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  1. 1. Chapter 3 MENDELIAN GENETICS Objectives: 1. Understand how traits are passed on from parents to offspring. 2. Understand the relationship between genotype and phenotype. 3. Determine your blood type and family pedigree. Please fill out the chart as best as possible before coming to lab. Introduction All organisms on earth exhibit wide variation in size, shape, coloration, or texture. Even within species, the observable variation can be quite great, with a single character (a feature or structure that is inherited, such as flower coloration) able to have multiple traits (a particular variant of a character, such as red or blue flowers). But where does this variation come from, and how are traits passed from parent to offspring? This is the realm of genetics, or the study of inherited traits, their transmission between generations, and the genes that control those traits. The modern science of genetics was started by Johann Gregor Mendel, an Austrian monk who worked with pea plants and who, through carefully planned breeding experiments and meticulous documentation of the traits of the parents and offspring of those experiments, discovered the particulate model of inheritance (the "gene" model). This model of inheritance states that traits are passed on as discrete, heritable units that retain their separate identities in offspring. This is in contrast to the blending model of inheritance, where the traits of the parents are mixed together in the offspring, giving rise to traits that are intermediate between those of the parents. Mendel's work was different from other researchers at the time in that he used a quantitative approach to study inheritance. He published the mathematical records of his results in 1866, but they were ignored and forgotten until the beginning of the 20th century, when researchers began to study the role of chromosomes in inheritance. From his results, Mendel made several generalizations, which we now refer to as Mendel's Laws of Inheritance: 1. Law of Segregation - every individual carries genes that control visible characters. Alternate versions of these genes are called alleles: each allele represents an alternate form of a trait (ex. Mendel's pea plants had two different alleles for flower color...purple and white). An organism will receive two copies of an allele, one from each parent. Some alleles are always expressed in the offspring, and are termed dominant; some alleles can be masked by the dominant allele, and are termed recessive. The two alleles for each character segregate (separate) from one another during gamete formation.
  2. 2. 2. Law of Independent Assortment - each pair of alleles for a given character will segregate independently, and at random, from alleles for other characters during gamete formation. By performing elaborate experimental tests, Mendel made these two generalizations from seven different characters of pea plants. In these tests, he either followed single traits or multiple traits through several generations. Today, you are going to look at traits similar to those studied by Mendel. First, however, we need to introduce some pertinent terminology. Genotype - since traits can have both dominant and recessive forms, it is important to distinguish between an organism’s appearance and the alleles it carries. The genotype is the genetic makeup of an organism. An organism can be either heterozygous (carries two different alleles) for a trait or homozygous (carries two identical alleles) for a trait. Phenotype – These are the organism's physical and physiological traits. It is not always indicative of its genotype. For example, in Mendel's pea plants, purple flower color is dominant to white flower color. Thus, two plants, one with two copies of the purple allele (homozygous dominant) and one with a copy of both purple and white alleles (heterozygous), will have identical phenotypes (purple flowers) but different genotypes (PP and Pp, respectively). Exercise 1. Monohybrid cross A monohybrid cross is a mating that involves differences in a single trait. Mendel derived his Law of Segregation from looking at many monohybrid crosses. In a monohybrid cross, Mendel obtained the first generation (parental generation, P1) from pure-breeding (i.e., homozygous) stocks that differed for a single trait, such as flower color (purple and white). The offspring resulting from the original cross are the first filial or generation (F1) and have a phenotype identical to the parent with the dominant trait (purple). When Mendel used two F1 individuals as parents in a second cross, the second generation of offspring (F2) showed a phenotypic ratio of 3 purple:1 white. 1. Obtain an ear of corn labeled "monohybrid". Each kernel on the ear of corn is an F2 individual from cross of two F1 individuals that were heterozygous for purple kernel color. One parental individual (P1) was homozygous for purple kernel color (PP) and the other was homozygous for yellow kernel color (pp). Count the number of kernels of each color on 3 rows of the ear and record Number of purple kernels =___________ Number of yellow kernels =___________ total number of kernels =_________ What is the phenotypic ratio =_________ What color is recessive? ________
