9. Y-linked traits are rare and are passed from fathers to sons only.
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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.
b. a normal XX individual
c. an XY individual with a block in testosterone synthesis 4. List the events that must take place for a fetus to develop as a
female.
5. Cite evidence that may point to a hereditary component to homosexuality.
Review Questions
1. How is sex expressed at the chromosomal, gonadal, phenotypic, and gender identity levels?
2. How do genes in the pseudoautosomal region of the Y chromosome differ from genes in the male-specific region (MSY)?
3. What are the phenotypes of the following individuals?
a. a person with a mutation in the SRY gene, rendering it nonfunctional
2. In severe Hunter syndrome, lack of the enzyme iduronate sulfate sulfatase leads to buildup of certain carbohydrates swelling the liver, spleen, and heart. In mild cases, deafness may be the only symptom. Intellect is usually unimpaired, and life span can be normal. Hunter syndrome is X-linked recessive. A man with mild Hunter syndrome has a child with a woman who is a carrier.
a. What is the probability that a son inherits Hunter syndrome?
b. What is the chance that a daughter inherits Hunter syndrome?
c. What is the chance that a daughter is a carrier?
3. Amelogenesis imperfecta (MIM 301200) is X-linked dominant. Affected males have extremely thin enamel on each tooth. Female carriers have grooved teeth from uneven deposition of enamel. Why might the phenotype differ between the sexes?
Applied Questions
1. To answer the following questions, consider these population data on sex ratios:
Selected sex ratios at birth Selected sex ratios after age 65
Nation Sex ratio Nation Sex ratio
Costa Rica 970 Rwanda 620
Tanzania 1,000 South Africa 630
Liechtenstein 1,010 France 700
South Africa 1,020 United States 720
United States 1,050 Qatar 990
Sweden 1.060 Montserrat 1,060
Italy 1,070 Bangladesh 1,160
China 1,130 Nigeria 990
a. In Rwanda, South Africa, France, and the U.S., males die, on average, signifi cantly younger than females. What types of information might explain the diff erence?
b. In Costa Rica, how many males at birth are there for every 100 females?
c. In which country listed do males tend to live the longest?
disorders that involve imprinting, one transmitted from the mother and one from the father, and use MIM to describe them.
Web Activities
4. Visit the National Center for Biotechnology Information (NCBI) website. Identify an X-linked disorder, then find it in MIM and describe it.
5. At the Imprinted Gene Catalogue website, click on “search by species name” and then click on “complete list.” Find two
Case Studies and Research Results
6. For each case description, identify the principle at work from the list that follows. More than one answer per case may apply.
A. Y-linked inheritance B. X-linked recessive
C. X-linked dominant inheritance D. Sex-limited inheritance E. Sex-infl uenced inheritance
F. X inactivation or manifesting heterozygote G. Uniparental disomy
H. Imprinting abnormality
a. In a three-generation family, sixteen members have speech-language disorder (MIM 602081) and cannot speak.
The gene that is mutant is called FOXP2. The speechless family members inherited both copies of the gene from their mothers and none from their fathers.
6. Why is it unlikely one would see a woman who is homozygous for an X-linked dominant condition?
7. What is the basis of sex ratio at birth?
8. Traits that appear more frequently in one sex than the other may be caused by genes that are inherited in an X-linked, sex- limited, or sex-influenced fashion. How might you distinguish among these possibilities in a given individual?
9. Why are male calico cats very rare?
10. How might X inactivation cause patchy hairiness in women who have congenital generalized hypertrichosis, even though the disease-causing allele is dominant?
11. How does X inactivation even out the “doses” of X-linked genes between the sexes?
12. Cite evidence that genetic contributions from both parents are necessary for normal prenatal development.
13. Prader-Willi and Angelman syndromes are more common in children conceived with certain assisted reproductive technologies ( in vitro fertilization and intracytoplasmic sperm injection) than among the general population. What process may these procedures disrupt?
b. Six-year-old LeQuan inherited Fabry disease (MIM 301500) from his mother, who is a heterozygote for the causative mutation. The gene, on the X chromosome, encodes a lysosomal enzyme. LeQuan would die before age 50 of heart failure, kidney failure, or a stroke, but fortunately he can be treated with twice-monthly infusions of the enzyme. His mother, Echinecea, recently began experiencing recurrent fevers, a burning pain in her hands and feet, a rash, and sensitivity to cold. She is experiencing mild Fabry disease.
c. The Chandler family has many male members who have a form of retinitis pigmentosa (RP) in which the cells that capture light energy in the retina degenerate, causing gradual visual loss. Several female members of the family presumed to be carriers because they have aff ected sons are tested for RP genes on chromosomes 1, 3, 6, and the X, but do not carry these RP genes. Many years ago, Rachel married her cousin Ross, who has the family’s form of RP.
