3. Homozygosity for lethal recessive alleles stops development before birth, eliminating an offspring class.
4. A gene can have multiple alleles because its sequence can deviate in many ways. Different allele combinations may produce different variations of the phenotype.
5. Heterozygotes of incompletely dominant alleles have phenotypes intermediate between those associated with the two homozygotes. Codominant alleles are both expressed in the phenotype.
6. In epistasis, one gene affects the phenotype of another.
7. An incompletely penetrant genotype is not expressed in all individuals who inherit it. Phenotypes that vary in intensity among individuals are variable in expressivity.
8. Pleiotropic genes have several expressions.
9. In genetic heterogeneity, two or more genes specify the same phenotype.
10. A phenocopy is a characteristic that appears to be inherited but is environmentally caused.
5.3 Mitochondrial Genes
11. Only females transmit mitochondrial genes; males can inherit such a trait but cannot pass it on.
12. Mitochondrial genes do not cross over, and they mutate more frequently than nuclear DNA.
13. The 37 mitochondrial genes encode tRNA, rRNA, or proteins involved in protein synthesis or energy reactions.
14. Many mitochondrial disorders are heteroplasmic, with mitochondria in a single cell harboring different alleles.
5.4 Linkage
15. Genes on the same chromosome are linked and, unlike genes that independently assort, produce many individuals with parental genotypes and a few with recombinant genotypes.
16. Linkage maps depict linked genes. Researchers can examine a group of known linked DNA sequences (a haplotype ) to follow the inheritance of certain chromosomes.
17. Knowing whether linked alleles are in cis or trans, and using crossover frequencies from pooled data, one can predict the probabilities that certain genotypes will appear in progeny.
18. Genetic linkage maps assign distances to linked genes based on crossover frequencies. Today many genetic markers spanning the genome are used to compare large groups of individuals, one of which has a particular phenotype. These genome-wide association studies are used in disovering genetic variants that contribute to multifactorial conditions.
www.mhhe.com/lewisgenetics9
Answers to all end-of-chapter questions can be found at www.mhhe.com/lewisgenetics9. You will also find additional practice quizzes, animations, videos, and vocabulary flashcards to help you master the material in this chapter.
Review Questions
1. Explain how each of the following phenomena can disrupt Mendelian phenotypic ratios.
a. lethal alleles b. multiple alleles c. incomplete dominance d. codominance e. epistasis
f. complete penetrance g. variable expressivity h. pleiotropy
i. a phenocopy j. genetic heterogeneity
2. How does the relationship between dominant and recessive alleles of a gene differ from epistasis?
3. Why can transmission of an autosomal dominant trait with incomplete penetrance look like autosomal recessive inheritance?
4. How does inheritance of ABO blood type exhibit both complete dominance and codominance?
5. How could two people with albinism have a child who has normal skin pigment?
6. How do the porphyrias exhibit variable expressivity, pleiotropy and genetic heterogeneity?
7. How can epistasis explain incomplete penetrance?
8. The lung condition emphysema may be caused by lack of an enzyme, or by smoking. Which cause is a phenocopy?
9. List three ways that mtDNA differs from DNA in a cell’s nucleus.
10. Describe why inheritance of mitochondrial DNA and linkage are exceptions to Mendel’s laws.
11. How does a pedigree for a maternally inherited trait differ from one for an autosomal dominant trait?
12. If researchers could study pairs of human genes as easily as they can study pairs of genes in fruit flies, how many linkage groups would they detect?
13. Describe three types of genetic markers.
14. The popular media often use words that have precise meanings in genetics, but more general common meanings. Explain the two types of meanings of “linked”
and “marker.”
Applied Questions
1. For each of the diseases described in situations a through i, indicate which of the following phenomena (A–H) is at work.
More than one may apply.
A. lethal alleles B. multiple alleles C. epistasis
D. incomplete penetrance E. variable expressivity F. pleiotropy
G. a phenocopy H. genetic heterogeneity
a. A woman has severe neurofi bromatosis type 1. She has brown spots on her skin and several large tumors beneath her skin. A genetic test shows that her son has inherited the disease-causing autosomal dominant allele, but he has no symptoms.
b. A man would have a widow’s peak, if he wasn’t bald.
c. A man and woman have six children. They also had two stillbirths—fetuses that died shortly before birth.
d. Mutations in any of at least three genes cause familial ALS.
e. A woman with dark brown skin uses a bleaching cream with a chemical called hydroquinone that darkens her fi ngertips and ears, just like alkaptonuria.
