Essentials-of-Biology

222 PART TWO Genetics 13.1 Gene Mutations Learning Outcomes Upon completion of this section, you should be able to 1. Explain how mutations cause variation in genes. 2. Describe how transposons may cause mutation. 3. Distinguish between point and frameshift mutations. A gene is a sequence of DNA bases that codes for a cellular product, most of- ten a protein (see  Section 11.2). The information within a gene may vary slightly—these variations are called alleles. In our discussion of patterns in inheritance (see Section 10.1), we examined a few examples of alleles in traits (for instance, those associated with flower color or wing shape). However, many alleles code for variations in proteins that are not easily observed, such as susceptibility to a medication.  How are new alleles formed? A new allele is the result of a mutation, or a change in the nucleotide sequence of DNA. Mutations may have nega- tive consequences, as is the case with the alleles for cystic fibrosis (see Sec- tion 10.2) However, mutations can increase the diversity of organisms by creating an entirely new gene product with a positive function for the organ- ism. In fact, mutations play an important role in the process of evolution (see Section 15.1) Figure 13.1 Transposons may cause gene mutations. Causes of Gene Mutations a. A purple-coding gene ordinarily codes for a purple pigment. b. A A gene mutation can be caused by a number of factors, including errors in transposon “jumps” into the purple-coding gene. This mutated gene DNA replication, a transposon, or an environmental mutagen. Mutations due to is unable to code for purple pigment, and a white kernel results. DNA replication errors are rare: They occur with a frequency of 1 in 100 mil- c. Indian corn displays a variety of colors and patterns due to lion cell divisions on average in most eukaryotes. This average is low due to transposon activity. DNA polymerase, the enzyme that carries out replication (see Section 11.1) and proofreads the new strand against the old strand, detecting and correcting (c): © Mondae Leigh Baker any mismatched pairs. Normal gene codes for Mutagens are environmental influences that cause mutations. Differ- purple ent forms of radiation, such as radioactivity, X-rays, ultraviolet (UV) light, a. pigment and chemical mutagens, such as pesticides and compounds in cigarette Mutated gene smoke, may cause breaks or chemical changes in DNA. If mutagens bring cannot about a mutation in the DNA in an individual’s gametes, the offspring of that transposon code for individual may be affected. If the mutation occurs in the individual’s body b. purple cells, cancer may result. The overall rate of mutation is low, however, be- pigment cause DNA repair enzymes constantly c. monitor for any irregularity and rem- edy the problem. Transposons  are specific DNA sequences that have the remarkable abil- ity to move within and between chromo- somes (Fig. 13.1). Their movement often disrupts genes, rendering them nonfunctional. These “jumping genes” have now been discovered in almost ev- ery group of organisms, including bac- teria, plants, fruit flies, and humans.

CHAPTER 13 Both Water and Land: Animals 223 Types and Efects of Mutations 13.1 CONNECTING THE CONCEPTS The effects of mutations vary greatly. The severity of a mutation usually de- Mutagens, replication errors, and pends on whether it affects one or more codons on a gene. In general, we know transposons can produce a mutation has occurred when the organism has a malfunctioning protein that mutations. leads to a genetic disorder or to the development of cancer. But many muta- tions go undetected because they have no observable effect or have no detect- able effect on a protein’s function. These are called silent mutations (Fig. 13.2). Point mutations involve a change in a single DNA nucleotide, and the severity of the results depends on the particular base change that occurs. A single base change can result in a change in the amino acid at that location of the gene. For example, in hemoglobin, the oxygen-transporting molecule in blood, a point mutation causes the amino acid glutamic acid to be replaced by a valine (Fig. 13.2). This changes the structure of the hemoglobin protein. The abnormal hemoglobin stacks up inside the red blood cells, causing them to become sickle-shaped, resulting in sickle-cell disease. A frameshift mutation is caused by an extra or missing nucleotide in a DNA sequence. Frameshift mutations are usually much more severe than point mutations, because codons are read from a specific starting point. Therefore, all downstream codons are affected by the addition or deletion of a nucleotide. For instance, if the letter C is deleted from the sentence THE CAT ATE THE RAT, the “reading frame” is shifted. The sentence now reads THE ATA TET HER AT— which doesn’t make sense. Likewise, a frameshift mutation in a gene often renders the protein nonfunctional, because its amino acid sequence no longer makes sense (Fig. 13.2). The movement of a transposon can cause a frameshift mutation. Check Your Progress 13.1 1. Explain how mutations are related to alleles.  