Chapter 14: Mendel and the Gene Idea If you have completed a first-year high school biology course, some of this chapter will serve as a review for the basic concepts of Mendelian genetics. For other students, this may be your first exposure to genetics. In either case, this is a chapter that should be carefully mastered. Spending some time with this chapter, especially working genetics problems, will give you a solid foundation for the extensive genetics unit in the chapters to come. Overview : 1.
In the 1800s the most widely favored explanation of genetics was blending. Explain the concept of blending, and then describe how Mendel’s particulate (gene) theory was different.
•The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and a nd yellow paint blend to make green) •The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) •Mendel documented a particulate mechanism through his experiments with garden peas Concept 14.1 Mendel used the scientific approach to identify two laws of inheritance 2.
One of the keys to success for Mendel was using pea plants. Explain how using pea plants allowed Mendel to control mating; that is, how did this approach let Mendel be positive about the exact characteristics of each parent?
•Advantages of pea plants for genetic g enetic study: 1. There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits; 2. Mating of plants can be controlled; 3. Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels); and 4. Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another
3. Define the following terms. Then, consider your own generation would your Mom ’s grandparents be? Your Mom? You?
family.
Which
P generation: Mom’s grandparents F1 generation: Your generation: Your Mom F2 generation: You generation: You 4.
Explain how Mendel’s simple cross of purple and white flowers did the following:
1. Refuted blending: Each purple and white flower were distinct; 2. Determined dominant and recessive characteristics: F2, 705 purple-flowered plants and 224 white-flowered plants. 3. Demonstrated the merit of experiments that covered multiple generations: We are able see the outcome generationally and over time. 5. Alternate versions of the same same gene, like purple and white white flower flower color, are termed
•These alternative versions of a gene are now called alleles
•The first concept is that alternative versions of genes account for variations in inherited characters •For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers •These alternative versions of a gene are now called alleles 6.
On the figure at below, label the allele for both purple and white flower color, a homologous pair , and the locus of the flower color gene.
7.
In sexually reproducing organisms, why are there exactly two chromosomes in each homologue?. •Each gene resides at a specific locus on a specific chromosome
•Mendel made this deduction without knowing about the role of chromosomes •The two alleles at a locus on a chromosome may be identical, as in the true breeding plants of Mendel’s P generation •Alternatively, the two alleles at a locus may differ, as in the F 1 hybrids Mendel’s model consists of four concepts. Describe each concept in the appropriate space below. Indicate which of the concepts can be observed during meiosis by placing an asterisk by the concept. Description of Concepts Mendel’s Four Concepts 1st Concept •The first concept is that alternative versions of genes account for variations in inherited characters nd 2 Concept •The second concept is that for each character an organism inherits two alleles, one from each parent 8.
3rd Concept
4th Concept Law of Segregation
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•The third concept is that if the two alleles at a locus differ, then one (the dominant allele ) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance •The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Using Figure 14.5 as your guide, provide the missing notations for the figure
below. (P, F 1, F2).
a. What is the F2 phenotypic and genotypic ratio? 3:1
b. Which generation is completely heterozygous? F2 c. Which generation has both heterozygous and homozygous offspring? F2
10.
In pea plants, T is the allele for tall plants, while t is the allele for dwarf plants. If you have a tall plant, demonstrate with a test cross how could it be determined if the plant is homozygous tall or heterozygous tall. •Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous •The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual •If any offspring display the recessive phenotype, the mystery parent must be heterozygous
11. Explain the difference between a monohybrid (one character) cross and a dihybrid cross (two characters) . •Mendel derived the law of segregation by following a single character •The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character •A cross between such heterozygotes is called a monohybrid cross
•Mendel identified his second law of inheritance by following two characters at the same time •Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters •A dihybrid cross, a cross between F 1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently 12.
As you start to work word problems in genetics, two things are critical: the parent’s genotype must be correct, and the ga metes must be formed correctly. Using Figure 14.8 as your guide, explain how the gametes are derived for the following cross. (You should have four different gametes). YyRr × YyRr YR Yr Ry ry 13.
Complete the cross, given in questions 12, by placing the gametes in a Punnett square . Then provide the phenotypic ratio of the offspring.
14. Explain Mendel’s law of independent assortment . •Using a dihybrid cross, Mendel developed the law of independent assortment •The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation •Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes •Genes located near each other on the same chromosome tend to be inherited together Before leaving this concept, it would be helpful to complete the three problems in the 14.1 Concept Check on page 269 of your textbook. The problems are worked and explained in the Answer section on page A-10 at the back of the book. Concept 14.2 The laws of probability govern Mendelian inheritance 15.
An event that is certain to occur has a probability of zero, while an event that is certain not to occur has a probability of zero.
16. In probability, what is an independent event ? •Mendel’s laws of segregation and independent assortment reflect the rules of probability
•When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
•In the same way, the alleles of one gene segregate into gametes independently of another gene’s allele 17. State the multiplication rule and give an original example. •The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities •Probability in an F 1 monohybrid cross can be determined using the multiplication rule •Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele 18. State the addition rule and give an original example. •The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities •The rule of addition can be used to figur e out the probability that an F 2 plant from a monohybrid cross will be heterozygous rather than homozygous 19.
What is the probability that a couple will have a girl, a boy, a girl, and a boy in this specific order? Same as the coin toss below.
Concept 14.3 Inheritance patterns are often more complex than those predicted by simple Mendelian genetics 20.
