Causes of Variation Mode of Description of protein Interaction Resulting in Phenotype Inheritance

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1 Name: Period: Variation: It s Not That Simple For many traits, variation is not as simple as either having the dominant or recessive trait decided by two alleles of a gene. Rather, variation can result from different kinds of interactions. These interactions can take place among the proteins produces by two or more alleles, or they can also be the result of the proteins produced by more than one gene. These interactions can result in a range of variations in that trait. In this activity, you will analyze several different traits. You will do this by using the information provided in six trait scenarios and in the table below. You will then determine the kinds of interactions that take place among the proteins that result in the traits you observe. Your Task 1. First read over the chart below describing the six different types of variation. 2. Secondly, read the descriptions of the six traits, and using the information from the chart answer the questions. 3. Once you are finished you will work through the attached codominance and incomplete dominance problems. Causes of Variation Mode of Description of protein Interaction Resulting in Phenotype Inheritance Complete dominance One gene has two alleles that may encode different forms of that protein (functional or nonfunctional). This results in different phenotypes. When an individual is heterozygous for the gene (has one dominant allele and one recessive allele), only one phenotype will appear (dominate). Incomplete dominance Codominance Multiple alleles Pleiotropic Polygenic One gene has two or more alleles. The phenotype is the result of the interaction of the proteins coded by each allele. The phenotype may appear as a blending of the two products. When the individual goes through meiosis, the alleles continue to separate independently. One gene has two alleles. The phenotype is the result of the both proteins coded for by of the alleles. This is similar to incomplete dominance, except here both phenotypes show up at the same time instead of blending together as in incomplete dominance. One gene has more than two possible alleles. They all encode different forms of a protein. The phenotype is dependent on which two alleles are present in the organism. It also depends on the patterns in which the products of these alleles interact. The patterns may be dominant, incompletely dominant, or codominant. One gene has two alleles. The one gene may have multiple effects on the phenotype of an organism. Both alleles of the gene encode different proteins that serve different functions. The failure to produce a functional protein alters many characteristics of the organism. A polygenic trait is the product of the interactions of the proteins made from several different genes and their alleles. Because of this we often see many different phenotypes for one trait.

2 Trait 1 Sickle-cell anemia is the result of non-functional hemoglobin. An individual who is homozygous for sickle-cell trait will show classic symptoms of sickle-cell anemia. These are intense joint pain, shortness of breath, anemia, and the characteristic sickle shape of the red blood cells. In an individual who is heterozygous for sickle cell, the alleles for both types of hemoglobin (A-functional hemoglobin and S-non-functional hemoglobin) are present in every red blood cell. But the sickling phenotype only appears under conditions of oxygen deprivation, such as during exercise at high altitudes. 2. Explain the phenotype of individuals who are homozygous and those who are heterozygous for this gene. Trait 2 The Andalusian variety of chicken has both black and white feathers. It is produced by crossing a black feathered parent and a white feathered parent. Color production in these chickens depends on a single gene. 2. Explain how it is possible for the chicken to make black and white feathers? Trait 3 Human height is determined by a number of factors, including diet. But even if all individuals were fed the same diet, height among individuals would show continuous variation. Height is determined by a number of gene products. These include levels in production of hormones, such as growth hormone. They also include growth capacity of structural components, such as cartilage, connective tissue, skeletal muscles, and bone. 2. Explain how it is possible for people to have so many different heights. Trait 4 The color of human skin is determined by genetic factors. But the shade may vary depending on environmental factors. Skin color is almost entirely the result of the amount of a protein called melanin pigment and its distribution in the outer layer of skin. The products of at least two genes have been shown to determine the levels of melanin produced. Several other genes appear to be involved in how this melanin is distributed in the skin. 2. Describe how this mode of inheritance allows for different shades of skin color. Trait 5 There are four major blood groups in humans (A, B, AB, and O). These blood groups are determined by a gene

3 that encodes a protein used to make a polysaccharide (a large sugar molecule). This polysaccharide is found on the surface of red blood cells. This gene has three alleles. Each encodes a variant of this enzyme. The variants result in the synthesis of the different forms of the polysaccharide that characterize the blood as being type A, B, AB, or O. The table below lists the phenotype (blood type) of an individual and the possible genotypes that result in this phenotype. 2. Describe how the different alleles produce the resulting phenotype. Phenotype A B AB O Possible Genotypes AA or AO BB or BO AB OO Trait 6 A single gene in rats controls the production of a protein involved in forming cartilage. Cartilage is the tough elastic tissue in vertebrate animals that provides some of the organism s structural support. A rat carrying two alleles resulting in an altered protein displays a whole complex of birth defects. These include thickened ribs, a narrowing of the passage through which air moves to and from the lungs, a loss of elasticity in the lungs, blocked nostrils, a blunt snout, and a thickening of the heart muscle. These effects generally result in death. 2. Explain how this one gene affects so many different traits.

4 Incomplete and Codominance Worksheet Answer the following questions. Provide a punnett square to support your answers where indicated. Express probabilities as percentages. For instance, a probability of one chance in ten would be 10%. 1. Explain the difference between incomplete dominance and codominance: 2. In some chickens, the gene for feather color is controlled by codominance. The allele for black is B and the allele for white is W. The heterozygous phenotype is known as erminette (black and white feathers). a. What is the genotype for black chickens? b. What is the genotype for white chickens? c. What is the genotype for black and white chickens? 3. If two erminette chickens were crossed, what is the probability that: a. They would have a black chick? % b. They would have a white chick? % c. They would have a black and white chick? % 4. A black chicken and a white chicken are crossed. What is the probability that: a. They would have a black chick? % b. They would have a white chick? % c. They would have a black and white chick? % 5. In snapdragons, flower color is controlled by incomplete dominance. The two alleles are red (R) and white (W). The heterozygous genotype is expressed as pink. a. What is the phenotype of a plant with the genotype RR? b. What is the phenotype of a plant with the genotype WW? c. What is the phenotype of a plant with the genotype RW?

5 6. A pink-flowered plant is crossed with a white-flowered plant. What is the probability of producing: a. A pink-flowered plant? % b. A red-flowered plant? % c. A white-flowered plant? % 7. What cross will produce the most pink-flowered plants? Show a Punnett square to support your answer. Parents: X 8. Human hair color is controlled by multiple alleles, one gene with four different allele options (with some incomplete dominance): HBr = brown HBd = blonde hr = red hbk = black The possible genotypes and phenotypes: HBdHBd or HBdhbk = blonde HBdHBr = light brown HBdhR = strawberry blonde HBrHBr or HBr hbk = brown HBrhR = auburn hrhr or hrhbk = red hbkhbk = black 9) What do you think your phenotype and genotype for hair color are? 10) If someone with auburn hair has children with someone with red hair (but whose mother had black hair), what are the genotype and phenotype probabilities for their children? Parents: X