12 MENDEL, GENES, AND INHERITANCE

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1 12 MENDEL, GENES, AND INHERITANCE Chapter Outline 12.1 THE BEGINNINGS OF GENETICS: MENDEL S GARDEN PEAS Mendel chose true-breeding garden peas for his experiments Mendel first worked with single-character crosses Mendel s single-character crosses led him to propose the principle of segregation Mendel could predict both classes and proportions of offspring from his hypotheses Mendel tested the independence of different genes in crosses Mendel s research founded the field of genetics Sutton s chromosome theory of inheritance related Mendel s genes to chromosomes 12.2 LATER MODIFICATIONS AND ADDITIONS TO MENDEL S HYPOTHESES In incomplete dominance, dominant alleles do not completely mask recessive alleles In codominance. the effects of different alleles are equally detectable in heterozygotes In multiple alleles, more than two alleles of a gene are present in a population In epistasis, genes interact, with the activity of one gene influencing the activity of another gene In polygenic inheritance, a character is controlled by the common effects of several genes In pleiotropy, two or more characters are affected by a single gene Learning Objectives After reading the chapter, you should be able to: 1. Know Mendel s principles of dominance, segregation, and independent assortment. 2. Understand how to solve genetics problems that involve monohybrid and dihybrid crosses. 3. Understand the variations that can occur in observable patterns of inheritance including: codominance, incomplete dominance, epistasis, and pleiotropy. 4. Be able to follow the concept of linkage groups and the probability of crossing over. 5. Discuss how the environment contributes to variations in gene expression. 6. Provide all the factors contributing to an individual s phenotypic expression. Key Terms blending theory of inheritance characters trait self-pollinate cross-pollination true-breeding F 1 generation P generation F 2 generation alleles dominance dominant segregate Principle of Segregation homozygote homozygous heterozygote heterozygous monohybrid monohybrid cross genotype phenotype probability product rule Punnett square testcross dihybrid dihybrid cross independent assortment Principle of Independent Assortment chromosome theory of inheritance locus incomplete dominance codominance multiple alleles Woelker Mendel, Genes, and Inheritance 102

2 epistasis polygenic inheritance pleiotropy Lecture Outline 12.1 The Beginnings of Genetics: Mendel s Garden Peas A. Until about 1900, most believed in the blending theory of inheritance where parent traits are blended evenly in offspring. Yet if blending takes place, why don t extremes such as very tall or very short disappear? Why do bole eyes turn up in offspring of brown-eyed parents? B. Mendel, an Augustinian monk, provided the first answers to these questions and many more. 1. Mendel had studied mathematics, chemistry, zoology, and botany. C. Mendel studied a variety of heritable characteristics called characters. Variation in a character (such as flower color) is called a trait. 1. Mendel observed that rather than blending evenly many parental traits appear unchanged in offspring. 2. Mendel s methods illustrate as well as any experiments in the history of science how rigorous scientific work is conducted through observation, making a hypothesis, and testing with experiments. D. Mendel chose true-breeding garden peas for his experiments. 1. The garden pea (Figure 12.3; Pisum sativum) are plants that self-fertilize, and breeding can be controlled artificially. Fertilization from a different plant is called cross-pollination or simply a cross. 2. True-breeding plants (pure-breeding) are plants that when self-fertilized the traits are passed on without change from one generation to the next. E. Mendel first worked with single-character crosses. 1. Flower color (white and purple) was among the seven characters Mendel selected for study. 2. Pollen from purple-flowered plants was placed on the flowers of white plants, and pollen from white plants was placed on the flowers of purple plants. a. The first generation generated from these crosses is called the F 1 generation (F stands for filial; filius = son). b. The parents are called the P generation. c. All offspring had purple flowers. 3. Mendel then allowed the purple F 1 generation to produce offspring, producing an F 2 generation. The white flowers reappeared. The ratio was 3:1 or about 75% purple flowers and 25% white flowers. 4. Mendel made similar crosses that involved six other characters with pairs of traits and got the same results. F. Mendel s single character crosses led him to propose the principle of segregation. 