Application: Inheritance Lab

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Inheritance Lab Background Background on Mendelian Genetics When traits are the result of a single gene with a few distinct alleles, you may use the logic of Mendelian genetics to predict the genotypes of offspring. To apply Mendelian genetics, you must understand the following terms: genotype, phenotype, dominant allele, recessive allele, heterozygous, homozygous, and Punnett square. Here is an example of a Punnett square. In this example, we will assume that having freckles is a simple single allele example of Mendelian genetics and that the dominant allele is freckles (F) and the recessive allele is no freckles (f). A heterozygous parent with a genotype of Ff mates with a homozygous parent who has an ff genotype. This is an example:

f f F Ff Ff f ff ff

Based on what you know about Mendelian genetics, what percentage of the offspring in the Punnett square above will display freckles? You may also use Mendelian genetics to infer possible genotypes of parents based on the phenotype of a child. For example, if a child displays freckles, what are his or her possible genotypes? (Hint: There are two possible genotypes.) Based on this answer, what genotypes might the parents have? (Hint: There are more than two possible genotypes for the parents.) Continuing with the above example, imagine that you do not know your birth parents and have no siblings, but that you do have freckles. Thus, your phenotype is F, but what is your genotype? Both an FF and an Ff individual would display freckles, so your genotype could be either of these. Using Mendelian genetics, you can infer the different possible pairings of parental phenotypes that would lead to your genotypes. For instance, let’s examine the case in which you are a heterozygote for freckles and consider what possible parental crosses could have resulted in the Ff genotype. Any of the following parental crosses are possible:

FF x Ff FF x ff Ff x Ff

Does this make sense? If not, run a Punnett square on each cross. (See pages 157–158 in your course text for how to use a Punnett square. You may also practice using a Punnett square by referring to the Punnett square calculator listed in the Optional Resources section.) You can also predict which of the above crosses would be most likely by considering which of these pairings is most likely to give an Ff offspring (e.g., 25%, 50%, or 100% probability). You will use this logic by identifying several particular phenotypes and then infer your parents’ possible genotypes.

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BACKGROUND ON PHENOTYPES For this lab, you will identify your phenotype for a variety of physical characteristics, and infer your possible genotypes based on the phenotype. Then you will infer possible genotypes for each of your parents. Save the Inheritance Lab Report document to your computer so you may complete an electronic version of the report. You submit this to your Instructor for your Application Assignment for this week. The following are the phenotypes you will identify in the lab report. When identifying the possible genotypes, use the letters listed below for the dominant and recessive alleles. EARLOBES Having free earlobes is a dominant trait (E); having attached earlobes is a recessive trait (e). Explanation: A free earlobe hangs below the point where the ear attaches to the head. An attached earlobe attaches directly to the side of the head.

DIMPLES Having dimples is a dominant trait (D); not having dimples is recessive (d). Explanation: Dimples are natural indentations in the face on either side of the mouth. (A person may have just one dimple on one side of the mouth.)

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TONGUE ROLLING The ability to roll up the sides of the tongue is dominant (T); not having the ability to roll up the sides of the tongue is recessive (t).

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TOE LENGTH ON FOOT Having a second toe longer than the foot’s big toe is a dominant trait (F); having a second toe shorter than the foot’s big toe is a recessive trait (f). Explanation: The second toe in the above statement refers to the toe that is adjacent (next to) the big toe on your foot. If the second toe is longer than the big toe, you have the dominant trait; if the second toe is shorter than the big toe, you have the recessive trait.

WIDOW’S PEAK Having a distinct point in the hairline at the top of the face is a dominant trait (W); not having a distinct point in the hairline at the top of the face is a recessive trait (w).