Chapter 5 INTERACTIONS OF GENES AND THE ENVIRONMENT

Transcription

1 Chapter 5 INTERACTIONS OF GENES AND THE ENVIRONMENT Chapter Summary Up to this point, the traits you have been studying have all been controlled by one pair of genes. However, many traits, including some human disorders, are produced in a cooperative fashion by the action of two or more gene pairs. A polygenic trait is one that is controlled in this manner. Polygenic traits are also characterized by the fact that they show a range of phenotypes varying from a small or slightly affected phenotype to a large or severely affected phenotype. The variation occurs within populations along normal bell-shaped distribution patterns. Examples of polygenic traits are human height and hypertension. In working with polygenes, geneticists talk about total numbers of alleles, as well as whether they are dominant or recessive. The more dominant alleles you have at all the polygenic loci that govern a particular trait, the greater the impact on your phenotype. When looking at the children of certain parents, another feature of polygenic traits is that the children tend to be an average of the parental extremes, a concept called regression to the mean. Polygenic traits are often affected by the environment of the individual. Such traits may then be called multifactorial. The simplest way to look at this is to consider height. You can never be taller than the height dictated by the number of dominant alleles you possess for height (genotype), but if you were malnourished as a child (environment), you may never grow to reach your genetic potential. Other polygenic traits may involve disorders; where these are concerned, the number of dominant alleles determine whether or not you develop the disease phenotype. If your total number puts you in the diseased category, you have crossed the threshold for that disorder. The greater the number of dominant alleles, the more severely the disease will be expressed. Some diseases can be helped by altering the body s environment. For

2 34 Chapter 5 example, even if you are genetically prone to hypertension, restricting your salt intake can help to lower your blood pressure. Twins, both fraternal and identical, are often used in studies to determine the relative influence of heredity and environment on the phenotype. Because identical twins have nearly identical genes, the only variable factor is environment. If identical twins are not always identical in phenotype (concordant) with respect to some trait or disease, then it may be deduced that an environmental influence exists. The debate over the relative influence of heredity and environment (sometimes called nature versus nurture) has generated a great deal of interest, especially when the trait being debated is intelligence. Current research involves identification of genes controlling different aspects of intellectual function. Some of these traits associated with intelligence are apparently polygenic. Learning Objectives By the end of this chapter, you should have an understanding of: a. the distinction between continuous and discontinuous traits. b. the characteristics of traits that demonstrate continuous variation. c. regression to the mean. d. the measurement of heritability and twin studies. e. the complex relationship between genotype and environment in polygenic traits. f. current information on the genetic control of intelligence.

3 Interactions of Genes and the Environment 35 Key Terms discontinuous variation continuous variation polygenic trait multifactorial traits complex traits regression to the mean lipoproteins heritability genetic variance environmental variance correlation coefficients dermatoglyphics monozygotic (MZ) intelligence quotient (IQ) dizygotic (DZ) concordance leptin essential hypertension familial hypercholesterolemia general cognitive ability quantitative trait loci Multiple Choice Circle the letter of the best answer. 1. The genes that control height in humans are different from many other genes because height genes a. have multiple alleles b. are incompletely penetrant c. produce continuously varying phenotypes d. have only dominant alleles 2. By definition, a polygenic trait that also is influenced by environmental factors a. is multifactorial b. is discontinuous c. is not affected by the number of dominant alleles present in the genotype d. has low heritability 3. Kristen and Todd are at the phenotypic extremes for a polygenic trait. Their offspring a. will favor one parent or the other b. will appear in four groups in a 9:3:3:1 ratio c. will appear to be an average of their two parents d. will be bell-shaped 4. When four loci are involved in determining a polygenic trait, the largest number of offspring will have a. seven out of eight dominant alleles b. four out of eight dominant alleles c. two out of eight dominant alleles d. no dominant alleles

4 36 Chapter 5 5. If you have a genetic predisposition to a disease, a. you have the disease from birth b. you will develop the disease as you get older c. you have only a few dominant alleles for the disease d. the development of the disease will be affected by environmental factors 6. If you have an aunt or uncle with a polygenic disorder a. you will eventually develop that disorder b. you have more dominant alleles than the average member of your population c. you will be more likely to develop the disorder than if your mother or father were affected d. you are not at risk to have children with the disorder. 7. The total number of dominant alleles controlling the phenotype of a polygenic trait a. is standard within a population b. is obtained by adding the dominant alleles at each locus c. may be found by multiplying every dominant allele times two d. may never be more than the total number of loci 8. Monozygotic twins a. are the result of the union of one sperm and one egg b. are formed when two sperm fertilize one egg c. result when one sperm fertilizes two eggs d. have 50 percent of their genetic traits in common 9. Dizygotic twins a. must be the same sex b. must be different sexes c. came from eggs ovulated in different cycles d. are genetically no more similar than any two children in the family e. more than one of the above 10. Leptin a. is a hormone produced by the pregnant uterus b. is produced by mutated genes possessed by diabetic mice c. helps female athletes put on muscle d. is a signal from fat cells to the brain 11. You would expect individuals with polygenic traits to a. have a variety of phenotypes b. separate easily into two or three phenotypic classes c. be very independent of environmental changes d. be indistinguishable from incompletely dominant traits 12. The concordance values for albinism are expected to be approximately

