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Chapter 15 The Chromosomal Basis of Inheritance AP Biology Overview: Locating Genes Along Chromosomes • Mendel’s “hereditary factors” were genes, though this wasn’t known at the time – Today we can show that genes are located on chromosomes – The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes The Chromosome Theory of Inheritance • Mitosis and meiosis were first described in the late 1800s – Made possible by improved techniques in microscopy – Brought to light parallels between chromosome behavior and the behavior of Mendel’s proposed hereditary factors • The chromosome theory of inheritance states: – Mendelian genes have specific loci (positions) on chromosomes – It is chromosomes that undergo segregation and independent assortment Behavior of Chromosomes and Mendel’s Laws • The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment Fig. 15-2 P Generation Yellow-round seeds (YYRR) Y – Behavior of homologous Y R r R Green-wrinkled seeds ( yyrr) y y r Meiosis chromosomes during meiosis Fertilization can account for segregation R of alleles at each genetic locus R y r LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. Y r R R y Y y 1 R r Y y r R Y y Anaphase I for independent assortment R r Y y Metaphase II r R Y y 2 2 Y Y Gametes R located on different 1/ 4 YR F2 Generation chromosomes r Metaphase I 1 chromosomes can account of alleles for 2 or more genes LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis Y Behavior of nonhomologous y r Y – r All F 1 plants produce yellow-round seeds (YyRr) F1 Generation to different gametes y R Y Gametes R y r r 1/ 4 Y Y y r r 1/ 4 Yr yr y y R R 1/ 4 yR An F1 F1 cross-fertilization 3 3 9 :3 :3 :1 Morgan’s Experimental Evidence: Scientific Inquiry • The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist • Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors – Several characteristics make fruit flies a convenient organism for genetic studies: • They breed at a high rate – A single mating produces 100s of offspring • A generation can be bred every two weeks • They have only four pairs of chromosomes – Include 3 pairs of autosomes and one pair of sex chromosomes • Females: XX • Males: XY Wild Type vs. Mutant Varieties • Morgan faced the tedious task of carrying out many matings and then microscopically searching for naturally occurring variant offspring – Morgan noted wild type, or normal, phenotypes that were common in the fly populations • – Traits alternative to the wild type are called mutant phenotypes • – Ex) Red eyes in Drosophila Due to alleles assumed to have originated as mutations in the wild-type allele Morgan invented a notation for symbolizing alleles in Drosophila • Gene takes its symbol first mutant discovered – • w = allele for white eyes A superscript + identifies the allele for the wild-type trait – w+ = allele for red eyes from the Morgan’s Experiments • In one experiment, Morgan mated male flies with white eyes (mutant) with female Fig. 15-4 flies with red eyes (wild type) EXPERIMENT – The F1 generation all had red eyes • – – Suggested that the wild-type allele dominant The F2 generation showed the 3:1 red:white eye ratio • All females had red eyes • Half of the males had red eyes and half had white eyes Morgan determined that the white-eyed mutant allele is located on X chromosome • • A single copy of the mutant allele conferred white eyes in males since there was no wild-type allele to offset the recessive allele A female could have white eyes only both of her X chromosomes carried the reccessive mutant allele P Generation isAll offspring F1 Generation had red eyes RESULTS F2 Generation CONCLUSION P Generation w+ X X w+ X Y w Eggs F1 Generation w+ Sperm w+ w+ w w+ Eggs F2 Generation w w+ w Sperm w+ w+ w+ w w w+ if Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair • Morgan’s finding supported the chromosome theory of inheritance – Proved that a specific gene is carried on a specific chromosome (eye color gene is located on the X chromosome) – Also indicated that genes located on sex chromosomes exhibit unique inheritance patterns Concept Check 15.1 • 1) Which one of Mendel’s laws relates to the inheritance of alleles for a single character? Which law relates to the inheritance of alleles for two characters in a dihybrid cross? • 2) What is the physical basis of Mendel’s laws? • 3) Propose a possible reason that the first naturally occurring mutant fruit fly Morgan saw involved a gene on a sex chromosome. Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance The Sex Chromosomes • In humans and some other animals, there is a chromosomal basis of sex determination – Fig. 15-5 There are two varieties of sex chromosomes • A larger X chromosome (XX = female) • A smaller Y chromosome (XY = male) – X Only the ends of the Y chromosome have Y regions that are homologous with the X chromosome • These regions allow X and Y chromosomes in males to pair and behave like homologous chromosomes during meiosis Segregation of Sex Chromosomes • The 2 sex chromosomes segregate during meiosis in both testes and ovaries – Each gamete contains one sex chromosome • • In females, each egg contains one X chromosome In males, half the sperm contain an X Fig. 15-6a 44 + XY chromosome and half contain a 44 + XX Parents Y chromosome – Sex is determined based on which of these sperm fertilizes the egg cell – There is a 50/50 chance of either 22 + 22 + or Y X Sperm + 44 + XX or 22 + X Egg 44 + XY Zygotes (offspring) (a) The X-Y system sperm fertilizing an egg • In humans, the anatomical signs of sex begin to emerge in 2 month old embryos – The SRY gene on the Y chromosome codes for the development of testes – In the absence of SRY, the gonads develop into ovaries Sex Determination in Other Animals • Other animals have different methods of sex determination – X-O system: there is only one type of sex chromosome (X) • • – 22 + 22 + or Y X Sperm Egg or 44 + XY Zygotes (offspring) Characteristic of grasshoppers, cockroaches, and some other insects (a) The X-Y system Sex is determined by presence or absence of a second X chromosome in sperm 22 + – Females: XX – Males: XO Characteristic of birds and some fish and insects XX 22 + X 76 + ZW 76 + ZZ 32 (Diploid) 16 (Haploid) (b) The X-0 system Z-W system: sex chromosome present in egg determines gender • 44 + XX 22 + X + (c) The Z-W system – Females: ZW – – Males: ZZ Haplo-diploid system: no sex chromosomes • Characteristic of most bees and ants (d) The haplo-diploid system – Females (diploid): develop from fertilized egg – Males (haploid): develop from unfertilized egg Inheritance of Sex-Linked Genes • The sex chromosomes also have genes for many characters unrelated to sex – A gene located on either sex chromosome is called a sex-linked gene – In humans, sex-linked usually refers to a gene on the larger X chromosome • Sex-linked genes follow specific patterns of inheritance – For a recessive sex-linked trait to be expressed: • A female needs two copies (homozygous) of the allele • A male needs only one copy (hemizygous) of the allele Inheritance of Sex-Linked Genes: Males vs. Females • Sex-linked recessive disorders are much more common in males than in females Fig. 15-7 – Ex) The chance of a female inheriting two recessive alleles is much less than the probability of a male inheriting a single recessive allele XNXN Sperm Xn XnY (a) Sperm XN Y Eggs XN XNXn XNY XN XNXn XNY Xn (b) Sperm Xn Y Eggs XN XNXN XNY XNXn XNY XNXn XnY Y Eggs XN XNXn XNY XnXN XnY Xn (c) XnXn XnY X-Linked Recessive Disorders • Some disorders caused by recessive alleles on the X chromosome in humans: – Color blindness – Duchenne muscular dystrophy: disease characterized by progressive weakening of muscles and loss of coordination • Results from absence of a key muscle protein called dystrophin • Affects 1/3500 males born in the US – – Affected individuals rarely live past their early 20s Hemophilia: caused by absence of one or more proteins required for blood clotting • Bleeding is therefore prolonged since a firm clot is slow to form • Affected individuals can be treated with intravenous injections of the missing protein X Inactivation in Female Mammals • In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development – Cells of males and females thus have the same effective dose (one copy) of each gene – The inactive X condenses into a compact object called a Barr body • Most of the genes on this chromosome are not expressed • Barr body chromosomes in the ovaries are reactivated in cells that give rise to eggs so that each female gamete has an active X X Inactivation in Female Mammals: Tortoiseshell Cats • Females consist of a “mosaic” of two cell types as a result of random and independent X inactivation of each embryonic cell present • – About half contain cells with an active X derived from father – The remaining cells contain an active X derived from mother A female heterozygous for a particular gene on the X chromosome will therefore be Fig. 15-8 X chromosomes a mosaic for that character Early embryo: – Allele for black fur Ex) Results in mottled coloration of tortoiseshell cats • Orange patches formed in regions where orange alleles are active Two cell populations in adult cat: Active X Cell division and X chromosome inactivation Black patches are found in areas where black allele is active • Only (heterozygous) females can be tortoiseshell Active X Inactive X Black fur • Allele for orange fur Orange fur Concept Check 15.2 • 1) A white-eyed female Drosophila is mated with a red-eyed (wild-type) male, the reciprocal cross of the one shown in Figure 15.4 (pp. 289). What phenotypes and genotypes do you predict for the offspring? • 2) Neither Tim nor Rhoda has Duchenne muscular dystrophy, but their firstborn son does have it. What is the probability that a second child of this couple will have the disease? What is the probability if the second child is a boy? A girl? • 3) During early embryonic development of female carriers for color blindness, the normal allele is inactivated by chance in about half of the cells. Why, then, aren’t 50% of female carriers color-blind? Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome Linked Genes • Each chromosome has hundreds or thousands of genes – Genes located on the same chromosome that tend to be inherited together are called linked genes – These linked genes do not assort independently and therefore deviate from the results of typical breeding experiments How Linkage Affects Inheritance • Morgan also performed experiments with fruit flies to see how linkage affects inheritance of two characters – Morgan crossed flies that differed in traits of body color and wing size • Wild-type flies have gray bodies and normal-sized wings • Mutant flies have black bodies and much smaller wings called vestigial wings – Mutant alleles are recessive to the wild-type alleles – Fig. 15-UN1 Parents in testcross b vg b+ vg+ b vg b vg b+ vg+ b vg Neither gene is located on a sex chromosome Most offspring or b vg b vg Morgan’s Observations • Morgan observed a much higher proportion of parental phenotypes that would be expected due to independent assortment – He therefore concluded that body color and wing size are usually inherited together in specific combinations • These combination were generally those combinations seen in the parental generation – Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) – He noted that these genes do not assort independently, and reasoned that they were on the same chromosome Genetic Recombination • However, nonparental phenotypes were also produced – This suggested that body color and wing size genes are only partially linked genetically • Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent Genetic Recombination and Linkage • The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination – Mendel observed that combinations of traits in some offspring differ from either parent • Offspring with a phenotype matching one of the parental phenotypes are called parental types • Offspring with nonparental phenotypes (new combinations of traits) are Fig. 15-UN2 called recombinant types, or recombinants – When 50% of all offspring are recombinants, Gametes from yellow-round heterozygous parent (YyRr) there is said to be a 50% frequency of YR recombination • A 50% frequency recombination Gametes from greenwrinkled homozygous recessive parent ( yyrr) yr Yr yR of yr YyRr yyrr Yyrr yyRr is observed for any two genes on different chromosomes Parentaltype offspring Recombinant offspring Recombination of Linked Genes: Crossing Over • In Morgan’s Drosophila testcross, most of the offspring had parental phenotypes for body color and wing size – – This suggested that the 2 genes were on the same chromosome • The occurrence of parental genotypes at a greater frequency than 50% indicates that genes are linked • In this case, about 17 % of offspring were recombinants • This suggested that, although genes can be linked, the linkage can be incomplete, as evident from recombinant phenotypes Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome • That mechanism was the crossing over of homologous chromosomes Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry • Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome – Assumed that crossing over is a random event, with the chance of crossing over approximately equal at all points along a chromosomes – Predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency • The greater the distance between 2 genes, the more points there are between them where crossing over can occur Linkage Maps • A linkage map is a genetic map of a chromosome based on recombination frequencies – Distances between genes can be expressed as map units • – • One map unit, or centimorgan, represents a 1% recombination frequency Map units indicate relative distance and order, not precise locations of genes Drosophila data: – Involved a map of 3 genes: eye color (cn), body color (b), and wing size (vg) • 9% recombination frequency for cn and b • 9.5% between cn and vg • 17% between b and vg • b-vg recombination frequency (17%) is slightly less than the sum of b-cn and cn-vg frequencies (9+9.5=18.5%) – This occurs because of the few times that a crossover occurs between both b-cn and cn-vg – These crossovers cancel each other out Physically Linked Genes That Are Genetically Unlinked • Some genes on a chromosome are so far apart that a crossover between them is virtually certain – Observed frequency of recombination in crosses such as these can have a maximum value of 50% • This result is indistinguishable from that for genes on different chromosomes • In such a case, the physical connection between genes on the same chromosome is not reflected – Such genes are physically linked, but genetically unlinked, and behave (assort independently) as if found on different chromosomes – These genes are mapped by adding the recombination frequencies from crosses involving a set of closer pairs of genes lying between the two distant genes Linkage Maps of Fruit Fly Genes • Sturtevant used recombination frequencies to make linkage maps of fruit fly genes Fig. 15-12 Mutant phenotypes – Found that these genes Short aristae Black body Cinnabar Vestigial eyes wings Brown eyes clustered into 4 groups of linked genes – 0 48.5 57.5 67.0 104.5 Correlated with the observation that there are 4 pairs of chromosomes in Drosophila • Other methods allow geneticists to Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes construct cytogenetic maps – These maps locate genes with respect to chromosomal features (ex: stained bands) Concept Check 15.3 • 1) When two genes are located on the same chromosome, what is the physical basis for the production of recombinant offspring in a testcross between a dihybrid parent and a double-mutant (recessive) parent? • 2) For each type of offspring of the testcross in Figure 15.9 (pp. 293), explain the relationship between its phenotype and the alleles contributed by the female parent. • 3) Genes A,B, and C are located on the same chromosome. Testcrosses show that the recombination frequency between A and B is 28% and between A and C is 12%. Can you determine the linear order of these genes? Explain. Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders Large-Scale Chromosomal Alterations • Large-scale chromosomal alterations can affect an organism’s phenotype – Can occur as a result of physical and chemical disturbances, as well as due to errors in meiosis – Often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Abnormal Chromosome Number • In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis – Can also occur if sister chromatids fail to separate during meiosis II • As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy Fig. 15-13-3 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 n+1 n–1 n–1 n+1 n–1 n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II n Aneuploidy • Aneuploidy results from the fertilization of gametes in which nondisjunction occurred – Offspring with this condition have an abnormal number of a particular chromosome • A monosomic zygote has only one copy of a particular chromosome – Results when fertilization involves a gamete that has no copy of a particular chromosome, leading to a missing (2n-1) chromosome in the zygote • A trisomic zygote has three copies of a particular chromosome – Cell has 2n+1 chromosomes – These abnormal doses of genes can result in conditions like Down syndrome (trisomy 21) Polyploidy • Polyploidy is a condition in which an organism has more than two complete sets of chromosomes – Triploidy (3n) is three sets of chromosomes • – Can occur by fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes Tetraploidy (4n) is four sets of chromosomes • Can result from failure of a 2n zygote to divide after replicating its chromosomes in preparation for mitosis – • • Subsequent normal mitotic divisions would then produce a 4n embryo Polyploidy is common in plants, but not animals – Bananas are triploid and wheat in hexaploid (6n) – Known to occur among fishes, amphibians, and a mammalian species of rodent Polyploids are more normal in appearance than aneuploids Alterations of Chromosome Structure • • Breakage of a chromosome can lead to four types of changes in chromosome structure: – Deletion removes a chromosomal segment – Duplication repeats a segment – Inversion reverses a segment within a chromosome – Translocation moves a segment from one chromosome to another Deletion and duplications are especially likely to occur during meiosis – Nonsister chromatids sometimes exchange unequal-sized segments of DNA during crossing over • The products of such nonreciprocal crossovers are one chromosome with a deletion and the other chromosome with a duplication Human Disorders Due to Chromosomal Alterations • Alterations of chromosome number and structure are associated with some serious disorders – The effects of aneuploidy are generally so disastrous to development that embryos are spontaneously aborted before birth – Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond • These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy Down Syndrome (Trisomy 21) • Down syndrome is an aneuploid condition that results from three copies of chromosome 21 (trisomy 21) – Causes characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation • Affected individuals tend to have a shorter average life span, though some live to middle age and beyond • Most are also sexually underdeveloped and sterile Fig. 15-16 – Affects about one out of every 700 children born in the US – The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained • 0.04% for children born to mothers under 30 • More than 0.92% for children born to mothers over 40 Aneuploidy of Sex Chromosomes • Nondisjunction of sex chromosomes produces a variety of aneuploid conditions – Appear to cause less upset to genetic balance than aneuploid conditions involving autosomes • • Y chromosome carries relatively few genes • Extra copies of X chromosome become inactivated as Barr bodies Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals – Affected individuals have male sex organs but abnormally small testes, resulting in sterility • Monosomy X, called Turner syndrome, produces X0 females – Affected females are phenotypically normal but sterile – This is the only known viable monosomy in humans Disorders Caused by Structurally Altered Chromosomes • Many deletions in human chromosomes cause severe problems – The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5 – Affected children are mentally retarded, with a small head and unusual facial features, and have a catlike cry • • Individuals usually die in infancy or early childhood Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes – OccursFig.when a reciprocal translocation happens during mitosis of cells 15-17 destined to become WBCs • Exchange of a large portion of chromosome 22 with a small fragment from the tip of chromosome 9 produces a short, easily recognizable chromosome 22, called the Philadelphia