Chromosomes and Human Inheritance. Chapter 12

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1 Chromosomes and Human Inheritance Chapter 12

2 Impacts, Issues: Strange Genes, Tortured Minds Exceptional creativity often accompanies neurobiological disorders such as schizophrenia, autism, chronic depression, and bipolar disorder Examples: Lincoln, Woolf, and Picasso

3 12.1 Human Chromosomes In humans, two sex chromosomes are the basis of sex human males have XY sex chromosomes, females have XX All other human chromosomes are autosomes chromosomes that are the same in males and females

4 Sex Determination in Humans Sex of a child is determined by the father Eggs have an X chromosome; sperm have X or Y

5 Sex Determination in Humans The SRY gene on the Y chromosome is the master gene for male sex determination Triggers formation of testes, which produce the male sex hormone (testosterone) Without testosterone, ovaries develop and produce female sex hormones (estrogens)

6 Sexual Development in Humans

7 diploid germ cells in female diploid germ cells in male meiosis, gamete formation in both female and male: eggs sperm X Y X X fertilization: X X X XX XX Y XY XY sex chromosome combinations possible in the new individual Fig. 12-2a, p. 186

8 Fig. 12-2bc, p. 186

9 At seven weeks, appearance of uncommitted duct system of embryo At seven weeks, appearance of structures that will give rise to external genitalia Y chromosome present Y chromosome absent Y chromosome present Y chromosome absent testes ovaries 10 weeks 10 weeks ovary penis vaginal opening uterus penis vagina testis birth approaching b c Fig. 12-2bc, p. 186

10 Animation: Human sex determination

11 Karyotyping Karyotype A micrograph of all metaphase chromosomes in a cell, arranged in pairs by size, shape, and length Detects abnormal chromosome numbers and some structural abnormalities Construction of a karyotype Colchicine stops dividing cells at metaphase Chromosomes are separated, stained, photographed, and digitally rearranged

12 Karyotyping

13 Fig. 12-3a, p. 187

14 Fig. 12-3b, p. 187

15 Animation: Karyotype preparation

16 12.1 Key Concepts Autosomes and Sex Chromosomes All animals have pairs of autosomes chromosomes that are identical in length, shape, and which genes they carry Sexually reproducing species also have a pair of sex chromosomes; the members of this pair differ between males and females

17 12.2 Autosomal Inheritance Patterns Many human traits can be traced to autosomal dominant or recessive alleles that are inherited in Mendelian patterns Some of those alleles cause genetic disorders

18 Autosomal Dominant Inheritance A dominant autosomal allele is expressed in homozygotes and heterozygotes Tends to appear in every generation With one homozygous recessive and one heterozygous parent, children have a 50% chance of inheriting and displaying the trait Examples: achondroplasia, Huntington s disease

19 Autosomal Recessive Inheritance Autosomal recessive alleles are expressed only in homozygotes; heterozygotes are carriers and do not have the trait A child of two carriers has a 25% chance of expressing the trait Example: galactosemia

20 Autosomal Inheritance

21 Fig. 12-4a, p. 188

22 Fig. 12-4b, p. 188

23 Animation: Autosomal dominant inheritance

24 Animation: Autosomal recessive inheritance

25 Galactosemia

26 Neurobiological Disorders Most neurobiological disorders do not follow simple patterns of Mendelian inheritance Depression, schizophrenia, bipolar disorders Multiple genes and environmental factors contribute to NBDs

27 12.3 Too Young to be Old Progeria Genetic disorder that results in accelerated aging Caused by spontaneous mutations in autosomes

28 Key Concepts Autosomal Inheritance Many genes on autosomes are expressed in Mendelian patterns of simple dominance Some dominant or recessive alleles result in genetic disorders

29 12.4 Examples of X-Linked Inheritance X chromosome alleles give rise to phenotypes that reflect Mendelian patterns of inheritance Mutated alleles on the X chromosome cause or contribute to over 300 genetic disorders

