《生物学》课程参考书籍文献（英文版，Chapter 11 - Chapter 19）Part IV Reproduction and Heredity、Part V Molecular Genetics

MEIOSIS Haploid gametes PROPHASE ierm-line cell TELOPHASE METAPHASE 小 ANAPHASE 小 FIGURE 12.15 How meiosis works. Meiosis consists of two rounds of cell division and produces four haploid cells Chapter 12 Sexual Reproduction and Meiosis 235

Chapter 12 Sexual Reproduction and Meiosis 235 MEIOSIS Germ-line cell Haploid gametes PROPHASE I II TELOPHASE I II ANAPHASE II I II I METAPHASE FIGURE 12.15 How meiosis works. Meiosis consists of two rounds of cell division and produces four haploid cells

2.3 The evolutionary origin of sex is a puzzle Why Sex? The Origin and maintenance of sex Not all reproduction is sexual. In asexual reproduction, There is no consensus among evolutionary biologists re- an individual inherits all of its chromosomes from a sin rding the evolutionary origin or maintenance of sex Conflicting hypotheses abound. Alternative hypotheses parent. Bacterial cells reproduce asexually, undergoing seem to be correct to varying de binary fission to produce two daughter cells containing organisms the same genetic information. Most protists reproduce asexually except under conditions of stress; then they The DNA Repair Hypothesis. If recombination is often switch to sexual reproduction. Among plants, asexual re- detrimental to an individual's progeny, then what benefit roduction is common, and many other multicellular or- promoted the evolution of sexual reproduction: Although ganisms are also capable of reproducing asexually. In ani- the answer to this question is unknown, we can gain some ls, asexual reproduction often involves the budding off insight by examining the protists. Meiotic recombination is a localized mass of cells, which grows by mitosis to often absent among the protists, which typically undergo form a new individual sexual reproduction nly occasionally. Often the fusion of Even when meiosis and the production of gametes two haploid cells occurs only under stress, creating a occur, there may still be reproduction without sex. The diploid zygote development of an adult from an unfertilized egg, called Why do some protists form a diploid cell in response parthenogenesis, is a common form of reproduction in to stress? Several geneticists have suggested that this oc- arthropods. Among bees, for example, fertilized eggs de- curs because only a diploid cell can effectively repair cer- velop into diploid females, but unfertilized eggs develop tain kinds of chromosome damage, particularly double- nto haploid males. Parthenogenesis even occurs among strand breaks in DNA. Both radiation and chemical the vertebrates. Some lizards, fishes, and amphibi lans are events within cells can induce such breaks. As organisms became larger and longer-lived, it must have become in- undergo a mitotic nuclear division without cell cleavage creasingly important for them to be able to repair such to produce a diploid cell, which then develops into an damage. The synaptonemal complex, which in early stage adult of meiosis precisely aligns pairs of homologous chromo- omes,may well have evolved originally as a mechanism Recombination can be destructive for repairing double-strand damage to DNA, using the undamaged homologous chromosome as a template to re- If reproduction can occur without sex, why does sex occur pair the damaged chromosome. A transient diploid phase at all? This question has generated considerable discussion provided an opportunity for such rep particularly among evolutionary biologists. Sex is of great yeast, mutations that inactivate the repair system for dou- evolutionary advantage for populations or species, whi ble-strand breaks of the chromosomes also prevent nefit from the variability generated in meiosis by random ing over, suggesting a common mechanism for orientation of chromosomes and by crossing over. How- synapsis and repair processes ever, evolution occurs because of changes at the level of in- dividual survival and reproduction, rather than at the popu- The Contagion Hypothesis. An unusual and interesting lation level, and no obvious advantage accrues to the alternative hypothesis for the origin of sex is that it arose as progeny of an individual that engages in sexual reprod g able el genetic elements. Suppose a replicating transpos a secondary consequence of the infection of eukaryotes by con d rocess in evolution. The segregation of chromo- able element were to infect a eukaryotic lineage. If it pos- ring meiosis tends to disrupt advantageous sessed genes promoting fusion with uninfected cells and binations of genes more often than it creates new, bet synapsis, the transposable element could readily copy itself ter adapted combinations; as a result, some of the diverse onto homologous chromosomes. It would rapidly spread by progeny produced by sexual reproduction will not be as infection through the population, until all members con- well adapted as their parents were. In fact, the more com- tained it. The bizarre mating type"alleles"found in many plex the adaptation of an individual organism, the less likely fungi are very nicely explained by this hypothesis. Each of that recombination will improve it, and the more likely that several mating types is in fact not an allele but an"id recombination will disrupt it. It is, therefore, a puzzle to iomorph. Idiomorphs are genes occupying homologous know what a well-adapted individual gains from participat- positions on the chromosome but having such dissimilar ng in sexual reproduction, as all of its progeny could main- sequences that they cannot be of homologous origin. The ain its successful gene combinations if that individual sim- idiomorph genes may simply be the relics of several ancient ply reproduced infections by transposable elements 236 Part IV Reproduction and Heredity