5 a. 100 percent for DZ twins and 25 percent for MZ twins b. 75 percent for MZ twins and 25 percent for DZ twins c. 75 percent for MZ twins and 25 percent for DZ twins d. 100 percent for MZ twins and 25 percent for DZ twins Interactions of Genes and the Environment 37 Fill-ins Supply the word or words to complete the statement The difference between height of pea plants and height in humans is that in pea plants, whereas in humans. 3. Two children in the same family have schizophrenia. This could be the result of heredity or. 4. Heritability can be explained as the contribution of to variation in phenotype. 5. Similarities in IQ among children who are genetically different but raised in the same family indicates the role that plays in the development of intelligence. 6. An IQ of 100 for a child of chronological age 10 indicates that the child s mental age is. 7. Skin color is a polygenic trait that seems to be controlled by genes at loci. Alice s genotype for a polygenic trait involving three loci is AABbCc. Use this information to answer the next four questions. 8. The gamete with the most dominant alleles that Alice can make is. 9. The gamete with the fewest dominant alleles that Alice can make is. 10. If Alice marries Ralph with genotype aabbcc, the most dominant alleles one of their children can have is. 11. Given the same marriage as question #10, the fewest dominant alleles one of their children can inherit is.

6 38 Chapter 5 Editing If the statement is true, label it T. If the statement is false, change the underlined word or phrase to make the statement true. Merely adding "no" or "not" is not an acceptable change. 1. If Chad, who has few dominant alleles for a polygenic trait, marries Diane, who has several dominant alleles for the same trait, their children will all look like their mother for this trait. 2. When a multifactorial trait such as neural tube defect is considered, researchers know that all observed variation within a population is due to the influence of genes. 3. The risk of cardiovascular disease is increased when HDL levels are high. 4. Familial hypercholesterolemia is an X-linked recessive disorder. 5. Dizygotic twins occur when the early embryo splits into two cell masses. 6. Widow s peak versus straight hairline is an example of a multifactorial trait. 7. Eye color within a family or population would be expected to have a heritability of Continuously varying traits are distinguished by having two distinct phenotypes. 9. Onset of tuberculosis has a concordance in MZ twins of 56 percent and a concordance in DZ twins of 22 percent. These concordance numbers indicate that your chance of developing TB is exclusively controlled by environment. 10. A pair of twins is concordant for a trait if both twins have the same appearance with respect to that trait. 11. Animal models have been used to study IQ. 12. Normal cognitive and intellectual function involves one or two genes at most. 13. Mouse genes controlling complex traits have been used to locate the same or related genes in humans. 14. Siblings adopted into different families still tend to have the similar levels of obesity. This indicates that the underlying cause of obesity is primarily environmental.

7 Interactions of Genes and the Environment 39 Short Discussion Questions/Problems 1. Assume five loci are responsible for a foot size, and that each dominant allele adds half a shoe size. Assume further that the smallest adult male shoe size is size 8. Use this information to answer the following: a. How many dominant alleles for foot size does Doug have if he wears a size 8 shoe? b. What is the largest shoe size possible given this relationship of genes to foot size? c. If 100 men picked at random have their feet measured for shoes, what shoe size should be the most common? The least common? d. To what number of dominant alleles does each shoe size correspond? e. Would you expect that women s foot sizes would be the same as men s? What size shoe does a woman with average feet wear? (If you don t know this bit of trivia, how could you find out?) 2. A polygenic trait has three loci. a. If Jeff (AABBCC) marries Kim (aabbcc), how many dominant alleles will be present in their F 1 children? b. If members of the F 1 generation have children, how many dominant alleles will the children in the largest phenotypic group have? 3. Explain the difference between multiple alleles of the same gene and polygenes. 4. Explain the difference between variable expressivity of a single gene and the variation occurring in a polygenic trait. 5. How do the data presented in Figure 5.18 illustrate the principle of regression to the mean? 6. What conclusions can be drawn from the fact that siblings adopted into different homes tend to be similar in their probability of obesity? 7. What genes have been found in mice to support the hypothesis that some obesity is genetic in nature? How do these genes function?

8 40 Chapter 5 8. A person with close relatives having a polygenic disorder such as hypertension is likely to have more dominant alleles than someone with no close hypertensive relatives. Should insurance companies be allowed to use this information in setting your insurance rates? Should this be grounds for being denied insurance? Defend your answers. 9. Assume that adult index finger length is controlled by three pairs of genes, each contributing one-eighth of an inch to a base finger length of 2.5 inches. a. How many different phenotypic classes are possible? b. If Kristin and Louie are heterozygous at all three loci, what is the shortest finger length that could occur in one of their children? What is the genotype of this child? c. What is the largest index finger one of Kristin and Louie s children might have? What is the genotype of this child? d. Which finger length will be most likely to occur? List some of the genotypes that will produce that finger length. e. What environmental factors or events might contribute to a person having a shorter finger than her/his genotype would indicate? f. If Sam has very short index fingers and Shawna has very long ones, what is the expected phenotype of their children? How does this illustrate regression to the mean? 10. When families with neural tube defects are examined, the heritability (H) falls between 0.0 and 1.0. What does this indicate about neural tube defects? a. How would this compare to the heritability of colorblindness in the same families? b. Pregnant women are being encouraged to increase their intake of folic acid to prevent neural tube defects. Will adequate levels of folic acid insure a baby without neural tube defects? Why or why not? 11. Harry and Larry are twins, as are Gary and Mary. Harry and Larry have always been referred to as identical, and Gary and Mary as fraternal. When blood typed, Harry and Larry are shown to be Type O and Type A respectively, whereas Gary and Mary are both Type B. Does this alter your opinion of whether Harry and Larry are identical? Of whether Gary and Mary are fraternal? Explain.

9 Interactions of Genes and the Environment Diabetes has a concordance value of 65 percent in MZ twins and 18 percent in DZ twins. What do these numbers indicate about the relative importance of genetics in the diabetic phenotype? What (if any) environmental factors could be involved in the development of the diabetic phenotype?