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 diploid
germ cells germ cells
in female in male
meiosis, gamete
formation in both eggs sperm
female and male:
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 At seven weeks, appearance
“uncommitted” duct system of of structures that will give
embryo rise to external genitalia
Y chromosome Y chromosome Y chromosome Y chromosome
present absent present 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. 12.2-12.3 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. 12-10a, p. 192

48. Fig. 12-10b, 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. 12-11, p. 193

56. Evolution of X and Y Chromosomes
from Homologous Autosomes

57. Ancestral reptiles Ancestral reptiles Monotremes Marsupials Monkeys Humans
(autosome pair) Y X Y X Y X Y X Y X
areas
that can
cross over
areas that
SRY cannot
cross over
A Before 350 B SRY gene C By 320–240 mya, the D Three more times, 170–130 mya,
mya, sex was evolves 350 mya. two chromosomes have the pair stops crossing over in
determined by Other mutations diverged so much that another region. Each time, more
temperature, not accumulate and they no longer cross changes accumulate, and the Y
by chromosome the chromosomes over in one region. The chromosome gets shorter. Today, t
differences. of the pair diverge. Y chromosome begins he pair crosses over only at a small
to degenerate. region near the ends.
Fig. 12-12, 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 CHROMOSOME
alignments at NONDISJUNCTION alignments at NUMBER
metaphase I AT ANAPHASE I metaphase II anaphase II IN GAMETES
Fig. 12-13b, p. 194

64. n+1
n+1
n−1
n−1
chromosome CHROMOSOME
alignments at NONDISJUNCTION alignments at NUMBER
metaphase I AT ANAPHASE I metaphase II anaphase II IN GAMETES
Stepped Art
Fig. 12-13b, p. 194

65. Down Syndrome and Maternal Age

66. Fig. 12-14a, p. 195

67. Fig. 12-14b, 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. 12.5-12.6 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. 12.7-12.8 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