2. Summarize the role of transposons in causing mutations.  3. Explain why point mutations may be silent, but frameshift mutations rarely are.  No mutation 3′ 5′ C AC G TG GAG TGAG G TC T C C T C Val His Leu Thr Pro Glu Glu His His CACGTAGAGTGAGGTC T C C T C b. Normal red blood cell 7,400× c. Sickled red blood cell 7,400× (normal protein) Val His Leu Thr Pro Glu Glu Figure 13.2 Varying efects of a point mutation. Glu Val CACGTGGAGTGAGGTC ACC T C (abnormal protein) Val His Leu Thr Pro Val Glu a. Starting at the top: a normal sequence of bases in hemoglobin; next, the mutation is silent and has no efect; next, due to point mutation, the DNA Glu Stop CACGTGGAGTGAGGTATC C TC now codes for valine instead of glutamic acid, and the result is that (incomplete protein) Val His Leu Thr Pro Stop (b) normal red blood cells become (c) sickle-shaped; next, a frameshift a. mutation (notice the missing C) causes the DNA to code for termination, and the protein will be incomplete. (b and c): © Eye of Science/Science Source

224 PART TWO Genetics 13.2 Chromosomal Mutations Learning Outcomes Upon completion of this section, you should be able to 1. Distinguish between a chromosomal deletion and duplication. 2. Explain the consequences of a translocation or an inversion. 3. List examples of the efects of chromosomal mutations in humans. We have just reviewed the consequences of mutation at the level of the gene, but it is also possible for events, such as nondisjunction and mutation, to cause changes in the number of each chromosome in the cells or the structure of in- dividual chromosomes. In humans, only a few variations in chromosome number, such as Down syndrome, Turner syndrome, and Klinefelter syndrome, are typically seen. These are usually caused by nondisjunction events during meiosis (see Section 9.4). Changes in chromosome structure, or chromosomal mutations, are much more common in the population. Syndromes that result from changes in chromosome structure are due to the breakage of chromosomes and their failure to reunite prop- erly. Various environmental agents—radiation, certain organic chemicals, and even viruses—can cause chromosomes to break apart. Ordinarily, when breaks occur in chromosomes, the segments reunite to give the same sequence of genes. But their failure to do so results in one of several types of mutations: deletion, duplication, translocation, or inversion. Chromosomal mutations can occur during meiosis, and if the offspring inherits the abnormal chromosome, a syndrome may result. Deletions and Duplications Figure 13.3 Deletion. A deletion occurs when a single break causes a chromosome to lose an end piece a. When chromosome 7 loses an end or when two simultaneous breaks lead to the loss of an internal segment of a chro- piece, the result is Williams syndrome. b. These children, although unrelated, ab cd ef gh mosome. An individual who inherits a all experience the appearance, health, deletion normal chromosome from one parent and and behavioral problems that are characteristic of Williams syndrome. cd a chromosome with a deletion from the (b): © The Williams Syndrome Association other parent no longer has a pair of alleles b. a b efg + for each trait, and a syndrome can result. a. h Williams syndrome occurs when lost chromosome 7 loses a tiny end piece (Fig. 13.3). Children with this syndrome have a turned-up nose, a wide mouth, a small chin, and large ears. Although their academic skills are poor, they exhibit ex- cellent verbal and musical abilities. The gene that governs the production of the protein elastin is missing, and this affects the health of the cardiovascular system and causes their skin to age prematurely. Such individuals are very friendly but need an ordered life, perhaps because of the loss of a gene for a protein that is normally active in the brain. Cri du chat (cat’s cry) syndrome occurs when chromosome 5 is missing an end piece. The affected individual

