling and Simulation Practices
    • Using Computational Models to Understand a Concept
  • Computational Problem Solving Practices
    • Assessing Different Approaches/Solutions to a Problem
  • Data Practices
    • Analyzing Data
    • Creating Data
  • Systems Thinking Practices
    • Investigating a Complex System as a Whole
    • Understanding the Relationships within a System

Comments, Feedback, and Quesitons

The Mystery Of Sickle Cell: Modeling Evolution In Varying Populations

Teacher Notes


  • Computer per 1-2 students
  • NetLogo Program 5.6 or higher
  • NetLogo Sickle Cell Model

Some things to keep in mind concerning the NetLogo Sickle Cell Model:

  1. Unless the dispersal rate is changed, individuals do not move around the world, even though it may seem that way. They are mating with other individuals that are in close proximity to them, the offspring are “born” one step away from them. This is what leads to the look that there is actual motion. Keeping them from moving allows the model to better simulate how a mutation might travel locally before it is able to be spread out globally.
  2. Based on what was stated in number 1, a starting population of 50 will probably be too spread out for mating to occur, leading to a population that will likely go extinct.
  3. The default setting for reproductive age is 3, which requires individuals to survive for two rounds before they are able to mate. This is because most deaths from sickle cell disease and malaria would occur before an individual has made it to reproductive age. Due to a historically high mortality when it comes to sickle cell disease, the trait is really only kept around and transmitted by the carriers and not so much from the individuals with the disease. Although within the last 40 years, the average lifespan of individuals with sickle cell has risen from 14 years to now over 50 years.
  4. The model may start lagging if there are too many individuals on the field at one time, this seems to occur when the population starts exceeding 20,000. Although population size may be used as a variable in an experiment, it is best not to have a starting population about 2,000, unless there is heavy selective pressure.
  5. The number of mosquitos increases as selective pressure against normal individuals increases, this is to simulate individuals living in areas of variable mosquito infestation. The mosquitos are there simply as a visual reminder that they are the vectors for the spreading of the malaria protozoa, in the model they are not actually interacting with the individuals.
  6. As the population goes above the starting population number there is a gradual increase in overall selective pressure against all individuals in the population. This allows populations to reach a variable carrying capacity instead of growing exponentially throughout.
  7. All of the variables (sliders) work dynamically within the model and can be changed or manipulated on the fly, showing their immediate effect on the model. As such it is possible for students to set a very large max-generations and actively change variables throughout that run, allowing them to see how those variables affect the simulation. Keep in mind that even the max-generations can be changed on the fly. This is not the case for the proportion-HbA-allele which only shows its influence at setup.

How to use the Sickle Cell Disease Model:



The setup button will populate the simulation based on the variables that you have chosen with the sliders. You should always press this button before you click the go buttons and after you have set up all variables.


This button will start the simulation and the people in the world will start following the rules and behaviors that they are given. The simulation will run until any of the following: the number of generations chosen in the max-generations selector are exceeded by 1 or if all individuals in the world have died. 

If you click the Go button while the simulation is running, then it will pause the simulation.

Go (once)

Same as go except it only progresses the simulation by one generation each time it is pressed.




This sets the rule for when an individual is able to reproduce. People are born with an age of one and each generation that they survive, 1 year is added to their age. If you put the slider at one, then all individuals will be able to reproduce the same generation that they are born. 


This allows you to select what percentage of the hemoglobin alleles are “HbA”. Keep in mind that HbA is the normal hemoglobin allele while HbS is the mutated form of the allele that can lead to an individual being born with Sickle Cell Disease or being born as a carrier of the disease.


This allows you to choose the starting population size. Populations can grow well past their starting size depending on variables selected, selective pressure will increase as the population increase.


This allows for selective pressure to be directed against different phenotypes. Setting the slider at 100% will kill all individuals expressing that phenotype.


This controls how far individuals move from their initial position. If you choose 0, then the individual will not move at all. It may look as if people are moving when the model is running, but the motion you are seeing are individuals being born or dying. They will become mobile if the slider is set greater than 0.


Choose how many ticks/generations you want the model to run for before it stops. Setting it to zero, will allow it to run indefinitely or until you stop it.


When you press the mutate button it will add several individuals that are carriers of the HbS allele at a random place in the simulation. The number that is selected with the number-mutated slider will allow you to specify how many of those individuals will be added.


When you press the mutate button it will add several individuals that are carriers of the HbS allele for sickle cell disease at a random place in the simulation. The number that is selected here will allow you to specify how many of those individuals will be added.

HbA to HbS mutation rate

This controls the mutation rate from normal HbA allele to sickle cell causing HbS allele. While set at zero, HbA alleles will never mutate to HbS alleles. When moved closer to 1, the mutation may occur more frequently.

David S. Goodsell, the Scripps Research Institute.
Figure 1

By The National Heart, Lung, and Blood Institute (NHLBI)
Figure 2

By BruceBlaus (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)]
Figure 3


Figure 4


These are resources found in the student activity but may not be available to them due to YouTube restriction. You may want to use them for whole class discussion.

Video 1
Video 2