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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
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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.
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
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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
Interactions of Genes and the Environment 37
a.
b.
c.
d.
100 percent for DZ twins and 25 percent for MZ twins
75 percent for MZ twins and 25 percent for DZ twins
75 percent for MZ twins and 25 percent for DZ twins
100 percent for MZ twins and 25 percent for DZ twins
Fill-ins
Supply the word or words to complete the statement.
1-2. 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 ________.
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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
0.5.
8. 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.
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 F1 children?
b. If members of the F1 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?
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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.
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12. 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?