CHAPTER 13 Both Water and Land: Animals 225 has a small head, mental disabilities, and facial abnormalities. Abnormal de- a bc def gh velopment of the glottis and larynx results in the most characteristic symptom— the infant’s cry resembles that of a cat. duplication inversion In a duplication, a chromosome segment is repeated, so the individual a bc def f e dgh has more than two alleles for certain traits. An inverted duplication is known to a. occur in chromosome 15. The term inverted indicates that a segment runs in the direction opposite from normal. Children with this syndrome, called inv b. dup 15 syndrome, have poor muscle tone, mental disabilities, seizures, a curved spine, and autistic characteristics that include poor speech, hand flapping, and Figure 13.4 Duplication. lack of eye contact (Fig. 13.4). a. When a piece of chromosome 15 is duplicated and inverted, inv Translocation dup 15 syndrome results. b. Children with this syndrome have poor muscle tone and autistic characteristics. A translocation is the movement of a segment from one chromosome to an- (b): © Kathy Wise other, nonhomologous chromosome or the exchange of segments between non- homologous chromosomes (Fig. 13.5a). A person who has both of the involved Connections: Health chromosomes has the normal amount of genetic material and a normal pheno- type, unless the translocation breaks an allele into two pieces or fuses two Do we know what genes cause Down syndrome? genes together. The person who inherits only one of the translocated chromo- somes will have only one copy of certain alleles and three copies of other al- Because individuals who inherit Down leles. A genetic counselor begins to suspect a translocation has occurred when spontaneous abortions are commonplace and family members suffer from syndrome by a translocation event re- various syndromes. A special microscopic technique allows a technician to determine that a translocation has occurred. GART gene In 5% of Down syndrome cases, a translocation that occurred in a previ- ous generation between chromosomes 21 and 14 is the cause. As long as the two chromosomes are inherited together, the individual is normal. But in future generations a person may inherit two normal copies of chromosome 21 and the abnormal chromosome 14 that contains a segment of chromosome 21. In these cases, Down syndrome is not related to parental age but instead tends to run in the family of either the father or the mother. Some forms of cancer are also associated with translocations. One example is called chronic myeloid leukemia (CML). This translocation was first discovered in the 1970s when new staining techniques revealed a translo- cation of a portion of chromosome 22 to chromosome 9. This translocated chromosome is commonly called a Philadelphia chromosome. Individuals with ceive only a portion of chromosome 21, it has been possible to determine which as as genes on chromosome 21 are contributing to the symptoms of bt bt Down syndrome. One of the more significant genes appears c u translocation c u to be GART, a gene involved in the processing of a type of dv dv nucleotide called a purine. Extra copies of GART are believed ew ew to be a major contributing factor in the symptoms of intellec- fx fx gy yg tual disability shown in Down syndrome patients. Another hz zh gene that has been identified (COL6A1) encodes the protein a. b. 1,000× collagen, a component of connective tissue. Too many copies of this gene causes the heart defects common in Down syndrome. Figure 13.5 Translocation. a. Translocations represent the movement of genetic material between nonhomologous chromosomes. b. One example occurs between chromosomes 22 and 9, resulting in chronic myeloid leukemia (CML). The pink cells in this micrograph are rapidly dividing white blood cells. (b): © Jean Secchi/Dominique Lecaque/Roussel-Uclaf/CNRI/Science Source

226 PART TWO Genetics AA aa inverted crossing-over AA ga CML have a rapid growth of white blood cells (Fig. 13.5b), which often pre- BB bb segments BB fb vents the ability of the body to form red blood cells and reduces the effective- CC ee C ce ness of the immune system.  DD dd C D DC A Bc d d Another example is Alagille syndrome, caused by a translocation be- EE cc eE d D tween chromosomes 2 and 20. This translocation also often produces a deletion FF ff ab E on chromosome 20. The syndrome may also produce abnormalities of the eyes GG gg Ec and internal organs. The symptoms of Alagille syndrome range from mild to fF e Ff severe, so some people may not be aware they have the syndrome. In Burkitt’s F Gg lymphoma, translocations between chromosome 8 and either chromosomes 2, gG Gb 14, or 22 disrupt a gene that is associated with regulating the cell cycle, result- ing in the formation of very fast-growing tumors. a Inversion homologous nonsister duplication chromosomes chromatids and deletion An inversion occurs when a segment of a chromosome is turned 180° (Fig. 13.6). in both You might think this is not a problem because the same genes are present, but the reversed sequence of alleles can lead to altered gene activity if it disrupts the con- Figure 13.6 Inversion. trol of gene expression. (Left) A segment is inverted in the sister chromatids of