1. Mendel developed a set of hypotheses to explain the results of his crosses. a. The adult plants carry a pair of factors that govern the inheritance of the character. 2. In modern terminology these factors are called genes. Different traits of a character are alleles of the gene. 3. If an individual s pair of genes consists of different alleles, one allele is dominant over the other. This makes one dominant and one recessive. The recessive is expressed only when both alleles are recessive. 4. The pairs of alleles that control a character segregate as gametes are formed; half the gametes carry one allele and the other half carry the other allele, known as Mendel s principle of segregation. 5. Mendel s three hypotheses explained the results of the crosses (Figure 12.5). a. An individual that has two of the same allele is called a homozygote (homo = same). For truebreeding purple plants, the allele is designated with a PP, indicating homozygous dominant. 6. The plants with white flowers also had two of the same alleles, but this time they were for the white flower, indicated with a pp. 7. All of the plants in the F 1 generation of a cross between true-breeding purple (PP) and true-breeding white (pp) were heterozygous (hetero = different) indicated by Pp. A cross with a single character like this is called monohybrid. 8. According to Mendel s hypothesis, the F 1 generation Pp produce two different kinds of gametes, one with P and one with p (Figure 12.5). 9. Mendel s hypothesis explains how individuals may differ genetically but still look the same. Genotype refers to the genetic constitution of an organism, and phenotype refers to its outward appearances. 10. Thus Mendel s experiments supported his three hypothesis. a. The genes that govern genetic characters occur in pairs in individuals. Woelker 2009 Mendel, Genes, and Inheritance 103

3 b. If different alleles are present in an individual s pair of genes, one allele is dominant over the other. c. The two alleles of a gene segregate and enter gametes singly. G. Mendel could predict both classes and proportions of offspring from his hypotheses. 1. To understand Mendel s predictions we need to review mathematical rules that govern probability. 2. In probability, the likelihood of an outcome is predicted on a scale of 0 to 1 with 1 being certain and 0 being impossible. a. For example, flipping a coin has two possibilities 1 head/2 possible outcomes and 1 tail/2 possible outcomes therefore the probability of getting a head is ½ and tail is ½, and you must get either a head or tail ½ + ½ = 1. H. The product rule in probability 1. In flipping a coin, one flip does not affect another, meaning if you flip a coin once the chance of getting a head is ½, and if you flip it again the chance of getting a head is ½. However, the chance of getting two heads in a row is the product of the probabilities or ½ x ½ or ¼. I. The sum rule in probability. 1. The sum rule applies when there are two or more different ways of obtaining the same outcome, thus either A or B will occur. a. For example, in flipping a coin what is the probability of getting a head and a tail. If you get a head on the first flip ½ and a tail on the second flip (½), the probability is ½ x ½ = ¼. However, you could also get a tail on the first flip (½) and a head on the second flip (½), which is also a probability of ½ x ½ or ¼. Since either of these two gives you one head and one tail, you sum the probabilities ¼ + ¼ = ½. J. Probability in Mendel s crosses 1. The same rules apply to crosses. To get a homozygous in an F 2 generation, say a PP zygote, two P gametes must combine. In crosses one P comes from one parent (½), and the second P (½) comes from the other parent, giving a probability of ½ x ½ or ¼ (Figure 12.6 and 12.7). 2. In the case of heterozygous Pp in an F 2 generation, there are two ways it could happen (like the coin flip for a head or tail); thus the probabilities add ¼ + ¼ = ½. Note that the overall probabilities still add to 1; ¼ (pp) + ¼ (PP) + ½ (Pp) = The probability of getting a purple flower in a monohybrid cross in the F 2 generation is as follows. ¼ (for PP) + ½ (for Pp) is equal to ¾; therefore, we would expect 75% of the flowers to be purple or a 3:1 ratio (Figure 12.7). 4. What has been done (Figure 12.7) is stepped though a Punnett square method for determining the genotypes of offspring and their expected proportions. K. Mendel used a testcross to check the validity of his hypotheses. 1. By crossing a heterozygous purple (Pp) plant with a homozygous recessive (white pp), he was able to correctly predict the outcome of ½ purple and ½ white or a ratio of 1:1 (Figure 12.8). 