214.5 Heritable Changes in Chromosome Structure Major changes in chromosome structure include duplications, deletions, inversions, and translocationsMajor changes in chromosome structure have been evolutionarily importantMore frequently, such changes tend to result in genetic disorders
3DuplicationDuplications occur during prophase I of meiosis, when crossing over occurs unequally between homologous chromosomesduplicationRepeated section of a chromosomeFigure 14.9 Large-scale changes in chromosome structure.
4DeletionIn mammals, deletions usually cause serious disorders and are often lethaldeletionLoss of part of a chromosomeFigure 14.9 Large-scale changes in chromosome structure.
5InversionInversion may not affect a carrier’s health if it doesn’t disrupt a gene, but it may affect fertilityinversionStructural rearrangement of a chromosome in which a part becomes oriented in the reverse direction, with no molecular lossFigure 14.9 Large-scale changes in chromosome structure.
6Duplication and Deletion Figure 14.9 Large-scale changes in chromosome structure.A With a duplication, a section of a chromosome gets repeated.B With a deletion, a section of a chromosome gets lost.Fig. 14.9ab, p. 210
7InversionC With an inversion, a section of a chromosome gets flipped so it runs in the opposite orientation.Figure 14.9 Large-scale changes in chromosome structure.Fig. 14.9c, p. 210
8TranslocationIf a chromosome breaks, the broken part may attach to a different chromosome, or to a different part of the same oneMost translocations are reciprocal, or balanced, which means two chromosomes exchange broken partstranslocationStructural change of a chromosome in which a broken piece gets reattached in the wrong location
9Reciprocal Translocation Many reciprocal translocations have no adverse effects on health, but can affect fertility
10Reciprocal Translocation D With a translocation, a broken piece of a chromosome gets reattached in the wrong place. This example shows a reciprocal translocation, in which two chromosomes exchange chunks.Figure 14.9 Large-scale changes in chromosome structure.Fig. 14.9d, p. 210
11Some Disorders with Changes in Chromosome Structure Huntington’s disease: expansion mutations (duplications)Degeneration of the nervous systemCri-du-chat syndrome (deletion)Mental impairment; abnormal larynxBurkitt’s lymphoma (translocation)An aggressive cancer of the immune system
12Chromosome Changes in Evolution Most major alterations are harmful or lethal in humansEven so, many major structural changes have accumulated in chromosomes of all species over evolutionary timeSpeciation can and does occur by large-scale changes in chromosomes
13Evolution of the Y Chromosome X and Y chromosomes were once homologous autosomes in reptilelike ancestors of mammalsAbout 350 mya, a gene on one chromosome mutated – interfering with crossing over during meiosis – and mutations began to accumulate separately in the two chromosomesToday, the SRY gene (Y chromosome) determines male sex
14Evolution of the Y Chromosome Figure Evolution of the Y chromosome. Today, the SRY gene determines male sex. Homologous regions of the chromosomes are shown in pink; mya, million years ago.
15Evolution of the Y Chromosome (autosome pair)area that cannot cross overSRYAncestral reptiles>350 myaAncestral reptiles350 myaMonotremes320–240myaMarsupials170–130 myaMonkeys130–80 myaHumans 50–30 myaA Before 350mya, sex was determined by temperature, not by chromosome differences.Figure Evolution of the Y chromosome. Today, the SRY gene determines male sex. Homologous regions of the chromosomes are shown in pink; mya, million years ago.B The SRY gene begins to evolve 350 mya. The DNA sequences of the chromosomesdiverge as other mutations accumulate.C By 320–240 mya, the DNA sequences of the chromosomes are so different that the pair can no longer cross over in one region. The Y chromosome begins to get shorter.D Three more times, the pair stops crossing over in yet another region. Each time, the DNA sequences of the chromosomes diverge, and the Y chromosome shortens. Today, the pair crosses over only at a small region near the ends.Fig , p. 211
16Human EvolutionOne human chromosome matches two in chimpanzees and other great apesDuring human evolution, two chromosomes fused end to end and formed our chromosome 2Figure Human chromosome 2 compared with chimpanzee chromosomes 2A and 2B.