one Inversions usually do not cause problems, but they can lead to an in- homologue. Notice that, in the red chromosome, edc occurs instead creased occurrence of abnormal chromosomes during sexual reproduction. of cde. (Middle) The nonsister chromatids can align only when the Crossing-over between an inverted chromosome and the noninverted homo- inverted sequence forms an internal loop. After crossing-over, a logue can lead to recombinant chromosomes that have both duplicated and de- duplication and a deletion can occur. (Right) The nonsister chromatid leted segments. This happens because alignment between the two homologues at the left arrow has AB and ab alleles but not fg or FG alleles. The is only possible when the inverted chromosome forms a loop (Fig. 13.6). nonsister chromatid at the right arrow has gf and GF alleles but not AB or ab alleles. 13.2 CONNECTING THE CONCEPTS Check Your Progress 13.2 Mutations can arise from various 1. Explain why a deletion can potentially have a greater efect on an changes in chromosomal structure. organism than a duplication.  2. Distinguish between a translocation and an inversion.  3. Describe the types of chromosomal mutations that produce cri du chat and Alagille syndromes. 13.3 Genetic Testing Learning Outcomes Upon completion of this section, you should be able to 1. Explain the role of a karyotype in genetic counseling. 2. Summarize how genetic markers and DNA microarrays may be used to diagnose a genetic disorder. 3. Distinguish among the procedures used to test DNA, the fetus, and the embryo for speciic genetic disorders. Potential parents are becoming aware that many illnesses are caused by abnor- mal chromosomal inheritance or by gene mutations. Therefore, more couples are seeking genetic counseling, which helps determine the risk of inherited disorders in a family. For example, a couple might be prompted to seek coun- seling after several miscarriages, when several relatives have a particular med- ical condition, or if they already have a child with a genetic defect. The counselor helps the couple understand the mode of inheritance, the medical

CHAPTER 13 Both Water and Land: Animals 227 consequences of a particular genetic disorder, and the decisions they might wish to make. Various human disorders may result from abnormal chromosome num- ber or structure. When a pregnant woman is concerned that her unborn child might have a chromosomal defect, the counselor may recommend karyotyping the fetus’s chromosomes. Analyzing the Chromosomes Figure 13.7 Karyotype analysis. A karyotype is a visual display of pairs of chromosomes arranged by size, A karyotype can reveal chromosomal mutations. In this case, the shape, and banding pattern. Any cell in the body except red blood cells, which karyotype shows that the newborn will have Down syndrome lack a nucleus, can be a source of chromosomes for karyotyping. In adults, it is (trisomy 21). easiest to use white blood cells separated from a blood sample for this purpose. © CNRI/SPL/Science Source In fetuses, whose chromosomes are often examined to detect a syndrome, cells can be obtained by either amniocentesis or chorionic villus sampling. For ex- Connections: Scientiic Inquiry ample, the karyotype of a person who has Down syndrome usually has three copies of chromosome number 21 instead of two (Fig. 13.7). What do the colored bands on chromosomes in a karyotype represent? Amniocentesis is a procedure for obtaining a sample of amniotic fluid from the uterus of a pregnant woman. A long needle is passed through the ab- The colored bands in a karyotype ( Fig. 13.7) are not the natural dominal and uterine walls to withdraw a small amount of fluid, which also con- color of chromosomes. Instead, a procedure called luores- tains fetal cells (Fig. 13.8a). Tests are done on the amniotic fluid, and the cells cent immunohistochemistry in situ hybridization (FISH) is used are cultured for karyotyping. Karyotyping the chromosomes may be delayed as to label short pieces of DNA with a luorescent marker. These long as 4 weeks, so that the cells can be cultured to increase their number. pieces of DNA bind to their complementary sequences on the chromosomes. When exposed to speciic wavelengths of Blood tests and the age of the mother are considered when determining light, the markers emit diferent colors of light. This helps ge- whether the procedure should be done. There is a slight risk of spontaneous netic counselors more easily identify the chromosomes. A abortion (about 0.6%) due to amniocentesis, with the greatest risk occurring in tagged piece of DNA can also be developed for a speciic the first 15 weeks of pregnancy.  