Why Do We Look Different?

Changes in DNA are mutations which create variation between different organisms. When mutations happen in sex cells (sperm and eggs), they may be passed on to future generations and influence natural selection; mutations that occur in body cells may affect the cell or the entire organism.


DNA doesn’t always stay the same. Often, there are changes that happen to the DNA inside of a cell because of asbestos, cigarette smoking, ultraviolet radiation, or just random chance. These changes to DNA are called mutations. Some mutations in DNA are harmless and cause no problems for the organism or its offspring. Many mutations are harmful and can cause cancers in the organism or birth defects in offspring. Even other mutations cause the death of the cell because it can’t survive any more.


There are three main types of mutations: substitutions, insertions and deletions. Substitutions are mutations where one base is substituted for another, such as G for A. These can often be harmless because the protein that the gene ends up producing can be exactly the same.

If a gene suffers from an insertion mutation, then the entire gene


can be affected or even destroyed. An insertion is when one or more bases are inserted into the gene and it shifts all of the codons down by one or more bases.


Lastly, a deletion mutation is when one or more bases are removed from the gene. This again can destroy the entire gene because it can shift all of the codons up by one or more bases.

When any of these mutations happen in a body cell, they only affect the organism itself. However, when these mutations happen in a sex cell, they can affect the offspring. This is one of the key concepts behind natural selection – yes, back to evolution! See, if it weren’t for mutations, there would be no new genes, and all life would look just like the first, simple one-celled bacteria.

Different eye colors

Mutations are the source of new genes: it’s thought that all humans started off having brown eyes. A mutation in the gene for eye color caused some humans to have blue eyes. In the bright sun of Africa, it made no sense to have blue eyes, which are more sensitive to light. But when humans immigrated into Europe, which receives less direct sunlight, individuals with blue eyes were more fit and survived to reproduce more than the brown-eyed individuals. In fact, the emigration from Africa would have been impossible without mutations to the genes for skin color, hair type, digestion of different foods, and more!

However, the only way that these mutations were passed on from generation to generation is that the initial mutation happened in either a sperm or egg cell. If the gene for eye color had changed in a body cell, that only would have affected the individual – not its offspring!

Even though most mutations result in offspring that don’t survive to reproduce, the “good” mutations more than make up for the “bad” ones. These mutations that take hold in a population cause the genes of the population to change. To continue our example, when Africans first immigrated to Europe, almost nobody had blue eyes, and these individuals were limited to the southernmost areas of the continent. However, as time went on and the mutation for blue eyes spread through the population, the percentage went up; in some areas in northern Europe, 100% of the population had blue eyes. This change over time in the percentage of a particular gene in a population is called genetic drift. As you can see, peoples’ genes “drifted” from brown to blue eyes over time.

1. How can you avoid mutations that can cause cancer?
2. Describe genetic drift in your own words.
3. Differentiate a substitution, insertion and deletion.
Put it together
4. Summarize the relationship between mutations and natural selection.
5. Why is it that mutations in body cells do not affect offspring?
Think about it
6. Predict a mutation in humans that will spread through the population over the next fifty years. What is the mutation? Where did it start? How is it advantageous?
7. What is the role of chloroplasts?
8. Identify five organ systems and the problems that they solve.
9. What does dominant mean? Recessive?
10. Define a sex-linked trait in your own words.
Blood Typing

For this activity, you will be determining the possible blood types of individuals. What you need to know about blood types is that there are four major types, A, B, AB and O. Alleles A and B are co-dominant, meaning that they are equally dominant. The recessive allele is O. The chart below shows the possible genotypes and phenotypes for the ABO blood groups:

Genotype Phenotype
  1. Identify the genotypes for John, Harry, Howie, and Len
  2. Complete a Punnett square between Bob and Melanie. What must both Bob and Melanie’s genotypes be in order to have Howie? Remember that everyone gets one characteristic (letter) from their mother and one from their father. You may have to try different genotypes for Bob (who could be AA or AO) and Melanie (who could be BB or BO).
  3. Use the same process that you used in #2 to figure out what Claire’s genotype must be.
  4. What are the genotypic and phenotypic possibilities for Ron? In other words, what are all of the possible genotypes (two letters) and phenotypes (blood types) for Ron? Do Punnett’s Squares for each combination of genotypes that you think is possible.
  5. What is the probability that Bob and Melanie have a child who has AB blood?
  6. Tom P. Son faces charges in a paternity suit brought by Mary H. Lamb. Tom is blood type AB, Ms. Lamb is blood type O. The child is blood type O. Could Tom be the Father? Explain.
  7. Two individuals of homozygous type A and homozygous type B marry and have offspring. Describe the offspring in the first generation.
  8. A child has a blood type of AB, and the mother has a blood type of A. What are the possible genotypes of the father? Could the father be type O? Explain!!
Mutations and Punnett Squares

In this activity, you will explore what happens when a mutation occurs in a population.

  1. Make up a human trait. It can be silly, fictional or real. Write down the dominant version of this trait.
  2. Write down the opposite version of this trait, and let’s assume that it’s the recessive trait.
  3. What letter will you use to represent this trait? [Hint: Always use the first letter of the dominant trait.]
  4. Complete a Punnett’s square for two homozygous dominant people. What will the offspring look like?
  5. Suppose that there’s a nuclear accident and radioactive spiders bite 10 people in Cleveland. This causes a mutation and one allele (trait) becomes recessive for these people. That causes these mutants to be heterozygous. Complete a Punnett’s square between a normal (homozygous dominant) and mutant.
  6. What are the chances of the offspring being a mutant?
  7. Suppose that two mutants have children. Complete the Punnett’s square between two mutants.
  8. What are the chances that these offspring will show the mutation?
  9. What are the chances that they’ll be a carrier for the mutant allele (trait)?
  10. Summarize what has to happen for a recessive mutation to actually show up as a physical trait in a population.
  1. Pedigree 1

    Look at pedigree 1. It shows males (squares), females (circles), and the individuals who have the trait that we’re studying are shaded in.

    1. How many males? Females?
    2. How many have the trait? How many do not?
  2. Pedigree 2

    Pedigree 2 is for a recessive trait. This means that the individual who is shaded in shows the recessive trait.

    1. Using the letters “A” and “a”, write the possible genotypes of each individual next to their shape. You will notice that for the male child, there is more than one possibility! Hint: Start with what you know for sure!
    2. Show the Punnett’s square for the two parents, with percentages.
  1. Pedigree 3

    In pedigree 3, two generations have been skipped by the recessive trait. With a pen or pencil, trace the path of the recessive allele from the 1st generation to the fourth.

    1. What does this line tell you about the genotypes of these individuals?
    2. What can you conclude about recessive traits skipping generations?
Blood Type Problems
  1. Tom P. Son faces charges in a paternity suit brought by Mary H. Lamb. Tom is blood type AB, Ms. Lamb is blood type O. The child is blood type O. Could Tom be the father? Explain, using the appropriate Punnett’s Square.
  2. Two individuals of homozygous type A and homozygous type B marry and have offspring. Describe the genotypes of all of the offspring in the first generation. Show the appropriate Punnett’s Square.
  3. A child has a blood type of AB, and the mother has a blood type of A.
  • What are the possible genotypes of the father? Show the Punnett’s Squares.
  • Could the father be type O? Explain!!