30 X-Linked Inheritance Patterns More males than females have X-linked recessive genetic disorders Males have only one X chromosome and can express a single recessive allele A female heterozygote has two X chromosomes and may not show symptoms Males transmit an X only to their daughters, not to their sons

31 X-Linked Recessive Inheritance Patterns

32 Animation: X-linked inheritance

33 Some X-Linked Recessive Disorders Hemophilia A Bleeding caused by lack of blood-clotting protein Red-green color blindness Inability to distinguish certain colors caused by altered photoreceptors in the eyes Duchenne muscular dystrophy Degeneration of muscles caused by lack of the structural protein dystrophin

34 Hemophilia A in Descendents of Queen Victoria of England

35 Red-Green Color Blindness

36 Fig. 12-9a, p. 191

37 Fig. 12-9b, p. 191

38 Fig. 12-9c, p. 191

39 Fig. 12-9d, p. 191

40 12.4 Key Concepts Sex-Linked Inheritance Some traits are affected by genes on the X chromosome Inheritance patterns of such traits differ in males and females

41 12.5 Heritable Changes in Chromosome Structure On rare occasions, a chromosome s structure changes; such changes are usually harmful or lethal, rarely neutral or beneficial A segment of a chromosome may be duplicated, deleted, inverted, or translocated

42 Duplication DNA sequences are repeated two or more times; may be caused by unequal crossovers in prophase I

43 normal chromosome one segment repeated p. 192

44 Deletion Loss of some portion of a chromosome; usually causes serious or lethal disorders Example: Cri-du-chat

45 segment C deleted p. 192

46 Deletion: Cri-du-chat

47 Fig a, p. 192

48 Fig b, p. 192

49 Inversion Part of the sequence of DNA becomes oriented in the reverse direction, with no molecular loss

50 segments G, H, I become inverted p. 192

51 Translocation Typically, two broken chromosomes exchange parts (reciprocal translocation)

52 chromosome nonhomologous chromosome reciprocal translocation p. 192

53 Does Chromosome Structure Evolve? Changes in chromosome structure can reduce fertility in heterozygotes; but accumulation of multiple changes in homozygotes may result in new species Certain duplications may allow one copy of a gene to mutate while the other carries out its original function

54 Differences Among Closely Related Organisms Humans have 23 pairs of chromosomes; chimpanzees, gorillas, and orangutans have 24 Two chromosomes fused end-to-end

55 human chimpanzee gorilla orangutan Fig , p. 193

56 Evolution of X and Y Chromosomes from Homologous Autosomes

57 Ancestral reptiles (autosome pair) Ancestral reptiles Y X Monotremes Y X Marsupials Y X Monkeys Y X Humans Y X areas that can cross over SRY areas that cannot cross over A Before 350 mya, sex was determined by temperature, not by chromosome differences. B SRY gene evolves 350 mya. Other mutations accumulate and the chromosomes of the pair diverge. C By mya, the two chromosomes have diverged so much that they no longer cross over in one region. The Y chromosome begins to degenerate. D Three more times, mya, the pair stops crossing over in another region. Each time, more changes accumulate, and the Y chromosome gets shorter. Today, t he pair crosses over only at a small region near the ends. Fig , p. 193

58 12.6 Heritable Changes in the Chromosome Number Occasionally, new individuals end up with the wrong chromosome number Consequences range from minor to lethal Aneuploidy Too many or too few copies of one chromosome Polyploidy Three or more copies of each chromosome

59 Nondisjunction Changes in chromosome number can be caused by nondisjunction, when a pair of chromosomes fails to separate properly during mitosis or meiosis Affects the chromosome number at fertilization Monosomy (n-1 gamete) Trisomy (n+1 gamete)

60 Nondisjunction

61 Autosomal Change and Down Syndrome Only trisomy 21 (Down syndrome) allows survival to adulthood Characteristics include physical appearance, mental impairment, and heart defects Incidence of nondisjunction increases with maternal age Can be detected through prenatal diagnosis