gene, which helps researchers identify the chromosomal loca- tion of a gene of interest. Chorionic villus sampling (CVS) is a procedure for obtaining chorionic villi cells in the region where the placenta will develop. This procedure can be done as early as the fifth week of pregnancy. A long, thin suction tube is in- serted through the vagina into the uterus (Fig. 13.8b). Ultrasound, which gives a picture of the uterine contents, is used to place the tube between the uterine lining and the chorionic villi. Then, a sampling of the chorionic villi cells is obtained by suction. The cells do not have to be cultured, and karyotyping can be done immediately. But testing amniotic fluid is not possible, because no amniotic fluid is collected. Also, CVS carries a greater risk of spontaneous abortion than amniocentesis—0.7% compared with 0.6%. The advantage of CVS is getting the results of karyotyping at an earlier date. After a cell sample has been obtained, the cells are stimulated to divide in a culture medium. A chemical is used to stop mitosis during metaphase when chromosomes are the most highly compacted and condensed. The cells are then killed, spread on a microscope slide, and dried. In a traditional karyo- type, stains are applied to the slides, and the cells are photographed. Staining causes the chromosome to have dark and light cross-bands of varying widths, and these can be used, in addition to size and shape, to help pair up the chro- mosomes. Today, technicians use fluorescent dyes and computers to arrange the chromosomes in pairs. Following genetic testing, a genetic counselor can explain to prospective parents the chances that a child of theirs will have a disorder that runs in the family. If a woman is already pregnant, the parents may want to know whether the unborn child has the disorder. If the woman is not pregnant, the parents may opt for testing of the embryo or egg before she does become pregnant, as described shortly.

228 PART TWO Genetics amniotic cavity Figure 13.8 Testing for chromosomal mutations. To test a fetus for an alteration in the chromosome number or structure, fetal cells can be acquired by (a) amniocentesis or (b) chorionic villus sampling. a. During amniocentesis, a long needle chorionic is used to withdraw amniotic fluid villi containing fetal cells. b. During chorionic villus sampling, a suction tube is used to remove cells from the chorion, where the placenta will develop. Testing depends on the genetic disorder of interest. In some instances, it is appropriate to test for a particular protein, and in others, to test for the mu- tated gene. Testing for a Protein Some genetic mutations lead to disorders caused by a lack of enzyme activity. For example, in the case of methemoglobinemia, it is possible to test for the quantity of the enzyme diaphorase in a blood sample and, from that, determine whether the individual is likely homozygous normal, is a carrier, or has methe- moglobinemia. If the parents are carriers, each child has a 25% chance of hav- ing methemoglobinemia. This knowledge may lead prospective parents to opt for testing of the embryo or egg, as described later in this section. enzyme cleavage sites enzyme cleavage sites Testing the DNA normal a ected There are several methods of analyzing DNA for specific mutations, including allele allele testing for a genetic marker, using a DNA microarray, and direct sequencing of an individual’s DNA. fragments of DNA fragments of DNA Genetic Markers a. Normal fragmentation pattern b. Genetic disorder fragmentation pattern Testing for a genetic marker relies on a difference in the DNA due to the pres- ence of the abnormal allele. As an example, consider that individuals with Figure 13.9 Use of a genetic marker to test for a genetic sickle-cell disease or Huntington disease have an abnormality in a gene’s base sequence. This abnormality in sequence is a genetic marker. The presence of mutation. specific genetic markers can be detected using DNA sequencing. Another op- tion is to use restriction enzymes to cleave DNA at particular base sequences a. In this example, DNA from a normal individual has certain restriction (see Section 12.1). The fragments that result from the use of a restriction en- enzyme cleavage sites. b. DNA from another individual lacks one of zyme may be different for people who are normal than for those who are het- the cleavage sites, and this loss indicates that the person has a erozygous or homozygous for a mutation (Fig. 13.9). mutated gene. In heterozygotes, half of their DNA would have the cleavage site and half would not have it. (In other instances, gaining a DNA Microarrays cleavage site could be an indication of a mutation.) New technologies have made DNA testing easy and inexpensive. For example, it is now possible to place thousands of known disease-associated mutant alleles onto a DNA microarray, also called a gene chip—a small silicon chip containing many DNA samples, in this case, the mutant alleles (Fig. 13.10). Genomic DNA

