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CHAPTER 20
GENES AND MEDICAL GENETICS
BEHAVIORAL OBJECTIVES
1. Recognize the conventions biologists use to represent alleles using letters. [20.1, p.404, Fig. 20.2]
2. Distinguish between phenotype and genotype, dominant and recessive, and homozygous and heterozygous.
[20.1, p.404]
3. Understand how gametes carry only one gene for each trait. [20.2, po. 405, Fig. 20.3]
4. List a variety of simple recessive and dominant traits in humans. [20.2, p. 405, Fig. 20.4]
5. Solve one-trait autosomal genetics problems using a Punnett square. [20.2, p. 406, Fig. 20.5-20.6]
6. Describe disorders that are inherited as recessive genes. [20. 2, p.407, Fig. 20.7]
7. Describe disorders that are inherited as a dominant gene. [20.2, p.408, Fig. 20.8]
8. Explain how to interpret pedigree charts. [20.2, p.409, Fig. 20.9, 20.10, 20.15]
9. Discuss one trait inherited as a polygenic trait. [20.3, p.411, Fig. 20.11]
10. Explain what is meant by multiple alleles and give an example. [20.3, p.412, Fig, 20.12]
11. Explain what is meant by incomplete dominance and give an example. [20.4, p.413, Fig. 20.13]
12. Show how sex-linked traits are generally inherited on the X chromosome. [20.4, p.414, Fig. 20.14]
13. Discuss several sex-linked disorders. [20.4, pp.415-416, Figs. 20.15-20.16]
14. Distinguish between sex-linked and sex-influenced traits. [20.4, pp.415-418]
15. Understand and use the bold-faced and italicized terms included in this chapter. [Understanding Key Terms,
p.419]
EXTENDED LECTURE OUTLINE
20.1 Genotype and Phenotype
The genotype represents the actual genes of an individual. Alleles are alternate forms of a gene at the same gene
locus. Individuals can be homozygous dominant (EE) for a trait, heterozygous (Ee), or homozygous recessive (ee).
The phenotype refers to the outward expression of the genotype (e.g., red hair).
Mader VRL CD-ROM
Image 0383l.jpg (Fig. 20.1)
Image 0384l.jpg (Fig. 20.2)
Mader ESP Modules Online
Genetics/Classical Genetics/Introduction
Transparencies
288 (Fig. 20.2)
20.2 Dominant/Recessive Traits
Forming the Gametes
During oogenesis and spermatogenesis (collectively, gametogenesis), chromosome number is reduced to half, and
each gene pair for a trait is separated, so that the offspring receives one gene for each trait from each parent.
Figuring the Odds
A parent who is homozygous dominant or homozygous recessive for a trait can pass on only one type of gamete in
each case. Heterozygous parents can pass on either the dominant or the recessive gene for a given trait. A Punnett
square is useful for determining the possible outcomes of a genetic cross. A cross of two individuals who are both
homozygous for a trait results in a 100% chance of having an offspring who is homozygous for the trait.
A cross of two heterozygotes results in a 25% chance that the offspring will be homozygous recessive, a 50%
chance that the offspring will be heterozygous, and a 25% chance that the offspring will be homozygous dominant.
Each offspring has a 25% chance of being homozygous recessive or of being homozygous dominant.
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Recessive Disorders
Tay-Sachs Disease
Tay-Sachs disease is inherited as an autosomal recessive. Between four and six months of age, an affected
infant shows neurological impairment. The child gradually becomes blind, helpless, and paralyzed, and
usually dies by age four. Tay-Sachs results from a lack of hexosaminidase A and the storage of its
substrate, glycosphingolipid in lysosomes. This disease is most prevalent in Jewish people from central and
eastern European descent.
Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic disease among U.S. Caucasians. The thick mucus in
bronchial passageways and pancreatic ducts interferes with the functioning of these organs. The defect lies
in a chloride ion transport protein within plasma membranes. When chloride passes through, water
normally follows. In cystic fibrosis patients, a lack of water following through results in the thick mucus.
The gene for the defect is on chromosome 7.
Phenylketonuria (PKU)
Individuals with phenylketonuria lack an enzyme needed for the normal metabolism of phenylalanine.
Phenylketone thus accumulates in the urine. If the infant is not put on a phenylalanine-restrictive diet in
infancy until age seven, brain damage and severe mental retardation result.