Why Sex? Not all reproduction is sexual. In asexual reproduction, an individual inherits all of its chromosomes from a sin￾gle parent and is, therefore, genetically identical to its parent. Bacterial cells reproduce asexually, undergoing binary fission to produce two daughter cells containing the same genetic information. Most protists reproduce asexually except under conditions of stress; then they switch to sexual reproduction. Among plants, asexual re￾production is common, and many other multicellular or￾ganisms are also capable of reproducing asexually. In ani￾mals, asexual reproduction often involves the budding off of a localized mass of cells, which grows by mitosis to form a new individual. Even when meiosis and the production of gametes occur, there may still be reproduction without sex. The development of an adult from an unfertilized egg, called parthenogenesis, is a common form of reproduction in arthropods. Among bees, for example, fertilized eggs de￾velop into diploid females, but unfertilized eggs develop into haploid males. Parthenogenesis even occurs among the vertebrates. Some lizards, fishes, and amphibians are capable of reproducing in this way; their unfertilized eggs undergo a mitotic nuclear division without cell cleavage to produce a diploid cell, which then develops into an adult. Recombination Can Be Destructive If reproduction can occur without sex, why does sex occur at all? This question has generated considerable discussion, particularly among evolutionary biologists. Sex is of great evolutionary advantage for populations or species, which benefit from the variability generated in meiosis by random orientation of chromosomes and by crossing over. How￾ever, evolution occurs because of changes at the level of in￾dividual survival and reproduction, rather than at the popu￾lation level, and no obvious advantage accrues to the progeny of an individual that engages in sexual reproduc￾tion. In fact, recombination is a destructive as well as a con￾structive process in evolution. The segregation of chromo￾somes during meiosis tends to disrupt advantageous combinations of genes more often than it creates new, bet￾ter adapted combinations; as a result, some of the diverse progeny produced by sexual reproduction will not be as well adapted as their parents were. In fact, the more com￾plex the adaptation of an individual organism, the less likely that recombination will improve it, and the more likely that recombination will disrupt it. It is, therefore, a puzzle to know what a well-adapted individual gains from participat￾ing in sexual reproduction, as all of its progeny could main￾tain its successful gene combinations if that individual sim￾ply reproduced asexually. The Origin and Maintenance of Sex There is no consensus among evolutionary biologists re￾garding the evolutionary origin or maintenance of sex. Conflicting hypotheses abound. Alternative hypotheses seem to be correct to varying degrees in different organisms. The DNA Repair Hypothesis. If recombination is often detrimental to an individual’s progeny, then what benefit promoted the evolution of sexual reproduction? Although the answer to this question is unknown, we can gain some insight by examining the protists. Meiotic recombination is often absent among the protists, which typically undergo sexual reproduction only occasionally. Often the fusion of two haploid cells occurs only under stress, creating a diploid zygote. Why do some protists form a diploid cell in response to stress? Several geneticists have suggested that this oc￾curs because only a diploid cell can effectively repair cer￾tain kinds of chromosome damage, particularly double￾strand breaks in DNA. Both radiation and chemical events within cells can induce such breaks. As organisms became larger and longer-lived, it must have become in￾creasingly important for them to be able to repair such damage. The synaptonemal complex, which in early stages of meiosis precisely aligns pairs of homologous chromo￾somes, may well have evolved originally as a mechanism for repairing double-strand damage to DNA, using the undamaged homologous chromosome as a template to re￾pair the damaged chromosome. A transient diploid phase would have provided an opportunity for such repair. In yeast, mutations that inactivate the repair system for dou￾ble-strand breaks of the chromosomes also prevent cross￾ing over, suggesting a common mechanism for both synapsis and repair processes. The Contagion Hypothesis. An unusual and interesting alternative hypothesis for the origin of sex is that it arose as a secondary consequence