Fragile X Mutations Affect Boys and Their Grandfathers
14.1 Population Genetics Underlies Evolution
1. A population is a group of interbreeding members of the same species in a particular area. Their genes constitute the gene pool.
2. Population genetics considers allele, genotype, and phenotype frequencies to reveal microevolution. Phenotypic frequencies can be determined empirically, then used in algebraic expressions to derive other frequencies.
3. Genotype frequencies change if migration, nonrandom mating, genetic drift, mutations, or natural selection operate.
In Hardy-Weinberg equilibrium, frequencies are not changing.
14.2 Constant Allele Frequencies
4. Hardy and Weinberg proposed an algebraic equation to explain the constancy of allele frequencies. This would show why dominant traits do not increase and recessive traits do not decrease in populations. The Hardy-Weinberg equation is a binomial expansion used to represent genotypes in a population.
5. Hardy-Weinberg equilibrium is demonstrated by following gamete frequencies as they recombine in the next
generation. In equilibrium, these genotypes remain constant if evolution is not occurring. When the equation p 2 + 2 pq + q 2 represents a gene with one dominant and one recessive allele, p 2 corresponds to the frequency of homozygous
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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.
8. For a forensics case, why would tracking VNTR sequences provide a more reliable identification than tracking STRs?
9. Why are specific population databases needed to interpret DNA profiles?
10. How is the Hardy-Weinberg equation used to predict the recurrence of X-linked recessive traits?
11. What is the basis of assigning a probability value to a particular copy number variant?
12. Under what circumstances is analysis of repeats in mtDNA valuable?
13. Describe the following ways to identify or distinguish among individuals at the DNA level:
a. VNTRs b. STRs
c. SNPs (see chapters 7 and 12)
Review Questions
1. What is a population? List three populations.
2. “We like him, he seems to have a terrific gene pool,” say the parents upon meeting their daughter’s boyfriend. Why doesn’t their statement make sense?
3. Explain the differences among an allele frequency, a phenotypic frequency, and a genotypic frequency.
4. What does Hardy-Weinberg equilibrium mean?
5. Why is Hardy-Weinberg equilibrium more a theoretical state than a common, real situation for genes that affect the phenotype?
6. What are the conditions under which Hardy-Weinberg equilibrium cannot be met?
7. Why is knowing the incidence of a homozygous recessive condition in a population important in deriving allele frequencies?
15. How did the challenges differ for profiling DNA in evidence from the September 11, 2001 attacks, the 2004 tsunami, and in matching Holocaust victims to survivors?
16. Suggest a novel use of DNA profiling.
d. mtDNA e. RFLPs
14. Why do forensic scientists using DNA profiling have to be extra careful in their analyses when the victim and suspect are blood relatives?
11. Maple syrup urine disease (MSUD) (see Reading 2.1) is autosomal recessive and causes mental and physical retardation, difficulty feeding, and a sweet odor to urine. In Costa Rica, 1 in 8,000 newborns inherits the condition. What is the carrier frequency of MSUD in this population?
12. The amyloidoses are a group of inborn errors of metabolism in which sticky protein builds up in certain organs.
Amyloidosis caused by a mutation in the gene encoding a blood protein called transthyretin (MIM 176300) affects the heart and/or nervous system. It is autosomal recessive. In a population of 177 healthy African Americans, four proved, by blood testing, to have one mutant allele of the transthyretin gene. What is the carrier frequency in this population?
13. Ability to taste phenylthiocarbamide (PTC) (MIM 607751) is mostly determined by the gene PTC. The letters T and t are used here to simplify analysis. TT individuals taste a strong, bitter taste; Tt people experience a slightly bitter taste; tt individuals taste nothing.
A fifth-grade class of 20 students tastes PTC that has been applied to small pieces of paper, rating the experience as “very yucky” ( TT ), “I can taste it” ( Tt ), and “I can’t taste it” ( tt ). For homework, the students test their parents, with these results:
Of 6 TT students, 4 have 2 TT parents; and two have one parent who is TT and one parent who is Tt.
Of 4 students who are Tt, 2 have 2 parents who are Tt, and 2 have one parent who is TT and one parent who is tt.
Of the 10 students who can’t taste PTC, 4 have 2 parents who also are tt, but 4 students have one parent who is Tt and one who is tt. The remaining 2 students have 2 Tt parents.
Calculate the frequencies of the T and t alleles in the two generations. Is Hardy-Weinberg equilibrium maintained, or is this gene evolving?
Applied Questions
1. Give three examples of how population genetics can be observed in everyday life.
2. Why are Hardy-Weinberg calculations more complicated if a gene has many alleles that affect the phenotype?
3. How can evolution occur at a microscopic and macroscopic level?
4. Two couples want to know their risk of conceiving a child with cystic fibrosis. In one couple, neither partner has a family history of the disease; in the other, one partner knows he is a carrier. How do their risks differ?
5. How does calculation of allele frequencies differ for an X-linked trait or disorder compared to one that is autosomal recessive?
6. Why might understanding Hardy-Weinberg equilibrium be important in understanding epidemiology (the patterns of infectious diseases in populations)?
7. Profiling of Y chromosome DNA implicated Thomas Jefferson in fathering a child of his slave, discussed in chapter 1. What might have been a problem with the conclusion?
8. Glutaric aciduria type I (MIM 231680) causes progressive paralysis and brain damage. It is very common in the Amish of Lancaster County, Pennsylvania—0.25 percent of newborns have the disorder. What percentage of newborns are carriers for this condition?