62 Trisomy 21

63 n + 1 n + 1 n 1 n 1 chromosome alignments at metaphase I NONDISJUNCTION AT ANAPHASE I alignments at metaphase II anaphase II CHROMOSOME NUMBER IN GAMETES Fig b, p. 194

64 n + 1 n + 1 n 1 n 1 chromosome alignments at metaphase I NONDISJUNCTION AT ANAPHASE I alignments at metaphase II anaphase II CHROMOSOME NUMBER IN GAMETES Stepped Art Fig b, p. 194

65 Down Syndrome and Maternal Age

66 Fig a, p. 195

67 Fig b, p. 195

68 Change in Sex Chromosome Number Changes in sex chromosome number may impair learning or motor skills, or be undetected Female sex chromosome abnormalities Turner syndrome (XO) XXX syndrome (three or more X chromosomes) Male sex chromosome abnormalities Klinefelter syndrome (XXY) XYY syndrome

69 Turner Syndrome XO (one unpaired X chromosome) Usually caused by nondisjunction in the father Results in females with undeveloped ovaries

70 Key Concepts: Changes in Chromosome Structure or Number On rare occasions, a chromosome may undergo a large-scale, permanent change in its structure, or the number of autosomes or sex chromosomes may change In humans, such changes usually result in a genetic disorder

71 12.7 Human Genetic Analysis Charting genetic connections with pedigrees reveals inheritance patterns for certain alleles Pedigree A standardized chart of genetic connections Used to determine the probability that future offspring will be affected by a genetic abnormality or disorder

72 Studying Inheritance in Humans Genetic studies can reveal inheritance patterns or clues to past events Example: A link between a Y chromosome and Genghis Khan?

73 Defining Genetic Disorders and Abnormalities Genetic abnormality A rare or uncommon version of a trait; not inherently life threatening Genetic disorder An inherited condition that causes mild to severe medical problems, characterized by a specific set of symptoms (a syndrome)

74 Some Human Genetic Disorders and Genetic Abnormalities

75 Stepped Art Table 12-1, p. 196

76 Recurring Genetic Disorders Mutations that cause genetic disorders are rare and put their bearers at risk Such mutations survive in populations for several reasons Reintroduction by new mutations Recessive alleles are masked in heterozygotes Heterozygotes may have an advantage in a specific environment

77 A Pedigree for Huntington s Disease A progressive degeneration of the nervous system caused by an autosomal dominant allele

78 Constructing a Pedigree for Polydactyly

79 Animation: Pedigree diagrams

80 12.8 Prospects in Human Genetics Genetic analysis can provide parents with information about their future children Genetic counseling Starts with parental genotypes, pedigrees, and genetic testing for known disorders Information is used to predict the probability of having a child with a genetic disorder

81 Prenatal Diagnosis Tests done on an embryo or fetus before birth to screen for sex or genetic problems Involves risks to mother and fetus Three types of prenatal diagnosis Amniocentesis Chorionic villus sampling (CVS) Fetoscopy

82 Amniocentesis

83 Animation: Amniocentesis

84 Fetoscopy

85 Preimplantation Diagnosis Used in in-vitro fertilization An undifferentiated cell is removed from the early embryo and examined before implantation

86 After Preimplantation Diagnosis When a severe problem is diagnosed, some parents choose an induced abortion In some cases, surgery, prescription drugs, hormone replacement therapy, or dietary controls can minimize or eliminate symptoms of a genetic disorder Example: PKU can be managed with dietary restrictions

87 Genetic Screening Genetic screening (widespread, routine testing for alleles associated with genetic disorders) Provides information on reproductive risks Identifies family members with a genetic disorder Used to screen newborns for certain disorders Used to estimate the prevalence of harmful alleles in a population

88 Key Concepts Human Genetic Analysis Various analytical and diagnostic procedures often reveal genetic disorders What an individual, and society at large, should do with the information raises ethical questions

89 Animation: Deletion

90 Animation: Duplication

91 Animation: Inversion

92 Animation: Morgan s reciprocal crosses

93 Animation: Translocation

94 Video: Strange genes, richly tortured minds

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