CHAPTER 13 Both Water and Land: Animals 229 from the subject to be tested is labeled with a fluorescent dye, then added to the microarray. The spots on the microarray fluoresce if the DNA binds to the DNA probe array mutant alleles on the chip, indicating that the subject may have a particular disorder or is at risk of developing it later in life. An individual’s complete tagged DNA did genotype, including all the various mutations, is called a genetic profile. bind to probe With the help of a genetic counselor, individuals can be educated about their genetic profile. It’s possible that a person has or will have a ge- DNA probe netic disorder caused by a single pair of alleles. However, polygenic traits are more common, and in these instances, a genetic profile can indicate an increased or decreased risk for a disorder. Risk information can be used to design a program of medical surveillance and to foster a lifestyle aimed at tagged tagged DNA did not reducing the risk. For example, suppose an individual has mutations com- DNA bind to probe mon to people with colon cancer. It will be helpful for him or her to have an annual colonoscopy, so that any abnormal growths can be detected and re- testing subject's DNA moved before they became invasive. Figure 13.10 Use of a DNA microarray to test for mutated DNA Sequencing genes. Recent advances in the processes of DNA sequencing (see Section 12.1) have This DNA microarray contains many disease-associated mutant made it much more feasible economically to sequence the genome of an individ- alleles. Fluorescently labeled genomic DNA from an individual has ual to detect specific mutations associated with a disease. Whereas a few years been added to the microarray. Any luorescent spots indicate that ago the cost of sequencing an individual’s genome could be as high as $100,000, binding has occurred and that the individual may have the genetic that cost has decreased to almost $1,000. This decrease has ushered in an era of disorder or is at risk for developing it later in life. personal genomics, sometimes also referred to as personalized medicine. (photo): © Deco/Alamy There are several approaches to personal genomics. While it is possible to sequence the entire genome of an individual, it is often more practical to Connections: Health target specific genes and look for alleles that are known to increase the risk associated with a specific disease. This approach is called a genome-wide as- Are over-the-counter (OTC) genetic tests sociation study, and it has become more prevalent in the field of personalized accurate? medicine due to the increase in large genomic population studies. With advances in technology that al- One of the more interesting possibilities arising from personal genomics low DNA microarrays to be assem- is pharmacogenomics, or the selection of a drug based on information coming bled inexpensively, a number of directly from an individual’s genome. In many cases, such as cancer, certain companies are now ofering OTC ge- alleles associated with a gene will respond more effectively to a specific class netic tests. The Food and Drug Ad- of drugs. Thus, knowledge of the allelic combination of a patient can prove to ministration (FDA) does not regulate be very beneficial to the physician. OTC genetic tests, and there are a Testing the Fetus number of concerns regarding the va- © Images by Morgana/ lidity of the information obtained from Alamy RF If a woman is already pregnant, ultrasound can detect serious fetal abnormali- ties, and it is possible to obtain and test the DNA of fetal cells for genetic these tests. In addition, some people worry that the test re- defects. sults may be used to discriminate against individuals applying for insurance or jobs. It is recommended that genetic tests be Ultrasound performed by licensed labs, and only after all of the patient’s rights have been discussed. Ultrasound images help doctors evaluate fetal anatomy. An ultrasound probe scans the mother’s abdomen, and a transducer transmits high-frequency sound waves, which are transformed into a picture on a video screen. This picture shows the fetus inside the uterus. Ultrasound can be used to deter- mine a fetus’s age and size, as well as the presence of more than one fetus. Also, some chromosomal abnormalities, such as Down syndrome, Edwards syndrome (three copies of chromosome 18), and Patau syndrome (three cop- ies of chromosome 13), cause anatomical abnormalities during fetal develop- ment that may be detected by ultrasound by the twentieth week of pregnancy. For this reason, a routine ultrasound at this time is considered an essential part of prenatal care.