of the infection of eukaryotes by mobile genetic elements. Suppose a replicating transpos￾able element were to infect a eukaryotic lineage. If it pos￾sessed genes promoting fusion with uninfected cells and synapsis, the transposable element could readily copy itself onto homologous chromosomes. It would rapidly spread by infection through the population, until all members con￾tained it. The bizarre mating type “alleles” found in many fungi are very nicely explained by this hypothesis. Each of several mating types is in fact not an allele but an “id￾iomorph.” Idiomorphs are genes occupying homologous positions on the chromosome but having such dissimilar sequences that they cannot be of homologous origin. These idiomorph genes may simply be the relics of several ancient infections by transposable elements. 236 Part IV Reproduction and Heredity 12.3 The evolutionary origin of sex is a puzzle

Patemal gamete Matemal gamete Diploid offspring Homologous pairs Potential gametes FIGURE 12.16 Independent assortment increases genetic variability. Independent assortment contributes new gene combinations to the next generation because the orientation of chromosomes on the metaphase plate is random. In the cells shown above with three chromosome pairs, eight different gametes can result, each with different combinations of parental chromosomes. The Red Queen Hypothesis. One evolutionary ad- process generates diversity more quickly; and, as you will vantage of sex may be that it allows populations to see in later chapters, genetic diversity is the raw material of store" recessive alleles that are currently bad but have evolution, the fuel that drives it and determines its poten promise for reuse at some time in the future. Because tial directions. In many cases, the pace of evolution appears populations are constrained by a changing physical and to increase as the level of genetic diversity increases. Pro- biological environment, selection is constantly acting grams for selecting larger stature in domesticated animals against such alleles, but in sexual species can never get such as cattle and sheep, for example, proceed rapidly at of those sheltered in heterozygotes. The evolution of first, but then slow as the existing genetic combinations are most sexual species, most of the time, thus manages to exhausted; further progress must then await the generation constraints.This"treadmill evolution s hd biological of new gene combinations. Racehorse breeding provides a keep pace with ever-changing physical sometimes graphic example: thoroughbred racehorses are all descen called the"Red Queen hypothesis, "after the Queen of dants of a small initial number of individuals, and selection Hearts in Lewis Carroll's Tbrougb the Looking Glass, who for speed has accomplished all it can with this limited tells Alice, "Now, here, you see, it takes all the running amount of genetic variability-the winning times in major you can do, to ep in the same place.” aces ceased to improve decades ago. Paradoxically, the evolutionary process is thus both Miller's Ratchet. The geneticist Herman Miller pointed revolutionary and conservative. It is revolutionary in that out in 1965 that asexual populations incorporate a kind of the pace of evolutionary change is quickened by genetic mutational ratchet mechanism--once harmful mutations recombination, much of which results from sexual repro- arise, asexual populations have no way of eliminating them, duction. It is conservative in that evolutionary change is and they accumulate over time, like turning a ratchet. Sex- not always favored by selection, which may instead pre- ual populations, on the other hand, can employ recombina- serve existing combinations of genes. These conservative tion to generate individuals carrying fewer mutations, pressures appear to be greatest in some asexually repro- which selection can then favor. Sex may just be a way to ducing organisms that do not move around freely and eep the mutational load down. that live in especially demanding habitats. In vertebrates, on the other hand, the evolutionary premium appears to The Evolutionary Consequences of Sex have been on versatility, and sexual reproduction is the predominant mode of reproduction by an overwhelming While our knowledge of how sex evolved is sketchy, it is margin abundantly clear that sexual reproduction has an enormous impact on how species evolve today, because of its ability to apidly generate new genetic combinations. Independent The close association between homologous assortment(figure 12. 16), crossing over, and random fertil chromosomes that occurs during meiosis may have ization each help generate genetic diversity evolved as mechanisms to repair chromosomal damage Whatever the forces that led to sexual reproduction, its although several alternative mechanisms have also been evolutionary consequences have been profound. No genetic Chapter 12 Sexual Reproduction and Meiosis 237

