- Week 3 DiscussionDiscussion Topic Due October 7 at 11:59 PMFor this assignment, make sure you post your initial response to the Discussion Area by the due date assigned.
To support your work, use your course and text readings and also use outside sources. As in all assignments, cite your sources in your work and provide references for the citations in APA format.
Respond to at least two of your classmates. Participate in the discussion by asking a question, providing a statement of clarification, providing a point of view with a rationale, challenging an aspect of the discussion, or indicating a relationship between two or more lines of reasoning in the discussion by the end of the week.
Research during the 1930s led to the discovery of an important region in human chromosomes known as the telomere. The enzyme telomerase enables the replacement of the telomere region in a chromosome. Assure Jane Marlow that gene testing or DNA testing has become a widely used tool to identify many disorders and can be a valuable tool for diagnosing conditions early on so that appropriate steps can be taken to prevent further risks of developing diseases. Addresss the following:
- What is the function of telomeres with regard to cell division?
- If a patient has an abnormality in telomerase activity, how might this contribute to cancer?
- Duchenne muscular dystrophy (DMD) is an X-linked disease for which prenatal diagnostic testing can be performed.
- Has prenatal genetic testing increased the detection rate of this disease, compared to prior diagnostic methods?
- If so, how? If not, why?
- In case the fetus is diagnosed with DMD and parents decide to terminate the pregnancy, are there chances that the next child will also be a carrier of the disease?
- Jane Hill’s husband, Charlie Marlow, has hemophilia A when he marries Hill, who does not have the disorder. Jane’s parents also do not have hemophilia, but her brother Steve Hill does.
- What is the probability that the Marlows’ son will have the disorder?
- What is the probability that their daughter will have hemophilia?
- Are there chances that their daughter will be a carrier of this disorder?
CHAPTER 4 Mendel’s Laws The Musical Maxwells Figure 2 A partial pedigree of the Maxwell family. Different combinations of symbols can be filled in to represent inheritance of specific traits. Peter Maxwell is an aging rock star. Although by day he is a high school music teacher, on the weekends his genetic gifts express themselves on stage. Peter has inherited a lucky combination of traits—hyperextensible fingers (autosomal recessive, genotype hf//hf); a fabulous head of long, red hair just like his paternal grandfather Emmett had; and a high, clear, strong voice that echoes those of the finest rock bands of the 1980s, a trait called rock star voice (RSV), which is autosomal dominant. Over the years Peter has been widely sought after for tribute bands. Each of these traits is inherited on a different autosome. Alas, Ellie Maxwell, Peter’s wife, is not musical and is, in fact, just the opposite. She has an autosomal dominant condition called congenital amusia or tune (not tone) deafness. Ellie comes from a long line of people who cannot hear the pitch and rhythmic patterns of musical notes as a melody. Instead, they hear a disconnected string of sounds, and this is what they mimic when they attempt to sing. They are always off-key. Her father, an identical twin, was once nicely asked to leave a church choir because he couldn’t correct his off-key notes. Ellie’s paternal grandfather, too, was notorious for his loud, horrible singing. Although she was convinced this lack of talent was inherited, Ellie never sang to her children, for fear that they might imitate her. Another autosomal dominant trait in the family is distal symphalangism, for which Ellie can blame her tiny toenails and very stiff fingers. Her siblings Jill and Dan luckily escaped both the weird fingers and bad singing voice. CHAPTER 4 Mendel’s Laws The Musical Maxwells Figure 2 A partial pedigree of the Maxwell family. Different combinations of symbols can be filled in to represent inheritance of specific traits. Peter Maxwell is an aging rock star. Although by day he is a high school music teacher, on the weekends his genetic gifts express themselves on stage. Peter has inherited a lucky combination of traits—hyperextensible fingers (autosomal recessive, genotype hf//hf); a fabulous head of long, red hair just like his paternal grandfather Emmett had; and a high, clear, strong voice that echoes those of the finest rock bands of the 1980s, a trait called rock star voice (RSV), which is autosomal dominant. Over the years Peter has been widely sought after for tribute bands. Each of these traits is inherited on a different autosome. Alas, Ellie Maxwell, Peter’s wife, is not musical and is, in fact, just the opposite. She has an autosomal dominant condition called congenital amusia or tune (not tone) deafness. Ellie comes from a long line of people who cannot hear the pitch and rhythmic patterns of musical notes as a melody. Instead, they hear a disconnected string of sounds, and this is what they mimic when they attempt to sing. They are always off-key. Her father, an identical twin, was once nicely asked to leave a church choir because he couldn’t correct his off-key notes. Ellie’s paternal grandfather, too, was notorious for his loud, horrible singing. Although she was convinced this lack of talent was inherited, Ellie never sang to her children, for fear that they might imitate her. Another autosomal dominant trait in the family is distal symphalangism, for which Ellie can blame her tiny toenails and very stiff fingers. Her siblings Jill and Dan luckily escaped both the weird fingers and bad singing voice.