Dominant Disorders
Neurofibromatosis
Neurofibromatosis (von Recklinghausen disease) is inherited as an autosomal dominant. People with this
condition develop benign neurofibromas under the skin and in various organs. The effects can range from
mild to severe, and some neurological impairment if possible. The gene for this trait is a nested gene on
chromosome 17.
Huntington Disease
Huntington disease is also inherited as an autosomal dominant and is characterized by progressive
neurological degeneration of brain cells, resulting in personality disorders and muscle spasms. No treatment
exists, and death occurs a decade or so after the symptoms appear. Individuals are more likely to be
affected if the disorder is inherited from their fathers, a characteristic of genomic imprinting.
Pedigree Charts
Pedigree charts are a means of constructing a family tree and indicate which individuals are affected by a
trait. Since recessive and dominant traits exhibit different patterns of inheritance, pattern of inheritance can
be partially determined by examining a pedigree chart. Carriers are normal individuals capable of
producing affected children.
Mader VRL CD-ROM
Image 0385l.jpg (Fig. 20.3)
Image 0386al.jpg (Fig. 20.4a, b)
Image 0386bl.jpg (Fig. 20.4c, d)
Image 0386cl.jpg (Fig. 20.4e, f)
Image 0386dl.jpg (Fig. 20.4g, h)
Image 0387l.jpg (Fig. 20.5)
Image 0388l.jpg (Fig. 20.6)
Image 0389l.jpg (Fig. 20.7)
Image 0390l.jpg (Fig. 20.8)
Image 0391l.jpg (Fig. TA20.1)
Image 0392l.jpg (Fig. 20.9)
Image 0393l.jpg (Fig. 20.10)
Life Science Animations VRL 2.0
Principles of Inheritance/Genetics/ Monohybrid Cross Done by
Mendel
Principles of Inheritance/Genetics/ Independent Assortment
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289 (Fig. 20.3)
290 (Fig. 20.5)
291 (Fig. 20.6)
292 (Fig. 20.9)
293 (Fig. 20.10)
20.3 Beyond Simple Inheritance Patterns
Polygenic Inheritance
Polygenic traits are those governed by more than one gene pair. Several pairs of genes may be involved in
determining phenotype.
Skin Color
The inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of
polygenic inheritance.
Polygenic Disorders
Many human traits, like allergies, schizophrenia, cleft lip, and hypertension, among others, appear to be
inherited as polygenic traits. The expression of some genes is subject to environmental influences (e.g., a
child allergic to ragweed will never express that trait while living in the Arctic).
Multiple Allelic Traits
In multiple alleles, more than two alternative types exist for a gene pair. ABO blood grouping is an example.
ABO Blood Types
The ABO blood grouping represents surface marker proteins on red blood cells. A person can have a gene
for an A marker or a B marker, which are codominant, or lack an A or B marker, designated type O, which
is recessive.
Human ABO blood types can then be type A (which can be AA or AO), type B (BB or BO), type AB (AB),
or type O (OO).
Incompletely Dominant Traits
Patterns of dominance often go beyond simple dominant or recessive traits. Codominance means that both alleles are
expressed (type AB blood). Incomplete dominance is exhibited when the heterozygote shows not the dominant trait
but an intermediate phenotype, representing a sort of blending of traits (e.g., skin color or hair type).
Sickle-Cell Disease
Sickle-cell disease is an example of incomplete dominance. An individual with two genes for normal
hemoglobin has normal hemoglobin. A heterozygote has a normal gene and a gene for sickled hemoglobin.
An individual with two sickling genes has sickle-cell disease. What may have maintained this apparently
detrimental gene in equatorial Africa is that heterozygotes for this trait have a marked resistance to the
malarial parasite prevalent in the region.
Mader VRL CD-ROM
Image 0394al.jpg (Fig. 20.11a)
Image 0394bl.jpg (Fig. 20.11b)
Image 0395l.jpg (Fig. 20.12)
Image 0396l.jpg (Fig. 20.13)
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Principles of Inheritance/Genetics/ Inheritance of Sickle-Cell
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Transparencies
294 (Fig. 20.11b)
295 (Fig. 20.12)
296 (Fig. 20.13)
20.4 Sex-Linked Traits
Sex-linked traits are genes (traits) carried most frequently on the X chromosome. (The Y chromosome is too small.)
X-Linked Alleles
In X-linked traits, the gene is carried on the X chromosome. Since males have only one copy of the X chromosome,
they show the phenotype for the allele they possess and are thus much more likely than females to show a recessive
trait. A female must have two copies of a recessive trait (one on each X chromosome) to display it. If a female has
only one copy of a recessive gene, she is said to be a carrier and will pass the trait on to 50% of her sons, on average.