The Red Queen Hypothesis. One evolutionary ad￾vantage of sex may be that it allows populations to “store” recessive alleles that are currently bad but have promise for reuse at some time in the future. Because populations are constrained by a changing physical and biological environment, selection is constantly acting against such alleles, but in sexual species can never get rid of those sheltered in heterozygotes. The evolution of most sexual species, most of the time, thus manages to keep pace with ever-changing physical and biological constraints. This “treadmill evolution” is sometimes called the “Red Queen hypothesis,” after the Queen of Hearts in Lewis Carroll’s Through the Looking Glass, who tells Alice, “Now, here, you see, it takes all the running you can do, to keep in the same place.” Miller’s Ratchet. The geneticist Herman Miller pointed out in 1965 that asexual populations incorporate a kind of mutational ratchet mechanism—once harmful mutations arise, asexual populations have no way of eliminating them, and they accumulate over time, like turning a ratchet. Sex￾ual populations, on the other hand, can employ recombina￾tion to generate individuals carrying fewer mutations, which selection can then favor. Sex may just be a way to keep the mutational load down. The Evolutionary Consequences of Sex While our knowledge of how sex evolved is sketchy, it is abundantly clear that sexual reproduction has an enormous impact on how species evolve today, because of its ability to rapidly generate new genetic combinations. Independent assortment (figure 12.16), crossing over, and random fertil￾ization each help generate genetic diversity. Whatever the forces that led to sexual reproduction, its evolutionary consequences have been profound. No genetic process generates diversity more quickly; and, as you will see in later chapters, genetic diversity is the raw material of evolution, the fuel that drives it and determines its poten￾tial directions. In many cases, the pace of evolution appears to increase as the level of genetic diversity increases. Pro￾grams for selecting larger stature in domesticated animals such as cattle and sheep, for example, proceed rapidly at first, but then slow as the existing genetic combinations are exhausted; further progress must then await the generation of new gene combinations. Racehorse breeding provides a graphic example: thoroughbred racehorses are all descen￾dants of a small initial number of individuals, and selection for speed has accomplished all it can with this limited amount of genetic variability—the winning times in major races ceased to improve decades ago. Paradoxically, the evolutionary process is thus both revolutionary and conservative. It is revolutionary in that the pace of evolutionary change is quickened by genetic recombination, much of which results from sexual repro￾duction. It is conservative in that evolutionary change is not always favored by selection, which may instead pre￾serve existing combinations of genes. These conservative pressures appear to be greatest in some asexually repro￾ducing organisms that do not move around freely and that live in especially demanding habitats. In vertebrates, on the other hand, the evolutionary premium appears to have been on versatility, and sexual reproduction is the predominant mode of reproduction by an overwhelming margin. The close association between homologous chromosomes that occurs during meiosis may have evolved as mechanisms to repair chromosomal damage, although several alternative mechanisms have also been proposed. Chapter 12 Sexual Reproduction and Meiosis 237 Paternal gamete Diploid offspring Maternal gamete Homologous pairs Potential gametes FIGURE 12.16 Independent assortment increases genetic variability. Independent assortment contributes new gene combinations to the next generation because the orientation of chromosomes on the metaphase plate is random. In the cells shown above with three chromosome pairs, eight different gametes can result, each with different combinations of parental chromosomes

Chapter 12 http://www.mhhe.com/raven6ehttp://www.biocourse.com Questions Media resource 12.1 Meiosis produces haploid cells from diploid cell Meiosis is a special form of nuclear division that 1. What are the cellular products produces the gametes of the sexual cycle. It involves of meiosis called, and are they two chromosome separations but only one haploid or diploid? What is the hromosome replication cellular product of syngas alled. and is it haploid or diploid 12.2 Meiosis has three unique features The three unique features of meiosis ar 2. What three unique features homologous recombination, and reduction division distinguish meiosis from mitosis 12.3 The sequence of events during meiosis involves two nuclear divisions. The crossing over that occurs between homologues 3. What are synaptonemal Art Activity: Meiosis I · Because crossing over binds the homologues令 syn omplexes? How do they articipate in crossing over? At together, only one side of each homologue what stage during meiosis are accessible to the spindle fibers. Hence, the spindle they formed: fibers separate the paired homologues rather than the 4.How many chromatids are Meiosis sister chromatids present for each type of chromosome at the completion At the end of meiosis I, one homologue of each of crossing over? What two chromosome type is present at each of the two poles of ructures hold the chromatids the dividing nucleus. The homologues