Investigate Alien Genetics

Introduction

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?

Materials

• Alien Genotype Table and the Alien Phenotype Table
• Construction paper, different colors (orange and green must be included)
• Scissors
• Tape
• Glue
• Markers, crayons, or coloring pens
• Pencil
• Two coins

What Happened?

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.

Digging Deeper

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 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?

Related Resources

Project Ideas

Activities

Lesson Plans

Links

• Nature Education: Mendelian Genetics: Patterns of Inheritance and Single-Gene Disorders
• Scientific American: Identical Twins' Genes Are Not Identical
• Palomar College: Probability of Inheritance

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