They had six children. The three sons are all aff ected, but their daughters all have normal vision.
d. Simon’s mother and her sister are breast cancer survivors, and their mother died of the disease. Simon’s sister Maureen has a genetic test and learns that she, too, has inherited the BRCA1 gene. Simon has two daughters, but doesn’t want to be tested because he thinks a man cannot transmit a trait that aff ects a body part that is more developed in females.
e. Tribbles are extraterrestrial mammals that long ago invaded a starship on the television program Star Trek. A gene called frizzled causes kinky hair in female tribbles who inherit just one allele. However, two mutant alleles must be inherited for a male tribble to have kinky hair.
f. Prozac died at age 16 of Lowe syndrome (MIM 309000).
He was slightly mentally retarded, had visual problems (cataracts and glaucoma), seizures, poor muscle tone, and progressive kidney failure, which was ultimately fatal.
His sister Lunesta is pregnant, and wonders whether she is a carrier of the disease that killed her brother. She remembers a doctor saying that her mother Yaz was a carrier. Lunesta’s physician determines that she is a carrier
because she has cataracts, which is a clouding of the lenses. It has not yet aff ected her vision. When a prenatal test reveals that Lunesta’s fetus is a female, her doctor tells her not to worry about Lowe syndrome.
g. Mating among Texas fi eld crickets depends upon females responding to a male mating call. The sounds must arrive at a particular frequency to excite the females, who do not sing back in response. However, females can pass on a trait that confers frequency of singing.
h. When Winthrop was a baby, he was diagnosed with
“failure to thrive.” At 14 months of age, he suddenly took an interest in food, and his parents couldn’t feed him fast enough. By age 4, Winthrop was obese, with disturbing behavior. He was so hungry that after he’d eaten his meal and everyone else’s leftovers, he’d hunt through the garbage for more. Finally a psychiatrist who had a background in genetics diagnosed Prader-Willi syndrome. Testing showed that the allele for the Prader- Willi gene that Winthrop had inherited from his father was abnormally methylated.
i. Certain breeds of dogs have cryptoorchidism, in which the testicles do not descend into the scrotum. The trait is passed through females.
7. Reginald has mild hemophilia A that he can control by taking a clotting factor. He marries Lydia, whom he met at the hospital where he and Lydia’s brother, Marvin, receive their treatment. Lydia and Marvin’s mother and father, Emma and Clyde, do not have hemophilia. What is the probability that Reginald and Lydia’s son will inherit hemophilia A?
8. Harold works in a fish market, but the odor does not bother him because he has anosmia (MIM 301700), an X-linked recessive lack of sense of smell. Harold’s wife, Shirley, has a normal sense of smell. Harold’s sister, Maude, also has a normal sense of smell, as does her husband, Phil, and daughter, Marsha, but their identical twin boys, Alvin and Simon, cannot detect odors. Harold and Maude’s parents, Edgar and Florence, can smell normally. Draw a pedigree for this family, indicating people who must be carriers of the anosmia gene.
Hold the Anchovies: Genes and Diet Cause Gout
The pain of gout in one’s big toe is sudden and excruciating. Two to six million people in the United States have the condition, which results from deposition of uric acid crystals in a joint. Once known as “the disease of kings” because it was seen mostly among royalty, we now know that gout occurs when a genetically susceptible individual eats a diet rich in organ meats and fish. These foods are mostly muscle tissue, packed with mitochondria that have abundant nucleic acid purine bases that form uric acid.
The Egyptians first described gout in 2640 B.C., and in the fifth century B.C., Hippocrates called it “the unwalkable disease.” King Charles I of Spain gave up his vast empire in 1556 due to the painful condition. In 2006, Spanish researchers confirmed the king’s diagnosis by detecting uric acid deposits in the end joint of a finger that, for reasons unknown, had been preserved in a small box apart from the rest of the king.
Today gout provides a great example of the power of genome-wide association studies to identify inherited susceptibility to a painful condition. Such a study seeks associations by mining very large samples.
To search for gout susceptibility alleles, researchers scanned the genomes of 26,714 individuals enrolled in three major health studies in the United States and the Netherlands for 500,000 SNPs (single-base sites in the genome that vary in at least 1 percent of a population). Just three of the half million SNPs were common to those individuals who had high uric acid levels in the blood, which indicates a tendency toward developing gout. The SNPs were very rare in others.
Multifactorial Traits
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