Have you ever wondered why biological siblings look so much alike? Why do they often share physical traits, such as hair color or eye color? It all has to do with their genes. Genes are passed on from parents to their offspring. This means that you share some of the same genes with your biological parents and siblings! In this activity, you will use an alien model to demonstrate how genes or physical traits are passed on from parents to their offspring. How similar will your alien siblings look?
- Alien Genotype Table and the Alien Phenotype Table
- Construction paper, different colors (orange and green must be included)
- Markers, crayons, or coloring pens
- Two coins
Note: You can also draw the alien babies instead of building them from construction paper, in which case, the construction paper, scissors, tape, and the glue are not necessary.
Print out the Alien Genotype Table and the Alien Phenotype Table.
Read the Digging Deeper section to become familiar with the basic concepts of genetics.
Review the Alien Genotype Table and look at the different traits of the alien. Answer the following questions to make sure you understand the table correctly.Based on the table, is green skin color a dominant or recessive trait? How many toes would an alien that has a dominant toe gene variant have? Looking at the alien mom’s finger genotype, how many fingers does she have?
Look at the genotypes of both parents.What do you notice when looking at the genotypes of both parents for all traits?
Fill out the phenotype table for both of the alien parents.Based on their genotypes, what do the alien mom and alien dad look like?
Draw the alien mother and the alien father based on their phenotypes.
Now it is time to create an alien baby from these two parents. Take the two coins. One coin represents the alien dad, the other coin represents the alien mom. Toss the coin for each trait to find out which of the two alleles is passed on to the alien baby. Heads means the dominant allele is passed on, tails means the recessive allele is passed on.
After each coin toss, fill in the resulting genotype for the alien baby in the Alien Genotype Table. An example table is shown here.What alleles did the alien baby inherit from its mom? Which ones from its dad?
Once you are done with all traits, fill out the baby’s phenotype in the Alien Phenotype Table. An example table is shown here.Can you already tell from the table if the alien baby looks similar to or different from its parents?
Draw or create your alien baby from the construction paper. The phenotype of the baby alien tells you how the baby alien looks. You can choose any traits that are not listed on the table yourself, such as length of arms or legs, eye color, etc.. An example alien baby is shown here.How does the alien baby look? Which traits does it share with its parents?
In the Alien Phenotype Table, mark all the traits that the baby does not share with its parents.Does it have any traits that the parents do not have?
Create an alien baby sibling the same way as you did before.How different are the alien siblings from each other?
Draw a Punnett square that shows the likelihood of the alien baby to inherit a specific trait from its parents. Review the Digging Deeper section to see an example of a Punnett square.What is the likelihood that the alien baby will have the same phenotype as its parents for a single trait? What is the likelihood that the alien baby will show a different phenotype?
You probably noticed from the genotype table that both alien parents share the exact same genotype. They are both heterozygous for all traits, which means they have both a dominant and a recessive gene variant (allele) for each trait. The alien baby receives one gene variant from each parent. Because it is random which gene variant is passed on, there is a 50/50 chance of each gene being passed on to the alien baby. This is why you used the coin to determine which genes were passed on from the alien mother and father; for each coin toss, there was also a 50/50 chance of getting heads or tails.
When you created your alien baby, it probably looked pretty similar to its parents. How many traits the alien baby shares with its parents depends on your coin tosses. To change the phenotype of the alien baby for one trait, it has to have two recessive gene variants for that trait. This is only the case if you toss tails for both of the alien parents. If you toss heads for either one of them, the dominant gene variant takes over again and the alien baby shares the trait with its parents.
When you created a sibling for the alien baby, you might have noticed that although both baby aliens came from the same genetic material, the siblings do not look the same. They share traits, but they also have different traits from each other, just like biological siblings in real life. Most likely, each of the alien babies show some traits that the parents have and some that they do not have. If you drew the Punnett square, it should have shown that for each single trait there is a 75% chance of the baby having the same phenotype as its parent and only a 25% of having a different phenotype.
It was Gregor Mendel, a monk and scientist, who first discovered in the 1860’s that some traits are passed down from generation to generation in very clear and predictable patterns. Today, we know that offspring inherit half of their DNA from each parent. Thus, our genome contains two copies of every gene (one copy from the mother and one copy from the father). Many genes come in several different versions, called alleles. Alleles, or gene variants, arise when the DNA sequence of a gene is changed due to mutations. This means that no two people have the exact same set of genes (identical twins come the closest, but even they have differences. Siblings, who originate from the same genetic material (their parents’ DNA), share some traits, but others are different. This is because each parent has two copies of every gene and either copy can be passed on to their offspring. Which gene a given offspring will get is random, and thus can vary from sibling to sibling. As a result, each of the siblings will only share part of their genes, and thus traits, with the other siblings.
When you have inherited two identical alleles (or gene variants), you are said to be homozygous for that gene. People with two different alleles are heterozygous for that gene. The set of alleles a person has is called their genotype. A genotype determines the phenotype, the observable characteristics (or traits) that the genotype codes for. Some traits, called Mendelian traits, are due to a single gene. Two examples of Mendelian traits are a cleft chin or face freckles. In genetics, scientists often abbreviate such traits with letters. These letters are often chosen so they relate to the trait, like the letter F to represent face freckles. Sometimes a gene only has two alleles, one of which is dominant and the other recessive. If you have just one copy of a dominant allele, you will display that trait. Scientists denote a dominant allele by a capital letter (F versus f). You need two copies of a recessive allele to display that trait. Recessive alleles are usually denoted by a lowercase letter (f versus F). Because alleles are randomly assigned during sex cell production, offspring can end up with different combinations of alleles relative to one another. If you know the alleles of the parents, you can predict the probability of an offspring having a particular set of alleles. For Mendelian traits, scientists use a Punnett square diagram to visualize the alleles of parents and their offspring (Figure 1).
Figure 1. A Punnett square visualizes the possible combinations (green) of maternal alleles (red) with paternal alleles (blue). In this example, F is the dominant allele for face freckles, whereas f represents the recessive allele for no face freckles.
Figure 1 shows that parents with a dominant phenotype can have offspring with either the dominant or the recessive phenotype. Scientists can use these rules of inheritance to examine generations within a biological family and discover the mode of inheritance for a specific human trait. To do this, scientists create family trees, called pedigrees, showing as many generations of a biological family as they can and marking who has which phenotype. Human pedigrees are also a powerful screening tool for certain diseases. Exactly like physical traits, genetic disorders of human beings that originate from a gene defect can be dominant or recessive. By examining a pedigree for where genetic diseases arise, scientists can deduce how the condition is inherited.
For Further Exploration
- Create another generation of aliens from one of the babies you made and another alien with a random genotype. Then make a pedigree of their alien family and trace individual traits through the different generations.
- Introduce a random gene mutation in a baby alien, which would result in a new trait that has not been there before (e.g. blue color, three eyes, etc.). Then create more generations of aliens and find out how the mutation gets passed on.
- Change the genotypes of the parent aliens. What happens if one parent has only recessive or only dominant gene variants for all traits