2. In this case, a cross between an individual with a dominant phenotype with a homozygous recessive individual is called a testcross and is used to determine the genotype by the outcome of the offspring. 3. A testcross cannot be used in humans; however, it can be used in reverse by noting traits present in families over several generations. L. Mendel tested the independence of different genes in crosses. 1. His question was: Would alleles of different characters be inherited independently? 2. He crossed the hereditary characters of seed shape and seed color. 3. When testing two characters, it is called a dihybrid cross. 4. Figure 12.9 shows an example of a dihybrid cross; here the expected phenotypic ratios of 9:3:3:1 were achieved as expected. 5. These findings led to one further hypothesis: The alleles of the genes that govern the two characters segregate independently during formation of gametes termed the independent assortment or Mendel s Principle of Independent Assortment. 6. The effect of this assortment can be shown by the cross of RR YY parents that produce R Y gametes with rr yy parents that produce r y gametes. The offspring will each receive Rr and Yy. 7. If the alleles assort independently, then each of the Rr and Yy gametes will produce four types of gametes: RY, Ry, ry, ry (Figure 12.9). 8. Filling the cells of the diagram (Figure 12.9) gives 16 combinations of alleles. Counting the alleles produced allows an estimate of probability; for example, the genotype rr yy is only found in one cell; and therefore the probability is 1/16. Woelker Mendel, Genes, and Inheritance 104

4 9. These probabilities yielded the prediction of 9:3:3:1 ratio of phenotypes, and Mendel observed 315:101:108: Mendel s first three hypotheses provided coherent explanation of the pattern of inheritance for alternate traits of the same character. His fourth addressed the inheritance of traits with different characters. M. Mendel s research founded the field of genetics. 1. Mendel s techniques and conclusions were so advanced for his time that their significance was not immediately appreciated. a. Mendel s success was based partly on a good choice of experimental organisms, and he got lucky in his selection of characters. b. When Mendel first reported his findings, the structure and function of chromosomes and the patterns by which they are separated were unknown. c. The use of mathematical analysis was a new and radical departure from the usual biological techniques of his day. 2. Mendel reported his finding in Brunn and published in a natural history journal in His work was overlooked until the 1900s when de Vries, Correns, and Tschermak independently performed work similar to Mendel 34 years later. Mendel never received the recognition that he so richly deserved during his lifetime. 3. Mendel was unable to relate the behavior of his factors (genes) to cell structures because critical information about meiosis was not discovered until the 1890s. N. Sutton s chromosome theory of inheritance related Mendel s genes to chromosomes. 1. Sutton recognized the similarities between the inheritance of the genes discovered by Mendel and the behavior of chromosomes in meiosis and fertilization (Figure 12.10). 2. He drew the necessary parallels between genes and chromosomes. a. Chromosomes occur in pairs in sexually reproducing diploid organisms as do the alleles of each gene. b. Chromosome pairs are separated and delivered singly to gametes as are the alleles of each gene. c. The separation of chromosomes is independent of the separation of the other pairs (Figure 12.10). d. One member of each chromosome pair is derived in fertilization from the male parent, and the other member is derived from the female parent. e. From the above coincidence in behavior, Sutton correctly concluded that genes and their alleles are carried on the chromosomes, known as the chromosome theory of inheritance. 3. The parallel is shown in Figure The particular site on a chromosome at which a gene is located is called the locus (Figure 12.11). 5. All the genetics research since Mendel s basic hypotheses has shown that Mendel s conclusions apply to all types of organisms from yeast to fruit flies to humans. Many human disorders that cannot be seen easily also show simple inheritance patterns Later Modifications and Additions to Mendel s Hypotheses A. The rediscovery of Mendel s research produced an immediate burst of interest in genetics. B. It was demonstrated that more than two alleles of a gene may be present among all the members of a population. C. One gene can influence the activity of a different gene called epistasis. D. In incomplete dominance, dominant alleles do not completely mask recessive alleles. 