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PCC BIO 100IN - Part 3 of worksheet 5

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How did I get this face? Bio 100 Instructor Deborah Bird Worksheet #5 1Page | 1Part 3 of worksheet #5 How did I get this face? Mike Tveten drawings by Chris Bondante Modified by Deborah Bird OBJECTIVES - Apply the principles of segregation and independent assortment to the inheritance of human facial characteristics - Determine the phenotypes and genotypes for yourself and your parents - Define allele, gene, homozygous, heterozygous - Define and give an example of dominant and recessive alleles, codominance, incomplete dominance, X-linked (sex-linked) traits, and polygenic traits The purpose of this lab is to illustrate: 1. Mendel's Law of Segregation 2. Mendel's Law of Independent Assortment 3. the relationship between genotype and phenotype 4. a variety of patterns of inheritance INTRODUCTION As you learned when you studied meiosis in the last unit, you received one half of your genes from your mother and the other half from your father. The chromosomes went through segregation from one another when they separated and made sex cells. Each one of the sex cells carried a gene at a locus. When fertilization took place the homologous chromosomes were restored and a new set of genetic material was introduced into the egg from the sperm. The sperm cells were also segregated during meiosis and depending upon which egg was fertilized by what sperm actually gives you your individual genetic compliment called the genotype. Human variation happens not only because of the large number of traits that exist within our species, but also because of the random mixing of alleles that occurs during sexual reproduction. The expression of the genes which we see physically is called the phenotype. There are many patterns of inheritance – some are relatively simple and straightforward, while others are more complex. Many alleles (forms of a gene) are dominant, which means that they mask or hide the expression of other alleles, called recessive alleles. Recessive alleles are expressed only when their dominant counterparts are not present – in other words, when we inherit two copies of the recessive allele, one from each parent. Remember, when we have two copies of the same allele, our genotype is homozygous for that trait. For example if you have a round face and inherited a R from mom and a R from dad , you are homozygous dominant for that trait. Dominant traits can be expressed even if there is only one. The genotype Rr is still a round face. The large R is dominating the r’s expression. Even though you are heterozygous (Rr) for the trait. Recessive alleles are expressed only in the homozygous condition (rr).How did I get this face? Bio 100 Instructor Deborah Bird Worksheet #5 2Page | 2If our genotype carries one copy of the dominant trait and one copy of the recessive trait then it is called heterozygous. In individuals with two different alleles for a trait heterozygous for the trait), neither allele completely masks the presence of the other, and the result is a combination of both. Other traits show continuous variation throughout a population and do not result in a distinctly dominant or recessive phenotype. These traits are thought to be controlled by many genes and are called polygenic traits. Another kind of interaction between genes is called epistasis. An example of this is the interaction between the gene that codes for red hair and the gene that codes for dark versus blonde. Thus, a person may be a strawberry blonde, or have dark auburn hair, depending on how these two genes interact. Some traits are carried on the X chromosome, and there is no counterpart on the Y chromosome. Because females have two X chromosomes, a recessive allele might be masked by the presence of a dominant allele. However, because males have only one X chromosome, recessive traits are often expressed since there is no other allele to mask them. These traits are called sex-linked, or more accurately, X-linked. Other traits may be found only on the Y chromosome, so they are found only in males. These traits, such as beard growth, are sex-limited , which means their expression is confined to one gender or the other (in this example, males). Most of the traits used in this activity have been simplified and represent a simplified model. In this exercise you will be determining your phenotype and guessing at your genotype . The last column of the table in worksheet 5 is answered mother, father, or mixture. Before you do this exercise, you need to determine your phenotype, and make your best guess about your genotype Unless you have an identical twin, you are unique! Your appearance (your phenotype) is determined by your genes (your genotype). Even people with the same parents have slightly different genotypes and therefore different phenotypes. (Remember synapsis in meiosis makes this possible.) Identical twins have identical genotypes because the fertilized egg splits after fertilization and grows another individual. Filling out the Table Take a long, hard look at yourself in the mirror or have someone look at you. Then use the illustrations that follow to determine your phenotype for the illustrated traits, such as eye color, nose size, and freckles. Record your phenotype for each trait in Table 1 on worksheet number 5. Then try to determine your genotype, and record your genotype in the table. In a simple dominant/recessive pattern of inheritance, if you show the recessive phenotype, you know that you are homozygous recessive for that trait. However, if you show the dominant phenotype, you may be homozygous or heterozygous. For this exercise, assume you are heterozygous (that is, you carry both the dominant and recessive alleles, unless the phenotype tells you the genotype. See the example below. - Remember -- PHENOTYPE is the PHYSICAL FEATURES -- what you see, such as "rounded face" or "cleft chin." GENOTYPE is the GENES (alleles) present that produce that phenotype. For example, if the phenotype is "rounded face," then the genotype is either "RR" or "Rr." You would use “Rr” for this exercise. For air texture, curly isHow did I get this face? Bio 100 Instructor Deborah Bird Worksheet #5 3Page | 3CC, wavy is Cc, straight is cc. In this case we can tell the genotype from the phenotype. Then, determine what alleles (gene form) will be in your gametes. Remember, if you are heterozygous for a trait such as Rr you can produce gametes with one or the


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