still consist of together at this stage? two chromatids, which may differ from each other as a 5. How is the attachment of result of crossing over that occurred during synapsis spindle microtubules to No further DNA replication occurs before the second centromeres in metaphase I of nuclear division, which is essentially a mitotic division which occurs in metaphase of mitosis? What effect does this The sister chromatids of each chromosome are difference have on the separated, resulting in the formation of four daughte movement of chromosomes nuclei. each with half the number of chromosomes during anaphase I? that were present before meiosis 6. What mechanism Cytokinesis typically but not always occurs at this responsible for the ind ndent point. When it does, each daughter nucleus has one assortment of chromosomes? some 12. 4 The evolutionary origin of sex is a puzzle In asexual reproduction, mitosis produces offspring 7. What is one of the current Evolution of sex genetically identical to the parent. cientific explanations for the eview of cell Division Meiosis is thought to have evolved initially as a evolution mechanism to repair double-strand breaks in DNA, 8. By what three mechanisms in which the broken chromosome is paired with its homologue while it is being repaired Increase genetic variability does this increase in genetic The evolutionary significance of meiosis is that it ariability affect the evolution of generates large amounts of recombination, rapidly reshuffling gene combinations, producing variability upon which evolutionary processes can act 238 Part IV Reproduction and Heredity

238 Part IV Reproduction and Heredity Chapter 12 Summary Questions Media Resources 12.1 Meiosis produces haploid cells from diploid cells. • Meiosis is a special form of nuclear division that produces the gametes of the sexual cycle. It involves two chromosome separations but only one chromosome replication. 1. What are the cellular products of meiosis called, and are they haploid or diploid? What is the cellular product of syngamy called, and is it haploid or diploid? • The three unique features of meiosis are synapsis, homologous recombination, and reduction division. 2. What three unique features distinguish meiosis from mitosis? 12.2 Meiosis has three unique features. • The crossing over that occurs between homologues during synapsis is an essential element of meiosis. • Because crossing over binds the homologues together, only one side of each homologue is accessible to the spindle fibers. Hence, the spindle fibers separate the paired homologues rather than the sister chromatids. • At the end of meiosis I, one homologue of each chromosome type is present at each of the two poles of the dividing nucleus. The homologues still consist of two chromatids, which may differ from each other as a result of crossing over that occurred during synapsis. • No further DNA replication occurs before the second nuclear division, which is essentially a mitotic division occurring at each of the two poles. • The sister chromatids of each chromosome are separated, resulting in the formation of four daughter nuclei, each with half the number of chromosomes that were present before meiosis. • Cytokinesis typically but not always occurs at this point. When it does, each daughter nucleus has one copy of every chromosome. 3. What are synaptonemal complexes? How do they participate in crossing over? At what stage during meiosis are they formed? 4. How many chromatids are present for each type of chromosome at the completion of crossing over? What two structures hold the chromatids together at this stage? 5. How is the attachment of spindle microtubules to centromeres in metaphase I of meiosis different from that which occurs in metaphase of mitosis? What effect does this difference have on the movement of chromosomes during anaphase I? 6. What mechanism is responsible for the independent assortment of chromosomes? 12.3 The sequence of events during meiosis involves two nuclear divisions. • In asexual reproduction, mitosis produces offspring genetically identical to the parent. • Meiosis is thought to have evolved initially as a mechanism to repair double-strand breaks in DNA, in which the broken chromosome is paired with its homologue while it is being repaired. • The evolutionary significance of meiosis is that it generates large amounts of recombination, rapidly reshuffling gene combinations, producing variability upon which evolutionary processes can act. 7. What is one of the current scientific explanations for the evolution of synapsis? 8. By what three mechanisms does sexual reproduction increase genetic variability? How does this increase in genetic variability affect the evolution of species? 12.4 The evolutionary origin of sex is a puzzle. http://www.mhhe.com/raven6e http://www.biocourse.com • Art Activity: Meiosis I • Meiosis • Meiosis • Evolution of Sex • Review of Cell Division