9. Torsion dystonia (MIM 128100) is a movement disorder that affects 1 in 1,000 Jewish people of eastern European descent (Ashkenazim). What is the carrier frequency in this population?
10. The Finnish population has a 1 percent carrier frequency for a seizure disorder called myoclonus epilepsy (MIM 607876). Two people who have no relatives with the illness ask a genetic counselor to calculate the risk that they will conceive an affected child, based on their belonging to this population group. What is the risk?
Web Activities
14. Go to one of the biobank websites and describe a medical test or treatment that may be developed from its data.
Case Studies and Research Results
15. An extra row of eyelashes is an autosomal recessive trait that occurs in 900 of the 10,000 residents of an island in the south Pacific. Greta knows that she is a heterozygote for this gene, because her eyelashes are normal, but she has an affected
parent. She wants to have children with a homozygous dominant man, so that the trait will not affect her offspring.
What is the probability that a person with normal eyelashes in this population is a homozygote for this gene?
Identify the samples that the vet might have sent, and what information each could contribute to the case. (P.S.: This is a real case. Killer—not his real name—was found guilty on the basis of DNA testing, but was not punished because there were no human eyewitnesses. Rufus’ angry owners installed a fence.)
20. In a true crime that took place in Israel, a man knocked a woman unconscious with a cement block and then raped her. He was careful not to leave any hairs at the crime scene.
But he left behind eyeglasses with unusual frames, and an optician helped police locate him. The man also left a half- eaten lollipop at the scene. DNA from blood taken from the suspect matched DNA from cheek-lining cells collected from the base of the telltale lollipop at four repeat loci on different chromosomes. Allele frequencies from the man’s ethnic group in Israel are listed beside the profile pattern below:
Frequency .20 .30 .70 .10 .40 .20 1
2
3 4 Repeat
a. For which of the tested repeats is the person a homozygote? How do you know this?
b. What is the probability that the suspect’s DNA matches that of the lollipop rapist by chance? (Do the calculation.) c. The man’s population group is highly inbred—many people
have children with relatives. How does this information aff ect the accuracy or reliability of the DNA profi le? (P.S.—He was so frightened by the DNA analysis that he confessed!) 21. Simone, a recent college graduate, was spending 2 years
using her biology degree to work in a clinic in Burma, in a town called Laputta. A devastating monsoon sent 18-foot waves over many of the neighboring villages, and survivors poured into the clinic. It had been raining for 8 days straight.
Suddenly, a rush of water overwhelmed the clinic, taking equipment and children in its path. Simone tried to save the children but she, too, was washed away.
Simone’s family in the United States was frantic, especially when her body was never found. Her father, a geneticist, insisted that investigators attempt to identify Simone’s DNA among evidence of human remains collected at the clinic. Explain how this might be done.
22. The governor of New York has proposed that DNA samples be taken from all people suspected of having committed a crime.
What are the possible benefits and risks of such a program?
Forensics Focus
16. DNA dragnets have been so successful in catching criminals in several countries that some people in law enforcement have suggested storing DNA samples of everyone at birth, so that a DNA profile could be obtained from anyone at any time. Do you think that this is a good idea or not? Cite reasons for your answer.
17. On December 5, 1984, Theresa Fusco was raped and strangled near a roller-skating rink on Long Island, New York. Two similar crimes had occurred in previous months. Three young men were charged with the crime and then convicted, but proclaimed their innocence, maintaining that their confessions had been coerced and witnesses had lied. At their trial in 1990, defense lawyers requested DNA profiling, but the judge ruled that the technology was too unproven to use. In 2003, the case was reopened. Stored semen was taken from the “rape kit” and DNA profiled, leading to exoneration. The men had not killed Theresa Fusco but they had spent more than a decade in prison.
a. Why might the judge have refused to consider DNA testing in 1990?
b. List the types of cells that could have been used to settle this case.
c. What information on the three suspects would be needed to interpret DNA patterns?
d. Do you think it is fair to decide whether or not a science- based forensic test or tool can be used based on how well a judge, jury, lawyers, or the public—who may have little or no training in genetics—understands how it works?
e. In 1992, lawyers Barry Scheck and Peter Neufeld, of the Cardozo School of Law in New York City, founded the nonprofi t Innocence Project, described in the chapter opener. Consult the Innocence Project website, click on “Case Profi les,” select a case, and describe how DNA evidence exonerated a prisoner.
18. “Indirect genetic kinship analysis” is routinely used in forensic investigations of natural or manmade disasters. It uses a DNA profile for one person to lead to identification of a blood relative. For example, if 11 of the 13 STR alleles typically examined closely matched for a man whose brother was missing at the World Trade Center and a bone bit found there, the bone was assumed to have been from the brother.
This approach is also being tried in criminal investigations.
If a “cold hit” leads to a prisoner who couldn’t possibly have committed a crime (perhaps because he was locked up at the time), detectives investigate his closest relatives. Do you think that this is an ethical use of DNA profiling? What are the pros and cons of this approach?
19. Rufus the cat was discovered in a trash can by his owners, his body covered in cuts and bite marks and bits of gray fur clinging to his claws—gray fur that looked a lot like the coat of Killer, the huge, aptly-named hound next door. Fearful that Killer might attack their other felines, Rufus’ distraught owners brought his body to a vet, demanding a forensic analysis. The vet suggested that the hair might have come from a squirrel, but agreed to send appropriate samples to a veterinary genetic testing laboratory.
Changing Allele Frequencies
Chapter Contents
15.1 Nonrandom Mating 15.2 Migration