Explain how incomplete dominance is different from complete dominance , and give an example of incomplete dominance. •In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties Figure 14.10 Incomplete dominance in snapdragon color
•Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical 21. Compare and contrast codominance with incomplete dominance . •In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways •In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties 22.
Dominant alleles are not necessarily more common than recessive alleles in the gene pool. Explain why this is true. The Relation Between Dominance and Phenotype •A dominant allele does not subdue a recessive allele; alleles don’t interact •Alleles are simply variations in a gene’s nucleotide sequence •For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype
23. Explain what is meant when a gene is said to have multiple alleles . •Most genes exist in populations in more than two allelic forms •For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I A , I B , and i . •The enzyme encoded by the I A allele adds the A carbohydrate, whereas the enzyme encoded by the I B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither 24.
Blood groups are so important medically that you should be able to solve genetics problems based on blood types. The first step in accomplishing that is to understand the genotypes of each blood type. Before working any problems, complete this ABO blood type chart.
Genotype I A I A or I A i I B I B or I B i I A I B ii
Red See See See See
Blood Cell Appearance above. above. above. above.
Phenotype (Blood Group) A B AB O
25.
Question 2 in the 14.3 Concept Check is a blood type problem. Complete it here, and show your work.
26. What is pleiotropy ? •Most genes have multiple phenotypic effects, a property called pleiotropy Explain why this is important in diseases like cystic fibrosis and sickle- cell disease. •For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease 27. Explain epistasis . •In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus •For example, in mice and many other mammals, coat color depends on two genes •One gene determines the pigment color (with alleles B for black and b for brown) •The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair 28.
Explain why the dihybrid cross detailed in Figure 14.12 has 4 white mice instead of the 3 that would have been predicted by Mendel’s work.
Figure 14.12 An example of epistasis
29. Why is height a good example of polygenic inheritance ? •Height is a good example of polygenic inheritance as indicated below: 1. •Quantitative characters are those that vary in the population along a continuum; and 2. •Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype 30. 31.
Quantitative variation usually indicates polygenic inheritance.
Using the terms norm of reaction and multifactorial , explain the potential influence of the environment on phenotypic expression. •Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype •The norm of reaction is the phenotypic range of a genotype influenced by the environment •e.g. Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity (Figure 14.14 below) •Norms of reaction are generally broadest for polygenic characters •Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype
Concept 14.4 Many human traits follow Mendelian patterns of inheritance 32. Pedigree analysis is often used to determine the mode of inheritance (dominant or recessive, for exam ple). Be sure to read the “Tips for pedigree analysis” in Figure 14.15; then complete the unlabeled pedigree by indicating the genotypes for all involved. Recessive mode of inheritance for this.
33.
Explain why you know the genotype of one female in the third generation, but are unsure of the other. The F3 generation was not followed in this pedigree.
34.
Describe what you think is important to know medically about the behavior of recessive alleles. Many genetic disorders are inherited in a recessive manner.
35.
Students are expected to have a general knowledge of the pattern of inheritance and the common symptoms of a number of genetic disorders. Provide this information for the disorders listed below. a. •Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent •The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes •Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine b. •Sickle-cell disease affects one out of 400 African-Americans •The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells •Symptoms include physical weakness, pain, organ damage, and even paralysis c. •Achondroplasia is a form of dwarfism caused by a rare dominant allele
•Some human disorders are caused by dominant alleles •Dominant alleles that cause a lethal disease are rare and arise by mutation d. •Huntington’s disease is a degenerative disease of the nervous system •The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age 36.
Amniocentesis and chorionic villus sampling are the two most widely used methods for testing a fetus for genetic disorders. Use the unlabeled diagram below to explain the three main steps in amniocentesis and the two main steps of CVS.
37. What are the strengths and weaknesses of each fetal test? •In amniocentesis, the liquid that bathes the fetus is removed and tested, DAMAGING, weeks long to find results. •In chorionic villus sampling (CVS), a sample of the placenta is removed and tested, DAMAGING, several hours to find results •Other techniques, such as ultrasound and fetoscopy , allow fetal health to be assessed visually in utero, NOT DAMAGING, observation is instant 38.
Explain the symptoms of phenylketonuria , and describe how newborn screening is used to identify children with this disorder. The damage caused by PKU can begin weeks after the baby has started drinking breast milk or formula. Babies with PKU need foods low in phenylalanine to
prevent severe brain damage. Phenylalanine is found in most foods that have protein, such as milk, cheese, and meats. A phenylketonuria (PKU) test is done to check whether a newborn baby has the enzyme needed to use phenylalanine in his or her body. Phenylalanine is an amino acid that is needed for normal growth and development. If a baby's body does not have the enzyme that changes phenylalanine into another amino acid called tyrosine, the phenylalanine level builds up in the baby's blood and can cause brain damage, seizures, and intellectual disability. •Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States Testing Your Knowledge: Genetics Problems (pg. 284) Now you should be ready to test your knowledge. This chapter does not have a Self-Quiz, but rather asks you to do a series of problems. One of the ways to determine your understanding of Mendelian genetics is to work many genetic problems. Complete the questions from the problems at the end of the chapter. Before starting, it would be productive to read the “Tips for Genetic Problem s” on page 283. Work neatly, and show all work. As you know, you can check your solutions in your text. Questions: Read and find 2, 3, 5, 6, 7, 8, and 9 below; Do 1, 13, 16, 17, and 19. 2.
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