Solving genetics problems I
This assignment is worth 10 points. It does not require that you purchase any materials.
Important note: Do not attempt this activity until after you have completed the lessons on Mendel, laws of inheritance, monohybrid crosses and Punnett squares (Lessons 8A-8C).
• Be able to use Punnett squares to solve single-trait, two allele genetics problems involving complete dominance.
Human genetic traits
Figure A8-1: A demonstration of tongue rolling.
Can you roll your tongue into a tube, like the child pictured above? If so, you are in the majority. About three quarters of the population can roll their tongues, whereas one quarter cannot.
Figure A8-2: A demonstration of hand clasping. This woman favors the right thumb on top.
Now, bring your hands together such that your fingers are interlocked and one thumb is on top of the other. Which thumb is on top? Now try doing it with the other thumb on top. Feels weird, doesn’t it? Almost everyone has a strong preference for one way or the other. There does seem to be some genetic basis for the trait, but it is unclear how it is controlled or passed on to offspring.
Many introductory biology classes use human features to study inheritance of traits. They tend to focus on physical traits that are easily seen, such as presence or absence of dimples or freckles on the face, attached versus unattached ear lobes, eye color, hair color, tongue rolling, hand clasping, etc. There are dozens of traits that one can find that vary from person to person. However, recent studies have shown that although many of these traits may have at least some genetic basis, almost all of them are not controlled by a single gene with just two alleles. That means that they should not be used as examples in simple Mendelian genetics problems.
The fact of the matter is that most physical human traits are controlled by several different genes (not just one!), show continuous variation with intermediate forms, and are impacted by environmental conditions. Consider eye color. Several different genes interact with one another to create a whole range of pigmentation. In other words, there are not just blue eyes and brown eyes.
That said, there are some human genetic traits that do follow the simple one gene, two allele Mendelian model. Sickle cell disease, Tay-Sachs disease, and cystic fibrosis are all recessive genetic disorders that follow this simple model. One allele is completely dominant over the other allele, so inheritance is easily predicted for offspring. Keep in mind, however, that most human traits do not follow the simple Mendelian model.
If you would like to learn more about the human genetic trait controversy in biology education, please visit the following website by Dr. John H. McDonald at the University of Delaware:
• Myths of human genetics
You will solve genetics problems based on three different traits, all of which are not in reality controlled by genes in the human body. Nevertheless, you will likely relate to them as being traits that you observe in yourself and others. Keep in mind, however, that we are only pretending that they are controlled by single genes with just two alleles, and that they show complete dominance. The main purpose of this activity is to practice solving simple Mendelian genetics problems, thus using fake genetic traits is both practical and appropriate.
Our fake traits:
1. Channel surfing syndrome. Do you tend to watch one specific thing on TV at a time, or do you surf the channels to see what else is on without really watching one thing from beginning to end? If you are one that likes to surf the channels, then you are a channel surfer, whereas if you do not tend to surf the channels, or do not watch TV at all, then you are a non-channel surfer.
Let’s suppose that channel surfing is a trait controlled by a single gene with just two alleles, “C” and “c,” and that the allele “C” (big “C”) is completely dominant over the allele “c” (little “c”). Thus, CC and Cc are the two genotypes that code for the dominant phenotype, channel surfer, whereas cc is the genotype that codes for the recessive phenotype, non-channel surfer.
2. Starbucks trait. Do you visit Starbucks often, perhaps several times during a typical week? If so, then you are a Starbucks fan. Do you rarely go to Starbucks, if at all? Is it your last resort when it comes to getting coffee? That would make you a Starbucks avoider.
Let’s suppose that being a Starbucks fan is a trait controlled by a single gene with just two alleles, “R” and “r.” The allele “R” (big “R”) is completely dominant over the allele “r” (little “r”). Thus, RR and Rr are the two genotypes that code for the dominant phenotype, Starbucks fan, whereas rr is the genotype that codes for the recessive phenotype, Starbucks avoider.
3. Social media trait. Do you check your cell phone for texts, tweets, instagrams, and/or Facebook updates every chance you get, all day long? Do you respond to texts almost immediately after they are received? Do you find yourself spending a lot of your evenings at home catching up on your social media correspondence, and at times find it overwhelming? If you answered yes to one or more of these questions, then you are probably a social media addict. If not, you are a social media non-addict.
Let’s suppose that being a social media addict is a trait controlled by a single gene with just two alleles, “A” and “a.” The allele “A” (big “A”) is completely dominant over the allele “a” (little “a”). Thus, AA and Aa are the two genotypes that code for the dominant phenotype, social media addict, whereas aa is the genotype that codes for the recessive phenotype, non-addict.
Problems to solve
1. What would be the expected proportions of genotypes and phenotypes for the offspring of a cross between a woman who is homozygous recessive and a man who is heterozygous for the Starbucks trait?
2. What would be the expected proportions of genotypes and phenotypes for the offspring of a cross between a woman who is heterozygous and a man who is also heterozygous for the social media trait?
3. What would be the expected proportions of genotypes and phenotypes for the offspring of a cross between a woman who is homozygous dominant and a man who is homozygous recessive for the channel surfing syndrome trait?
How to submit your answers for credit and see the key
You have a lot of flexibility in how you submit your work. As long as you include your completed Punnett squares to show how you arrived at your answers, and provide the answers to the questions, including probabilities of both genotypes and phenotypes, clearly labeled which is which, you will get credit. Use the examples in Lesson 8C as your guide.
It is important to note, however, that you are only allowed to submit one file. The Assignment 8 dropbox will not allow you to submit any more than that. Once you make your submission, you cannot take it back or change it in any way. So, be sure that it is completely ready before you upload it to the dropbox. You are being graded on completion rather than the correctness of your answers. Thus, even if you get some things wrong, you can still get full credit. They key is that you show all of your work and provide everything that is asked for in the question.
You can either create a document, such as a MSWord file with tables for Punnett squares, or write out your Punnett squares and answers on a sheet of paper, photograph it, and upload it as a picture file. As long as I can see your work, it does not matter how you submit it, although as always it needs to be in an acceptable file format.
Immediately after you make your submission of your single file in the Assignment 8 dropbox, you will be given access to the key. In order to access the key, click on the “Content” link in D2L, select Module 8 from the table of contents, and then click on the link called “Genetics problems assignment key.” Once you have uploaded your file in the dropbox, it will appear just after the link called “Assignment 8: Instructions.” Check your answers! It is important that you know how to solve these types of problems. Expect to see them on your module quiz and proctored exam.
When you are ready, submit your work as a single document in the Assignment 8 dropbox, and compare your answers to the key.
Optional “Morphthing” activity
Do you want to know how you and your significant other’s child might look? Just for fun, you can visit the following website and create a simulated child based on the traits of two different parents. Although we have pointed out in this assignment the fact that facial features are not controlled by simple genetics, they are indeed heritable. Thus, this simulation does have some merit. Once again, this is just for fun and is completely optional. You will not get any extra credit as a result of completing it and please do not include it in your assignment submission.
If you are pleased with your results, and would like to share them with the class, feel free to post your picture and comments in the “General course questions” thread in the student help area.