Peter and Ellie have three children (Figure 2). Sean is lead guitarist in a rock band, thanks to his hyperextensible fingers and synesthesia (see Chapter 1). Unfortunately, he has inherited his mother’s tune deafness, which is especially frustrating because he has also inherited rock star voice. He can sing clear, beautiful, soaring notes—but the wrong ones. He has stopped singing since his attempts at harmony made his bandmates ill. Sean hopes he keeps his lustrous hair, which is like his father’s, and doesn’t somehow inherit Uncle Roger’s baldness. How awful it would be to watch those glorious red curls thin and then fall out! Sean’s sister Keri has rock star voice and distal symphalangism. Her stiff fingers don’t bother her, but she always covers her shrunken toenails. Maybe she’ll get fake ones. The only family peculiarity that little sister Anna shares is synesthesia. Peter’s sister Joan uses her hyperextensible fingers in her career as an eye surgeon. She likes to sing in the operating room, but it is good that her patients are often unconscious because she hasn’t inherited RSV. Peter’s and Joan’s older brother Roger has been unlucky genetically—in addition to the pattern baldness that worries Peter and Sean, Roger has an inherited heart condition called long QT syndrome. The first symptom can be a fatal disturbance of the heart’s rhythm (see extended pedigree, figure i).
QUESTIONS FOR RESEARCH AND DISCUSSION 6. Which approach to determining the likelihood of various genotypes occurring in the offspring of particular parents is easier for you—constructing a Punnett square, or using the product rule to calculate probabilities? 7. Compare and contrast the inheritance patterns of autosomal recessive and autosomal dominant traits.
- Discuss inherited and environmental contributions to Sean’s musical ability. 9. Which of the traits in the Maxwell family are listed in Mendelian Inheritance in Man and which are not? Inborn Athletes Ellie Maxwell’s nephews Caden and Jaden Cicero were large at birth—each twin weighed close to 8 pounds. This wasn’t terribly surprising, though, because their father, Jerry, is a weight lifter and construction worker with amazing strength. By their second well-baby checkup, the boys’ muscles looked so oversized that the pediatrician ordered a genetic test for a mutation in the myostatin gene, carried on chromosome 2. Mutations that disable this gene are responsible for “double muscles” in various other organisms, including mice, dogs, chickens, and prized beef cattle. Medical journals report that mighty-muscled Hercules, from Greek mythology, resembled a person with a myostatin mutation. The twins had indeed inherited an autosomal recessive myostatin mutation from each parent. The wild-type function of the gene is to stop stem cells from proliferating and making muscles grow too large. Without this genetic brake, stem cells in the muscle cells keep dividing. The double-muscle phenotype may threaten health in adulthood by causing heart muscle to overgrow. The pediatrician refers the family to a researcher who is studying genes that predispose people to exceptional strength and athletic ability. The investigation of the twins’ astounding strength and stamina reveals a subtler condition that they share—erythrocytosis—which clearly contributes to the overall phenotype. Their mother, Jill, inherited from her father Philip a dominant mutation in a gene on chromosome 19 that causes erythrocytosis. Her sister Ellie and brother Dan did not inherit it. Both Jill’s and her father’s blood, and also her uncle Craig’s, has too little erythropoietin (EPO), which is a hormone that stimulates bone marrow to release more progenitor cells that can differentiate as red blood cells. Under oxygen-poor conditions, such as at high altitude, secretion of EPO stimulates production of more red blood cells, ensuring a continual supply of oxygen to the muscles and other tissues. Philip, Craig, Jill, Caden, and Jaden have seemingly too little EPO, but the deficit exists because their bodies use the hormone as soon as it is synthesized, much more rapidly than other people. As a result, they have very large red blood cells that are packed with oxygen-carrying hemoglobin molecules. On a whole-body level, this gives these individuals great endurance. Jill, who never liked sports, had been unaware of her inherited advantage until she had to chase after her toddling athletes, whose muscles are not only enlarged, but packed with oxygen. The twins excel at many sports, and they can lift weights with the strength of a full-grown man. However, medical researchers caution that over time, erythrocytosis can cause high blood pressure and cardiovascular disease, as well as abnormal bleeding. Removing blood periodically can help to avert these problems, so the twins will have to be examined frequently.