f. In Labrador retrievers, the B allele confers black coat color and the b allele brown coat color. The E gene controls the expression of the B gene. If a dog inherits the E allele, the coat is golden no matter what the B genotype is. A dog of genotype ee expresses the B (black) phenotype.
g. Two parents are heterozygous for genes that cause albinism, but each gene specifi es a diff erent enzyme in the biochemical pathway for skin pigment synthesis. Their children thus do not face a 25 percent risk of having albinism.
h. Alagille syndrome (MIM 118450), in its most severe form, prevents the formation of ducts in the gallbladder, causing liver damage. Aff ected children also usually have heart murmurs, unusual faces, a line in the eye, and butterfl y- shaped vertebrae. Such children often have one otherwise healthy parent who has a heart murmur, unusual face, and butterfl y vertebrae.
i. Two young children in a family have terribly decayed teeth.
Their parents think it is genetic, but the true cause is a babysitter who puts them to sleep with juice bottles in their mouths.
2. If many family studies for a particular autosomal recessive condition reveal fewer affected individuals than Mendel’s law predicts, the explanation may be either incomplete penetrance or lethal alleles. How might you use haplotypes to determine which of these two possibilities is the cause?
3. A man who has type O blood has a child with a woman who has type A blood. The woman’s mother has AB blood, and her father, type O. What is the probability that the child is of blood type
a. O b. A
c. B d. AB?
4. Enzymes are used in blood banks to remove the A and B antigens from blood types A and B. This makes the blood type O.
a. Does this alter the phenotype or the genotype?
b. Removing the A and B antigens from red blood cells is a phenocopy of what genetic phenomenon?
5. Ataxia-oculomotor apraxia syndrome (MIM 208920), which impairs the ability to feel and move the limbs, usually begins in early adulthood. The molecular basis of the disease is impairment of ATP production in mitochondria, but the mutant gene is in the nucleus of the cells. Would this disorder be inherited in a Mendelian fashion? Explain your answer.
6. What is the chance that Greg and Susan, the couple with nail- patella syndrome, could have a child with normal nails and type AB blood?
7. A gene called secretor (MIM 182100) is located 1 map unit from the H gene that confers the Bombay phenotype on chromosome 19. Secretor is dominant, and a person of either genotype SeS e or Sese secretes the ABO and H blood type antigens in saliva and other body fluids.
This secretion, which the person is unaware of, is the phenotype. A man has the Bombay phenotype and is not a secretor. A woman does not have the Bombay phenotype and is a secretor. She is a dihybrid whose alleles are in cis. What is the chance that their child will have the same genotype as the father?
8. In prosopagnosia (MIM 610382), a person has “face blindness”—he or she cannot identify individuals by their faces. It is inherited as an autosomal dominant trait, and affects people to different degrees. Some individuals learn early in life—perhaps from a parent—to identify people by other features, such as voice or style of dress, and so appear not to have the condition. Only a small percentage of cases are inherited; most are the result of stroke or brain injury.
www.faceblind.org offers tests to help you imagine what it is like not to be able to recognize faces—not even your own.
Which of the following does face blindness demonstrate?
Explain your choices.
a. incomplete penetrance b. variable expressivity c. pleiotropy
d. phenocopy
9. Many people who have the “iron overload” disease hereditary hemochromatosis (MIM 235200; see section 20.2) are homozygous for a variant of the C282Y gene. How would you determine the penetrance of this condition?
10. A Martian creature called a gazook has 17 chromosome pairs. On the largest chromosome are genes for three traits—round or square eyeballs ( R or r ); a hairy or smooth tail ( H or h ); and 9 or 11 toes ( T or t ). Round eyeballs, hairy tail, and 9 toes are dominant to square eyeballs, smooth tail, and 11 toes. A trihybrid male has
15. For some of the porphyrias, attacks are precipitated by an environmental trigger. Using MIM, describe factors that can trigger an attack of any of the following:
a. acute intermittent porphyria b. porphyria cutanea tarda c. coproporphyria d. porphyria variegata
e. erythropoietic protoporphyria
Web Activities
11. Go to the Family Village website. Family Village is a
clearinghouse for disease information. Click on library. Explore the diseases, and identify one that exhibits pleiotropy.
12. Go to the United Mitochondrial Disease Foundation website and describe the phenotype of a mitochondrial disorder.
13. Browse the National Center for Biotechnology Information (NCBI) site, and list three sets of linked genes. Consult MIM to describe the trait or disorder that each specifies.
