Anthropology Lab#3
ATH 2100 READING (“Simple” Mendelian Inheritance)
DIRECTIONS: Please read the materials that follow and then complete Quiz 6 on PILOT.
By the time you finish reading these materials, you should be able to answer the following questions:
1. What is the difference between a simple and a complex trait?
2. What similarities do simple and complex traits share?
3. Are most human traits simple or complex?
4. What distinguishes a genotype from a phenotype?
5. What are the genetic versus non-genetic factors that influence complex traits?
In this week’s lab, you will examine variation in Mendelian traits with your classmates. From this information, you will determine the phenotypes and potential genotypes found in our classroom. Put simply, a phenotype is a physical trait that we can observe (e.g., presence of a widow’s peak or absence of mid-digital hair). Some people will call phenotypes “variants” or “traits” so pay attention when these words are used in class! The phenotype is controlled in large part by the genotype, which is the genetic composition of an organism, represented by pairs of alleles (e.g., AA, Aa, or aa). As an example, the genotype “aa” might control for the phenotype “dimples,” while the genotypes AA and Aa control for the phenotype “no dimples.”
SIMPLE AND COMPLEX TRAITS
Mendelian (or “simple”) traits are traits that exhibit a simple inheritance pattern with a limited amount of genetic and phenotypic variation. Mendelian traits, therefore, are controlled for by a single genetic locus and are physically expressed as traits that are usually either present or absent. Simple traits tend not to be influenced by the environment because they are not major contributors to natural selection (in which the environment is a key element).
Non-Mendelian (or “complex”) traits are traits that follow an inheritance pattern in which the genotype sets the genetic “potential” for a trait, while the physical and cultural environment impacts how the phenotype will be expressed (i.e., what you will look like, do, etc.). Close to 100% of HUMAN traits are inherited in a complex fashion, which makes it difficult for researchers to determine the underlying causes leading people to behave and even respond to treatment for diseases in different ways.
Simple inheritance can be easily tracked by looking at phenotypes- as you will learn in class, Mendel used the phenotypic differences in the shape and color of pea plant flowers, pea plant leaves, pea pods, and the peas inside the pods to track simple inheritance patters. On the other hand, complex inheritance patterns require much more sophisticated methods for locating genetic differences because the combination of allele pairs is only ONE part of what gives you your phenotype. In this week’s lab you will explore several “simple traits” like earlobe shape and the ability to roll your tongue.
The reality is that almost all of these traits have since been shown to be “complex” traits- but, they are easy to observe (as compared to your genotypic variation) and have few phenotypes, making them easy to track. As such, we will pretend that these traits are simple so that we can begin to understand the very basic processes that contribute to the expression of our genotypes (allele pairs) as phenotypes (what you can see with your eyes).
As you go into lab, remember, complex traits can also follow “regular” patterns of recessiveness and dominance (just like simple traits). The difference is that the genotype is only one part of the process. Other genetic factors like polygenism (multiple genes contributing to one effect/ set of phenotypes) and pleiotropy (a single gene leading to multiple “effects”/ several sets of phenotypes), and other dominance patterns like co-dominance (two different dominant phenotypes competing) or incomplete dominance (recessive phenotype expressed “equally” with the dominant phenotype) can lead to differences in the phenotype. Likewise, environmental factors ranging from the physical environment (like temperature, humidity, or barometric pressure) and the cultural environment (like behavioral differences that influence exposure to the physical environment or pressures that impact bodily processes (e.g., like stress or hormone levels)) can also influence your phenotype for a complex trait.
How can we track genotypic and phenotypic changes?
A Punnett square is a mathematical way of visualizing the probability of a pair of mating individuals to produce offspring of different genotypes (and phenotypes). Let’s try a practice Punnett square together:
Suppose that a man with the genotype AA (for brown hair) mates with a woman with the genotype aa (for red hair). Using a Punnett Square we split the alleles to perform our monohybrid cross (or, single trait mating event).
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A |
A |
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a |
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a |
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To complete the square we simply go column by column and row by row, matching alleles back into pairs that include one allele from “mom” and one allele from “dad.” Our final square reveals the following genotypes:
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A |
A |
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a |
Aa |
Aa |
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a |
Aa |
Aa |
All of the children (100%) are heterozygous (Aa) for hair color. If we use Mendel’s “principle of dominance,” namely that dominant alleles (e.g., A) are always expressed over recessive alleles (e.g., a) we can now determine the phenotypes of these children (since we know that A controls for brown hair and a controls for red hair). Since all the kids have one of dad’s brown hair alleles 100% are also brown haired.
Now, on your own, try the same experiment this time changing dad’s genotype to Aa (i.e., mom’s stays the same). What genotypes and phenotypes are produced?
Please note, you are not required to bring a copy of this reading to lab, but you are encouraged to do so in case you need to reference the materials as you answer lab questions. If you want to avoid large stacks of paper and the cost of printing, consider arranging a rotation among lab group members for printing and bringing copies to each lab.
Please remember to take Quiz 6 once you have completed this reading.