230 PART TWO Genetics 8-celled embryo Embryonic cell Many other conditions, such as spina bifida, can be diagnosed by an ul- is removed. trasound. Spina bifida results when the backbone fails to close properly around the spinal cord during the first month of pregnancy. Surgery to close a new- Cell is born’s spine in such a case is generally performed within 24 hours after birth. genetically healthy. Testing Fetal Cells Embryo develops Fetal cells can be tested for various genetic disorders. If the fetus has an incur- normally in uterus. able disorder, the parents may wish to consider an abortion. Figure 13.11 Testing the embryo. For testing purposes, fetal cells may be acquired through amniocente- sis or chorionic villus sampling, as described earlier in this section. In addi- Genetic diagnosis is performed on one cell removed from an eight- tion, fetal cells may be collected from the mother’s blood. As early as celled embryo. If this cell is found to be free of the genetic defect of 9 weeks into the pregnancy, a small number of fetal cells can be isolated from concern, and the seven-celled embryo is implanted in the uterus, it the mother’s blood using a cell sorter. Whereas mature red blood cells lack develops into a newborn with a normal phenotype. a nucleus, immature red blood cells do have a nucleus, and they have a © Elyse Lewin/Exactostock-1555/Superstock RF shorter life span than mature red blood cells. Therefore, if nucleated fetal red blood cells are collected from the mother’s blood, they are known to be from this pregnancy. Only about one of every 70,000 blood cells in a mother’s blood are fetal cells, and therefore the polymerase chain reaction (PCR) is used to amplify the DNA from the few cells collected. The procedure poses no risk to the fetus. Testing the Embryo and Egg As discussed in Section 29.2, in vitro fertilization (IVF) is carried out in labo- ratory glassware. A physician obtains eggs from the prospective mother and sperm from the prospective father and places them in the same receptacle, where fertilization occurs. Following IVF, now a routine procedure, it is pos- sible to test the embryo. Prior to IVF, it is possible to test the egg for any ge- netic defect. In any case, only normal embryos are transferred to the uterus for further development. Testing the Embryo If prospective parents are carriers for one of the genetic disorders discussed earlier, they may want assurance that their offspring will be free of the disorder. Genetic diagnosis of the embryo will provide this assurance. Following IVF, the zygote (fertilized egg) divides. When the embryo has six to eight cells, one of these cells can be removed for diagnosis, with no effect on normal development (Fig. 13.11). Only embryos that test negative for the genetic disorders of interest are placed in the uterus to continue developing. So far, thousands of children worldwide have been born free of al- leles for genetic disorders that run in their families following embryo test- ing. In the future, embryos that test positive for a disorder could be treated by gene therapy, so that those embryos, too, would be allowed to continue to term. Testing the Egg Unlike males, who produce four sperm cells following meiosis, meiosis in fe- males results in the formation of a single egg and at least two nonfunctional cells called polar bodies. Polar bodies, which later disintegrate, receive very little cytoplasm, but they do receive a haploid number of chromosomes and thus can be useful for genetic diagnosis. When a woman is heterozygous for a recessive genetic disorder, about half the polar bodies receive the mutated

CHAPTER 13 Both Water and Land: Animals 231 Figure 13.12 Testing the egg. Genetic diagnosis is performed on a polar body removed from an egg. If the egg is free of a genetic defect, it is used for IVF, and the embryo is transferred to the uterus for further development. © Elyse Lewin/Exactostock-1555/Superstock RF Polar body Egg Woman is is removed. heterozygous. IVF Polar body contains mutant allele. Genetically healthy egg used for IVF. Embryo develops normally in uterus. 13.3 CONNECTING THE CONCEPTS A variety of methods are available to analyze an individual for the presence of speciic mutations. allele, and in these instances the egg receives the normal allele. Therefore, if a Check Your Progress 13.3 polar body tests positive for a mutated allele, the egg probably received the normal allele (Fig. 13.12). Only normal eggs are then used for IVF. Even if the 1. Summarize how a karyotype can indicate potential sperm should happen to carry the mutation, the zygote will, at worst, be hetero- problems in a fetus.  zygous. But the phenotype will appear normal. 2. Explain the diferences between testing for a protein If gene therapy becomes routine in the future, it’s possible that an egg and testing the DNA for genetic disorders.  will be given genes that control traits desired by the parents, such as musical or athletic ability, prior to IVF. Such genetic manipulation, called eugenics, car- 3. Describe the diferences between ultrasound, ries many ethical concerns. amniocentesis, and chorionic villus sampling (CVS). List the limitations of each.  4. Predict how changes in base sequence can be used to test for genetic disorders.