14. Use MIM to identify a genetically heterogeneic condition, and explain why this description applies.
• 4 have round eyeballs, a smooth tail, and 11 toes • 4 have square eyeballs, a hairy tail, and 9 toes a. Draw the allele confi gurations of the parents.
b. Identify the parental and recombinant progeny classes.
c. What is the crossover frequency between the R and T genes?
offspring with a female who has square eyeballs, a smooth tail, and 11 toes on each of her three feet. She gives birth to 100 little gazooks, who have the following phenotypes:
• 40 have round eyeballs, a hairy tail, and 9 toes • 40 have square eyeballs, a smooth tail, and 11 toes • 6 have round eyeballs, a hairy tail, and 11 toes • 6 have square eyeballs, a smooth tail, and 9 toes
Case Studies and Research Results
16. Shiloh Winslow is deaf. In early childhood, she began having fainting spells, especially when she became excited. When she fainted while opening Christmas gifts, her parents took her to the hospital, where doctors said, as they had in the past, that there wasn’t a problem. As the spells continued, Shiloh became able to predict the attacks, telling her parents that her head hurt beforehand. Her parents took her to a neurologist, who checked Shiloh’s heart and diagnosed long QT syndrome with deafness, a severe form of inherited heartbeat irregularity (see Reading 2.2). Ten different genes can cause long QT syndrome. The doctor told them of a case from 1856: a young girl, called at school to face the headmaster for an infraction, became so agitated that she dropped dead. The parents were not surprised; they had lost two other children to great excitement.
The Winslows visited a medical geneticist, who discovered that each parent had a mild heartbeat
irregularity that did not produce symptoms. Shiloh’s parents had normal hearing. Shiloh’s younger brother Pax was also hearing-impaired and suffered night terrors, but had so far not fainted during the day. Like Shiloh, he had the full syndrome. Vivienne, still a baby, was also tested. She did not have either form of the family’s illness; her heartbeat was normal.
Today, Shiloh and Pax are treated with beta blocker drugs, and each has an implantable defibrillator to correct a potentially fatal heartbeat. Shiloh’s diagnosis may have saved her brother’s life.
a. Which of the following applies to the condition in this family?
i. genetic heterogeneity ii. pleiotropy
iii. variable expressivity iv. incomplete dominance v. a phenocopy
b. How is the inheritance pattern of this form of long QT syndrome similar to that of familial hypercholesterolemia?
c. How is it possible that Vivienne did not inherit either the serious or asymptomatic form of the illness?
d. Do the treatments for the condition aff ect the genotype or the phenotype?
17. Barnabas Collins has congenital erthyropoietic porphyria, and his wife Angelique is a carrier of ALA dehydratase deficiency.
What is the chance that if they have a child, he or she will have a porphyria?
Forensics Focus
18. “Earthquake McGoon” was 32 years old when the plane he was piloting over north Vietnam was hit by groundfire on May 6, 1954. Of the five others aboard, only two survived.
McGoon, actually named James B. McGovern, was well known for his flying in World War II, and for his jolliness. Remains of a man about his height and age at death were discovered
in late 2002, but could not be identified by dental records.
However, DNA sampled from a leg bone enabled forensic scientists to identify him. Describe the type of DNA likely analyzed, and what further information was needed to make the identification.
A Controversial Hypothesis: Mental Illness, Mom, and Dad
Mental illnesses have been extremely difficult to classify. They run in families yet not in predictable patterns, and attempts to pin such disorders as autism, depression, bipolar disorder, and schizophrenia on specific genes or environmental influences have been mired in complexity. A sociologist from London and an evolutionary biologist from Canada have turned fresh eyes to the challenge of classifying mental disorders.
Their view, which they call the “imprinted brain hypothesis,” places the disorders on a continuum, rather than in the separate bins of the psychiatrist’s diagnostic manual. Sociologist Christopher Badcock and biologist Bernard Crespi argue that the disorders stem not so much from which gene variants patients inherit, but whether the alleles come from the father or mother—a phenomenon called genomic imprinting.
The idea did not come from DNA sequencing, but from simple observation: Dr. Badcock noticed that the way that children with autism avoid making eye contact with people and speak of themselves in the third person seems opposite the way that people with paranoid schizophrenia imagine they are being followed by others and have deluded ideas of their own importance. This view echoes a genetic phenomenon called imprinting, in which certain genes are physically blocked from being expressed, by being covered in chemical methyl (CH 3 ) groups, if they come from one parent but not if they come from the other. Imprinting is well-studied in certain pairs of illnesses that have symptoms similar to autism if a particular gene variant comes from the father, yet symptoms more like schizophrenia or a mood disorder if inherited from the mother.
Matters of Sex
Chapter Contents