1 Patterns of Inberitance Concept Outline 13.1 Mendel solved the mystery of heredity Early Ideas about Heredity: The Road to Mendel. Before Mendel. the mechanism of inheritance was not known Mendel and the Garden Pea. Mendel experimented with heredity in edible peas counted his results What Mendel Found. Mendel found that alternative traits for a character segregated among second-generation progeny in the ratio 3: 1. Mendel proposed that information for a trait rather than the trait itself is inherited How Mendel Interpreted His Results. Mendel found that one alternative of a character could mask the other in heterozygotes, but both could subsequently be expressed in homozygotes of future generations Mendelian Inheritance Is Not Always Easy to Analyze variety of factors can influence the Mendelian gregation of allel 13.2 Human genetics follows Mendelian principles. Most Gene Disorders Are Rare. Tay-Sachs disease is due to a recessive allele FIGURE 13.1 Multiple Alleles: The ABO Blood Groups. The human Human beings are extremely diverse in appearance.The ABO blood groups are determined by three I gene alleles. differences between us are partly inherited and partly the result Patterns of Inheritance Can Be deduced from of environmental factors we encounter in our lives Pedigrees. Hemoph inked Gene Disorders Can Be Due to Simple Alterations of Proteins. Sickle cell anemia is caused by a single amino Erery liv inis creat ifis a product te ng eis Some Defects May Soon Be Curable. Cystic fibrosis share this history, only humans wonder about the may soon be cured by gene replacement therapy. processes that led to their origin. We are still far from understanding everything about our origins, but we have 13.3 Genes are on chromosomes learned a great deal. Like a partially completed jigsaw Chromosomes: The Vehicles of Mendelian puzzle, the boundaries have fallen into place, and much Inheritance.Mendelian segregation reflects the random of the internal structure is becoming apparent. In this assortment of chromosomes in meiosis Genetic Recombination. Crossover frequency reflect enigma of \ougroiry. Why do groups of people from di chapter, we will discuss one piece of the puzzle-th the physical distance between genes ferent parts of the world often differ in appearance(fi Human Chromosomes. Humans possess 23 pairs of ure 13.1)? Why do the members of a family tend to re- chromosomes, one of them determining the sex semble one another more than they resemble members of Human abnormalities due to Alterations in other families Chromosome Number. Loss or addition of hromosomes has serious consequences Genetic Counseling. Some gene defects can be detected arly in pregnancy. 239

239 13 Patterns of Inheritance Concept Outline 13.1 Mendel solved the mystery of heredity. Early Ideas about Heredity: The Road to Mendel. Before Mendel, the mechanism of inheritance was not known. Mendel and the Garden Pea. Mendel experimented with heredity in edible peas counted his results. What Mendel Found. Mendel found that alternative traits for a character segregated among second-generation progeny in the ratio 3:1. Mendel proposed that information for a trait rather than the trait itself is inherited. How Mendel Interpreted His Results. Mendel found that one alternative of a character could mask the other in heterozygotes, but both could subsequently be expressed in homozygotes of future generations. Mendelian Inheritance Is Not Always Easy to Analyze. A variety of factors can influence the Mendelian segregation of alleles. 13.2 Human genetics follows Mendelian principles. Most Gene Disorders Are Rare. Tay-Sachs disease is due to a recessive allele. Multiple Alleles: The ABO Blood Groups. The human ABO blood groups are determined by three I gene alleles. Patterns of Inheritance Can Be Deduced from Pedigrees. Hemophilia is sex-linked. Gene Disorders Can Be Due to Simple Alterations of Proteins. Sickle cell anemia is caused by a single amino acid change. Some Defects May Soon Be Curable. Cystic fibrosis may soon be cured by gene replacement therapy. 13.3 Genes are on chromosomes. Chromosomes: The Vehicles of Mendelian Inheritance. Mendelian segregation reflects the random assortment of chromosomes in meiosis. Genetic Recombination. Crossover frequency reflect the physical distance between genes. Human Chromosomes. Humans possess 23 pairs of chromosomes, one of them determining the sex. Human Abnormalities Due to Alterations in Chromosome Number. Loss or addition of chromosomes has serious consequences. Genetic Counseling. Some gene defects can be detected early in pregnancy. Every living creature is a product of the long evolu￾tionary history of life on earth. While all organisms share this history, only humans wonder about the processes that led to their origin. We are still far from understanding everything about our origins, but we have learned a great deal. Like a partially completed jigsaw puzzle, the boundaries have fallen into place, and much of the internal structure is becoming apparent. In this chapter, we will discuss one piece of the puzzle—the enigma of heredity. Why do groups of people from dif￾ferent parts of the world often differ in appearance (fig￾ure 13.1)? Why do the members of a family tend to re￾semble one another more than they resemble members of other families? FIGURE 13.1 Human beings are extremely diverse in appearance. The differences between us are partly inherited and partly the result of environmental factors we encounter in our lives