1. Incomplete dominance occurs when effects of recessive alleles can be detected to some extent in heterozygotes. For example, snapdragon plants have red or white flowers, and the offspring of a cross are all pink (Figure 12.13). The F 2 generation produced a ratio of 1:2:1. 2. When one allele is not completely dominate to the other, we use a superscript to signify the character. For example, the snapdragon plants C R or C W for the red or white allele. In this case, the phenotypic ratio is the same as the genotypic ratio. 3. Some human disorders show incomplete dominance, for example sickle-cell disease (as discussed in Why it Matters). 4. Familial hypercholesterolemia (involving a gene that codes for low-density lipoprotein, LDL) is another example of incomplete dominance. 5. Many alleles that appear to be completely dominant are actually incomplete in their effects when analyzed at the biochemical or molecular level. In the pigments that produce fur or flower color, often Woelker 2009 Mendel, Genes, and Inheritance 105

5 the heterozygotes produce enough color to make them look exactly the same as the homozygote dominate but they produce lower amounts of the pigment. 6. A similar situation occurs in humans who carry the recessive allele that causes Tay-Sachs disease. E. In codominance, the effects of different alleles are equally detectable in heterozygotes. 1. The inheritance of the human blood types M, MN, and N is an example; these are different from the familiar ABO. 2. The MN blood type does not affect blood transfusion and has relatively little medical importance. F. In multiple alleles, more than two alleles of a gene are present in a population. 1. Multiple alleles may be present if all individuals of a population are taken into consideration; for instance, one of the genes that plays a part in the acceptance or rejection of organ transplants in humans has more than 200 different alleles. 2. Multiple alleles present no real difficulty in genetic analysis because each diploid individual still has only two alleles. G. Human ABO blood group 1. This group exhibits both dominance and codominance. It was discovered by Karl Landsteiner in There are four blood types: A, B, AB, and O. 2. Landsteiner determined that in the wrong combinations red blood cells from one blood type agglutinated or clumped by an agent in the serum of another type. 3. The antigens responsible are the carbohydrate parts of the glycoproteins. People with blood type A have antigen A and have no antibodies against A but do have antibodies against B. People with blood type O have neither antigen and both antibodies. 4. The four blood types are produced by different combinations of multiple (three) alleles of a single gene (Figure 12.15). H. In epistasis, genes interact with the activity of one gene influencing the activity of another gene. 1. In epistasis (epi = on or over; stasis = standing or stopping), genes interact with one or more alleles of a gene at one locus, inhibiting or masking the effects of one or more alleles of a gene at a different locus. 2. Labrador retrievers may have black, chocolate brown, or yellow fur. One gene coding for an enzyme involved in melanin production determines black or chocolate brown; however, a dominant allele E coded for a gene permits pigment deposition. Therefore, in the recessive condition, pigment deposition is almost blocked, producing a yellow color. Thus, the E gene is epistatic to the B gene. 3. Epistasis by the E gene eliminates some of the expected classes from the crosses among the labs because many of the genotypes look exactly the same and you do not get a 9:3:3:1 ratio. 4. Researchers believe that gene interactions and epistasis are common in human biology. A specific example is insulin resistance. I. In polygenic inheritance, a character is controlled by the common effects of several genes. 1. Some characters produce more or less even gradation of types, forming a continuous distribution rather than on or off effects such as height, skin color, and body weight of people. 2. By defining classes of variation, such as human body height divided into 60, 61, 62 inches (each being classes), polygenic inheritance can be detected (Figure 12.17). 3. Polygenic inheritance is often modified by the environment; for example, poor nutrition during infancy can limit growth. 4. At first glance, the effects of polygenic inheritance might appear to support the idea that characteristics of parents are blended in their offspring. J. In pleiotropy, two or more characters are affected by a single gene. 1. In pleiotropy, a single gene affects more than one character of an organism; for example, in sickle-cell disease the altered hemoglobin leads to blood vessel blockage that can damage many tissues. Woelker Mendel, Genes, and Inheritance 106