  3. 3. Exercise 2. Dihybrid cross Mendel also investigated the inheritance of two traits simultaneously, which is known as a dihybrid cross. For example, one of the dihybrid crosses that Mendel performed was for seed color (green or yellow) and seed texture (smooth or wrinkled). Yellow seed color (Y) is dominant to green color (y) in peas, and smooth seed texture (S) is dominant to a wrinkled texture (s). When Mendel crossed homozygous yellow/smooth pea plants to homozygous green/wrinkled pea plants, the F1 generation all had yellow/smooth seeds. When two of the F1 generation individuals were crossed, the F2 plants had seeds that showed four different phenotypes, all in a particular proportion: 9 yellow/smooth: 3 yellow/wrinkled: 3 green/smooth: 1 green/wrinkled. 1. This time, get an ear of corn marked "dihybrid"; the kernels on this ear are the F2 generation of a dihybrid cross for seed color and starch content. In this cross, purple seed color (P) is dominant to yellow seed color (p), and starchy kernels (with a round, full appearance - S) are dominant to those with a low starch content (with a wrinkled appearance - s). Look at the ear of corn and see if you can recognize all four different phenotypes. The parents of this generation all had purple/starchy kernels, and were the result of a cross between a plant with purple/starchy kernels and one with yellow/sweet kernels. Count the number of kernels of each type on 4 rows and record the number. Number of purple/starchy =________ Number of purple/sweet =_________ Number of yellow/starchy =________ Number of yellow/sweet =________ total number of kernels =_________ Phenotypic ratio =_________ What is the genotype of the F1 individuals? _________ What are the genotypes of the original (P1) cross? ____________X____________
  4. 4. Exercise 3. Human blood type and family pedigree Another interesting way to look at inheritance is to look at blood types within a family, such as your own. This also presents an interesting departure in two ways from the typical situation of dominant/recessive alleles that you've learned previously. First, the A/B/O blood types that are found in humans are an example of multiple alleles. Many characters in human populations have more than two different alleles. This system has a total of three alleles and the gene in this case codes for an antigen that is found on the surface of the red blood cells (A antigen or B antigen). Second, it is an example of codominance, indicating that two or more alleles affect the phenotype of an individual in distinct and visible ways. The combination of having multiple alleles and codominance of two of those alleles generate four different phenotypes from six possible genotypes: Phenotype Genotype Interaction Antigens/Antibodies produced Type A blood IAIA or IAi A allele is dominant to A antigens on RBCs; Anti-B recessive allele antibodies in blood serum Type B blood IBIB or IBi B allele is dominant to B antigens on RBCs; Anti-A recessive allele antibodies in blood serum Type AB blood IAIB A and B alleles are A and B antigens on RBCs; no codominant antibodies produced Type O blood ii homozygous recessive No antigens on RBCs; Anti-A individual and Anti-B antibodies Blood type O is a universal donor for purposes of donating blood. Blood type O produces no antigens and, therefore, there is no reaction against this type of blood. Blood type AB is the universal acceptor. A person with this phenotype can accept any blood type. Blood type B produces a B antigen, and correspondingly, A type blood produces A antigen. Persons who do not already have these antigens in their blood naturally cannot be given blood containing these antigens. The body would immediately attack this blood by eliciting an immune response. Today we will be determining your blood type using a simple commercial blood typing kit. It will involve a finger stick with a needle, and is completely voluntary. Please let the instructor know if you prefer to not have this test performed. All individuals performing this part of the laboratory must first sign the liability waiver form. All used lancets are to be discarded in the 'sharps' container. Do not throw into the trash. Do not use the lancets multiple times; one lancet per person.
  5. 5. 1. Follow the instruction sheet that comes with the blood typing kit and any additional directions from your lab instructor. 2. Record your blood type along with that of your family in the pedigree chart: Table 1. Family blood type pedigree Maternal Maternal Paternal Paternal grandfather grandmother grandfather grandmother Mother Father Sibling Self Sibling Questions: 1. Freckles on the skin is a dominant trait that is controlled by a single gene in humans. a. What is the genotype of a person without freckles? b. If two parents with freckles have a child with no freckles, what is the probability that any other child that they have will have freckles? 2. A man with type A blood marries a woman with type B blood. Their child has type O blood. What are the genotypes of these three individuals? What other genotypes, and in what frequencies, would you expect in offspring from this marriage?