QUESTIONS FOR RESEARCH AND DISCUSSION 17. Discuss the evolutionary significance of a mutation that is seen in several species. 18. Explain how researchers deduced the wild type function of the myostatin gene from the phenotype. 19. Describe two mutations in the Maxwell family that contradict the widely held idea that all mutations are harmful. 20. Devise a way to depict the traits discussed in this section on the partial pedigree of the Maxwell family.
CHAPTER 5 Beyond Mendel’s Laws Long QT Syndrome Roger Maxwell is very health-conscious. He runs, swims, and hikes; follows a low-carbohydrate diet; and generally feels great. He sees a physician when he needs to, in addition to annual physical exams at the large company where he is an engineer. He’d never allow himself to get so out of shape that heart disease would be a risk. Because of his strict adherence to this healthy lifestyle, Roger is surprised when a medical intern, gazing at his yearly electrocardiogram (ECG) at his work physical, clearly picks up on something. “What? What are you looking at?” Roger blurts out while buttoning up his shirt. “Oh, it’s probably nothing.” But she doesn’t look like it’s probably nothing. “The heart murmur? My mom’s been telling me about it since childhood. Not a big deal. The doctors called it something last year, something I never heard of.” “Did you check it out?” asks the intern. “Nah. It wasn’t bothering me, so I forgot about it. Why? What’s wrong?” “Well, maybe you should ask the doctor to explain it again and suggest what to do.” “About what?” “The doctor will explain it. Please don’t worry, though,” says the intern as she rushes off to the next patient. Roger’s electrocardiograms had in fact been showing that he has had long QT syndrome, and not a heart murmur, for many years. The doctor explains that this is a problem with the heart’s rhythm, and not its valves. Roger goes home and Googles long QT syndrome right away. What he finds concerns him enough to alert his relatives. Long QT syndrome is a lengthening in the time that it takes the ventricles (the lower two heart chambers) to recover after a contraction, called the QT interval on an electrocardiogram. This delay is called torsade de pointes, and it causes lightheadedness upon standing or even fainting, as blood pressure drops when the heart rhythm becomes abnormal. If the arrhythmia turns into the more erratic condition called ventricular fibrillation, it can be deadly. Some cases of sudden cardiac arrest in people who apparently do not have heart disease are in fact due to long QT syndrome. This may have been the case with Roger’s aunt, Amelia, his mother’s sister, who died at age 34 of what was thought to be a heart attack, but, now Roger realizes, was more likely an arrhythmia. Still, with only one affected relative, Roger had never thought of his aunt’s early demise as a family history, especially since his mother is healthy. Roger reads that in people with some forms of long QT syndrome, fatal arrhythmia can be triggered by intense emotions or a sudden loud sound. The first recorded case of the condition was a little girl, who collapsed dead when her teacher
suddenly yelled at her. Her older brother had died in a similar circumstance. Suddenly, Roger remembers that his daughter Sheila faints very easily. She even passed out once at a rock concert because she got so excited. He’d never panicked over it because his mother fainted easily, too. A pattern was emerging. Long QT syndrome is caused by mutations in any of at least 10 genes that encode either proteins that form parts of ion channels (potassium, sodium, or calcium) or proteins that affect the functioning of these channels. Ion channels control the spread of nerve impulses and the resulting muscle contraction. The time for the heart’s recovery after a beat, called repolarization, extends the period when ions are trapped inside heart muscle cells because the channels are blocked, too slow to open, or too quick to close in people who are at elevated risk due to inheriting a mutation. People with long QT syndrome can experience arrhythmia if they take certain drugs that prolong the QT interval. These drugs include certain antibiotics, antidepressants, and diuretics (“water pills”). Roger reads about the different genes and drug combinations that cause long QT syndrome on Wikipedia, and then he scans www.genetests.org to find labs that test for all of them. Only then does he make an appointment with a cardiologist, and he arranges to have his blood and that of his daughter and mother sent to one of the labs. Two weeks later, he learns that they all have a dominant mutation in a gene called HERG (for “human ether-a-go-go”) that causes long QT syndrome type 2 (LQT2). Even healthy family members could have inherited the mutation, because 15 percent of people with long QT syndrome do not have symptoms. The concern is their increased risk of developing symptoms in the future—perhaps suddenly.
Synesthesia Revisited Peter Maxwell is lucky. In addition to his beautiful red shiny hair and musical gifts, he has escaped the family’s long QT syndrome and is a synesthete. However, he isn’t entirely convinced that he has his genes to thank that musical notes appear colored to him. Instead, he worries that it was his experimentation with LSD in the 1960s and its persistent effects that have caused what appears to be synesthesia—he can’t remember when he first noticed it.