X-Linked Disorders
Color Blindness
Three types of cones are in the retina: those that detect red, those that detect green, and those that detect
blue. Genes for blue cones are autosomal; those for red and green cones are on the X chromosome. Males
are much more likely to have red/green colorblindness than are females.
Muscular Dystrophy
Duchenne muscular dystrophy is X-linked and is characterized by progressive muscle deterioration during
childhood. The absence of the protein dystrophin causes the disorder.
Hemophilia
Hemophilia (bleeder’s disease) can be traced to Queen Victoria of England and is characterized by the
absence or minimal presence of one of two different clotting factors. Again, males are much more prone to
this trait than females and often require blood transfusions.
Sex-Influenced Traits
Some traits carried on autosomes such as male-pattern baldness, can be influenced by gender. In this instance, the
male hormone testosterone is the culprit.
Mader VRL CD-ROM
Image 0397l.jpg (Fig. 20.14)
Image 0398l.jpg (Fig. 20.15)
Image 0399l.jpg (Fig. 20.16)
Image 0400l.jpg (Fig. 20A)
Image 0401l.jpg (Fig. 20.17)
Image 0402l.jpg (Fig. TA20.2)
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Principles of Inheritance/Genetics/X-Linked Inheritance
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Genetics/Classical Genetics/Beyond Mendel
Transparencies
297 (Fig. 20.14)
298 (Fig. 20.15)
299 (Fig. 20.16)
300 (Fig. 20.17)
301 (Fig. TA20.1)
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SEVENTH EDITION CHANGES
New/Revised Text:
This was chapter 19 in the previous edition. This chapter has been fewer A heads. The new section 20.3
Beyond Simple Inheritance Patterns includes polygenic inheritance, multiple allelic traits, and incompletely
dominant traits. Four sets of Practice Problems have been added.
20.2 Dominant/Recessive Traits. Recessive Disorders are now discussed before dominant disorders.
Pedigree Charts makes it clear that with recessive genetic disorders, when both parents are affected, all children are
affected (and why); and with dominant genetic disorders, two affected parents can have an unaffected child (and
why). This information will help the student be able to understand and successfully answer the related practice
problems.
20.3 Beyond Simple Inheritance Patterns includes polygenic inheritance, multiple allelic traits, and
incompletely dominant traits.
New Bioethical Focus: Genetic Profiling
New/Revised Figures:
20.2 Genetic inheritance; 20.9 Autosomal recessive pedigree chart; 20.10 Autosomal dominant pedigree
chart; 20.12 Inheritance of blood type; 20.13 Incomplete dominance; 20.14 Cross involving an X-linked allele;
20.15 X-linked recessive pedigree chart; 20A Genetic profiling
STUDENT ACTIVITIES
“The Century of the Gene”
1. Ask students to read “Introduction: The Biotech Century” (Time, January 11, 1999, page 42). Then have them
read Michael Lemonick’s “Designer Babies” (pp.64-66). [Ask the library to hold this issue on reserve or make
copies of these articles for your students to use.] Use these articles to generate a discussion of which are
appropriate uses of this new technology and which are not.
Bioethics of Genetic Profiling
2. Read the Bioethical Focus for this chapter (p. 417) aloud to your students. According to this reading, genetic
profiling can be used in two ways: to benefit patients, and to discriminate against them. Initiate a discussion
with your students. Health insurance companies are already beginning to deny or cancel insurance for women
carrying the breast cancer gene. Is this ethical? Should other people insured through the same company pay
higher rates to help cover the claims of women who have breast cancer treatment? Would it be better to have the
insurance company pay for or require prophylactic removal of both breasts prior to cancer development? Should
women carrying this or another deleterious gene be sterilized? Address some of the other questions raised in
the Bioethical Focus.
Genetics of Your Students
3. Certain obvious human traits can demonstrate the idea of dominant and recessive traits, and can even be
discussed in the greater context of population genetics. Canvas your class to see how many fall into each of the
following categories:
Expression of Dominant Trait
Expression of Recessive Trait
Dark hair color
Light or red hair color
Farsightedness
Normal vision
Freckles
No freckles
Dark eyes
Blue eyes
Free earlobes
Attached earlobes
Six fingers or toes
Five fingers or toes
Cheek dimples
Lack of dimples
Full lips
Thin lips
Widow’s peak
Straight hair line
Tongue roller
Cannot roll tongue
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