17Human Evolution telomere sequence human chimpanzee Figure Human chromosome 2 compared with chimpanzee chromosomes 2A and 2B.humanchimpanzeeFig , p. 211
18ANIMATION: Deletion To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to playMac Users: CLICK HERE
19ANIMATION: Duplication To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE
20ANIMATION: Inversion To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to playMac Users: CLICK HERE
21Animation: Translocation To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE
2214.6 Heritable Changes in the Chromosome Number Occasionally, abnormal events occur before or during meiosis, and new individuals end up with the wrong chromosome numberConsequences range from minor to lethal changes in form and function
23NondisjunctionChanges in chromosome number are usually caused by nondisjunctionNondisjunction affects chromosome number at fertilization and causes genetic disorders among resulting offspringnondisjunctionFailure of sister chromatids or homologous chromosomes to separate during nuclear division
25Nondisjunction Metaphase I Anaphase I Telophase I Metaphase II Figure An example of nondisjunction during meiosis. Of the two pairs of homologous chromosomes shown here, one fails to separate during anaphase I. The chromosome number is altered in the resulting gametes.Metaphase IAnaphase ITelophase IMetaphase IIAnaphase IITelophase IIFig , p. 212
26Nondisjunction Metaphase I Anaphase I Telophase I Metaphase II Anaphase IITelophase IIFigure An example of nondisjunction during meiosis. Of the two pairs of homologous chromosomes shown here, one fails to separate during anaphase I. The chromosome number is altered in the resulting gametes.Stepped ArtFig , p. 212
27AneuploidyIn aneuploidy, an individual’s cells have too many or too few copies of a chromosome (result of nondisjunction)Most cases of autosomal aneuploidy are lethal in embryosaneuploidyA chromosome abnormality in which an individual’s cells carry too many or too few copies of a particular chromosome
28Types of Aneuploidy Trisomy: A normal gamete (n) fuses with an n+1 gameteNew individual is trisomic (2n+1), having three of one type of chromosome and two of every other typeMonosomy:An n-1 gamete fuses with a normal (n) gameteNew individual is monosomic (2n-1)
29PolyploidyPolyploid individuals have three or more of each type of chromosomePolyploidy is lethal in humans, but many flowering plants, and some insects, fishes, and other animals, are polyploidpolyploidHaving three or more of each type of chromosome characteristic of the species
30Disorders with Changes in Chromosome Number Disorder Main SymptomsDown syndrome Mental impairment; heart defectsTurner syndrome (XO) Sterility; abnormal ovaries and sexual traitsKlinefelter syndrome Sterility; mild mental impairmentXXX syndrome Minimal abnormalitiesXYY condition Mild mental impairment or no effect
31Autosomal Change and Down Syndrome The most common aneuploidy, trisomy 21, causes Down syndromeCharacteristics include upward-slanting eyes, slightly flattened facial features, and other symptomsTrisomic 21 individuals tend to have moderate to severe mental impairment and heart problems
33Down Syndrome Figure 14.13 Down syndrome, genotype and phenotype. Fig a, p. 213
34Down Syndrome Figure 14.13 Down syndrome, genotype and phenotype. Fig b, p. 213
35Change in Sex Chromosome Number A change in the number of sex chromosomes usually results in some degree of impairment in learning and motor skillsIn individual with trisomy (XXY, XXX, and XYY) these problems can be subtle and the cause may never be diagnosed
36Female Sex Chromosome Abnormalities Individuals with Turner syndrome have an X chromosome and no corresponding X or Y chromosome (XO)XO individuals are well proportioned but short; their ovaries do not develop properly, so they do not make enough sex hormones to become sexually matureIn XXX syndrome, having extra X chromosomes usually does not result in physical or medical problems
37Male Sex Chromosome Abnormalities Males with Klinefelter syndrome (XXY ) tend to be overweight, tall, and within normal range of intelligenceThey make more estrogen and less testosterone than normal males, which has feminizing effectsXYY males tend to be taller than average and have mild mental impairment, but are otherwise normal
38Key Concepts Changes in Chromosome Structure and Number A chromosome may undergo a large-scale, permanent change in its structure, or the number of autosomes or sex chromosomes may changeIn humans, such changes usually result in a genetic disorder
39Animation: Amniocentesis To play movie you must be in Slide Show ModePC Users: Please wait for content to load, then click to playMac Users: CLICK HERE
40ABC Video: Genetic Testing: Screening Embryos for Disease
4114.7 Genetic ScreeningProspective parents can estimate probability that a child will inherit a genetic disorder with genetic screening, in which pedigrees and genotype are analyzed by a genetic counselorSome disorders can be detected early enough to start countermeasures before symptoms develop
42Newborn Screening for PKU Most US hospitals now screen newborns for mutations in the gene for phenylalanine hydroxylase, a defect that can cause phenylalanine to accumulate to high levelsThe resulting imbalance inhibits protein synthesis in the brain, which results in severe neurological symptoms characteristic of phenylketonuria (PKU)
43Prenatal DiagnosisPrenatal genetic testing of an embryo or fetus can reveal genetic abnormalities or disorders before birthObstetric sonographyFetoscopyAmniocentesisChorionic villus sampling (CVS)An invasive procedure often carries a risk to the fetus
44Imaging a Fetus in the Uterus Obstetric sonography (ultrasound) forms images of the fetus’s developing limbs and internal organsFetoscopy yields higher-resolution images
45Imaging a Fetus in the Uterus Figure Imaging a fetus developing in the uterus.A An ultrasound image.Fig a, p. 214
46Imaging a Fetus in the Uterus Figure Imaging a fetus developing in the uterus.B A fetoscopy image.Fig b, p. 214
47Tests for Genetic Disorders With amniocentesis, fetal cells shed into the fluid inside the amniotic sac are tested for genetic disordersChorionic villus sampling tests cells of the chorion, which is part of the placenta
48Tests for Genetic Disorders Figure An 8-week-old fetus. With amniocentesis, fetal cells shed into the fluid inside the amniotic sac are tested for genetic disorders. Chorionic villus sampling tests cells of the chorion, which is part of the placenta.amniotic sacplacentaFig , p. 215
49Preimplantation Diagnosis Clump of cells formed by three mitotic divisions after in vitro fertilizationOne cell can be removed for genetic analysis to determine whether the embryo carries any genetic defectsFigure Clump of cells formed by three mitotic divisions after in vitro fertilization. All eight of the cells are identical and one can be removed for genetic analysis to determine whether the embryo carries any genetic defects.
50Key Concepts Genetic Testing Genetic testing provides information about the risk of passing a harmful allele to one’s offspringAfter conception, various methods of prenatal testing can reveal a genetic abnormality or disorder in a fetus or embryo
51Shades of Skin (revisited) People of Chinese descent carry an allele of the DCT gene which results in conversion of tyrosine to melaninDistribution of SLC24A5 and DCT genes suggests that (1) an African population was ancestral to both Chinese and Europeans, and (2) Chinese and European populations separated before their pigmentation genes mutated and their skin color changed