chromosome Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) Concept Check 15.4 • 1) More common than completely polyploid animals are mosaic polyploids, animals that are diploid except for patches of polyploid cells. How might a mosaic tetraploid – an animal with some cells containing four sets of chromosomes – arise? • 2) About 5% of individuals with Down syndrome have a chromosomal translocation in which a third copy of chromosome 21 is attached to chromosome 14. If this translocation occurred in a parent’s gonad, how could it lead to Down syndrome in a child? • 3) The ABO blood type locus has been mapped on chromosome 9. A father who has type AB blood and a mother who has type O blood have a child with trisomy 9 and type A blood. Using this information, can you tell in which parent the nondisjunction occurred? Explain your answer. Concept 15.5: Some inheritance patterns are exceptions to the standard chromosome theory Exceptions to the Standard Chromosome Theory • There are two normal exceptions to Mendelian genetics – One exception involves genes located in the nucleus – The other exception involves genes located outside the nucleus • In both of these cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance Genomic Imprinting • For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits Fig. 15-18 – Such variation in phenotype is called genomic imprinting – Paternal chromosome Maternal chromosome Involves the silencing of Normal Igf2 allele is not expressed certain genes that are stamped” with an imprint during gamete production • As a result, only either the male or Normal Igf2 allele is expressed Wild-type mouse (normal size) (a) Homozygote Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father female allele is expressed in every cell of an organism – Ex) In the mouse gene for Normal size mouse (wild type) Dwarf mouse (mutant) Normal Igf2 allele is expressed Mutant Igf2 allele is expressed Mutant Igf2 allele is not expressed Normal Igf2 allele is not expressed insulin-like growth factor 2 (Igf2), only the paternal allele is expressed (b) Heterozygotes Methylation and Genomic Imprinting • It appears that imprinting is the result of the methylation (addition of –CH3) of DNA – May directly silence genes • Ex) Heavily methylated genes are usually inactive – May activate the expression of an allele • Ex) Methylation results in paternal expression of Igf2 allele • Genomic imprinting is thought to affect only a small fraction of mammalian genes – Most imprinted genes are critical for embryonic development Inheritance of Organelle Genes • Not every eukaryotic gene is located on nuclear chromosomes – Extranuclear genes (or cytoplasmic genes) are genes found in organelles in the cytoplasm • Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules • These organelles can reproduce independently and thus transmit their genes to daughter organelles – Distribution of these genes to offspring does not follow the same rules or Mendelian inheritance patterns • Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg Extranuclear Genes in Plants • The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant – These coloration patterns are due to mutations in plastid genes that control pigmentation • Coloration of offspring is determined by maternal parent – Pattern depends on ratio of wild-type to mutant plastids inherited from the mother Mitochondrial Genes of Plants and Animals • Similar maternal inheritance is also the rule for mitochondrial genes in most animals and plants – Products of most mitochondrial genes help make up protein complexes of ETC and ATP synthase • Defects in one or more of these proteins reduce the amount of ATP the cell can make • Results in diseases that affect the muscular and nervous systems – Ex) Mitochondrial myopathy causes weakness, intolerance of exercise, and muscle deterioration – Ex) Leber’s hereditary optic neuropathy can produce sudden blindness Concept Check 15.5 • 1) Gene dosage, the number of active copies of a gene, is important to proper development. Identify and describe two processes that establish the proper dosage of certain genes. • 2) Reciprocal crosses between two primrose varieties, A and B, produced the following results: A female X B male = offspring with all green (nonvariegated) leaves B female X A male = offspring with spotted (variegated) leaves. Explain these results. • 3) Mitochondrial genes are critical to the energy metabolism of cells, but mitochondrial disorders caused by mutations in these genes are generally not lethal. Why not? You should now be able to: 1. Explain the chromosomal theory of inheritance and its discovery 2. Explain why sex-linked diseases are more common in human males than females 3. Distinguish between sex-linked genes and linked genes 4. Explain how meiosis accounts for recombinant phenotypes 5. Explain how linkage maps are constructed Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 6. Explain how nondisjunction can lead to aneuploidy 7. Define trisomy, triploidy, and polyploidy 8. Distinguish among deletions, duplications, inversions, and translocations 9. Explain genomic imprinting 10.Explain why extranuclear genes are not inherited in a Mendelian fashion Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings