1. Chapter 14 Human Inheritance
(Sections ) 1

2. 14.5 Heritable Changes in Chromosome Structure
Major changes in chromosome structure include duplications, deletions, inversions, and translocations
Major changes in chromosome structure have been evolutionarily important
More frequently, such changes tend to result in genetic disorders

3. Duplication
Duplications occur during prophase I of meiosis, when crossing over occurs unequally between homologous chromosomes
duplication
Repeated section of a chromosome
Figure 14.9 Large-scale changes in chromosome structure.

4. Deletion
In mammals, deletions usually cause serious disorders and are often lethal
deletion
Loss of part of a chromosome
Figure 14.9 Large-scale changes in chromosome structure.

5. Inversion
Inversion may not affect a carrier’s health if it doesn’t disrupt a gene, but it may affect fertility
inversion
Structural rearrangement of a chromosome in which a part becomes oriented in the reverse direction, with no molecular loss
Figure 14.9 Large-scale changes in chromosome structure.

6. Duplication 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

7. Inversion
C 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

8. Translocation
If a chromosome breaks, the broken part may attach to a different chromosome, or to a different part of the same one
Most translocations are reciprocal, or balanced, which means two chromosomes exchange broken parts
translocation
Structural change of a chromosome in which a broken piece gets reattached in the wrong location

9. Reciprocal Translocation
Many reciprocal translocations have no adverse effects on health, but can affect fertility

10. Reciprocal 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

11. Some Disorders with Changes in Chromosome Structure
Huntington’s disease: expansion mutations (duplications)
Degeneration of the nervous system
Cri-du-chat syndrome (deletion)
Mental impairment; abnormal larynx
Burkitt’s lymphoma (translocation)
An aggressive cancer of the immune system

12. Chromosome Changes in Evolution
Most major alterations are harmful or lethal in humans
Even so, many major structural changes have accumulated in chromosomes of all species over evolutionary time
Speciation can and does occur by large-scale changes in chromosomes

13. Evolution of the Y Chromosome
X and Y chromosomes were once homologous autosomes in reptilelike ancestors of mammals
About 350 mya, a gene on one chromosome mutated – interfering with crossing over during meiosis – and mutations began to accumulate separately in the two chromosomes
Today, the SRY gene (Y chromosome) determines male sex

14. Evolution 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.

15. Evolution of the Y Chromosome
(autosome pair)
area that cannot cross over
SRY
Ancestral reptiles
>350 mya
Ancestral reptiles
350 mya
Monotremes
320–240
mya
Marsupials
170–130 mya
Monkeys
130–80 mya
Humans 50–30 mya
A Before 350
mya, 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 chromosomes
diverge 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

16. Human Evolution
One human chromosome matches two in chimpanzees and other great apes
During human evolution, two chromosomes fused end to end and formed our chromosome 2
Figure Human chromosome 2 compared with chimpanzee chromosomes 2A and 2B.

17. Human Evolution telomere sequence human chimpanzee
Figure Human chromosome 2 compared with chimpanzee chromosomes 2A and 2B.
human
chimpanzee
Fig , p. 211

18. ANIMATION: Deletion To play movie you must be in Slide Show Mode
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19. ANIMATION: Duplication
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20. ANIMATION: Inversion To play movie you must be in Slide Show Mode
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21. Animation: Translocation
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22. 14.6 Heritable Changes in the Chromosome Number
Occasionally, abnormal events occur before or during meiosis, and new individuals end up with the wrong chromosome number
Consequences range from minor to lethal changes in form and function

23. Nondisjunction
Changes in chromosome number are usually caused by nondisjunction
Nondisjunction affects chromosome number at fertilization and causes genetic disorders among resulting offspring
nondisjunction
Failure of sister chromatids or homologous chromosomes to separate during nuclear division

24. Nondisjunction

25. Nondisjunction 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 I
Anaphase I
Telophase I
Metaphase II
Anaphase II
Telophase II
Fig , p. 212

26. Nondisjunction Metaphase I Anaphase I Telophase I Metaphase II
Anaphase II
Telophase 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.
Stepped Art
Fig , p. 212

27. Aneuploidy
In 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 embryos
aneuploidy
A chromosome abnormality in which an individual’s cells carry too many or too few copies of a particular chromosome

28. Types of Aneuploidy Trisomy:
A normal gamete (n) fuses with an n+1 gamete
New individual is trisomic (2n+1), having three of one type of chromosome and two of every other type
Monosomy:
An n-1 gamete fuses with a normal (n) gamete
New individual is monosomic (2n-1)

29. Polyploidy
Polyploid individuals have three or more of each type of chromosome
Polyploidy is lethal in humans, but many flowering plants, and some insects, fishes, and other animals, are polyploid
polyploid
Having three or more of each type of chromosome characteristic of the species

30. Disorders with Changes in Chromosome Number
Disorder Main Symptoms
Down syndrome Mental impairment; heart defects
Turner syndrome (XO) Sterility; abnormal ovaries and sexual traits
Klinefelter syndrome Sterility; mild mental impairment
XXX syndrome Minimal abnormalities
XYY condition Mild mental impairment or no effect

31. Autosomal Change and Down Syndrome
The most common aneuploidy, trisomy 21, causes Down syndrome
Characteristics include upward-slanting eyes, slightly flattened facial features, and other symptoms
Trisomic 21 individuals tend to have moderate to severe mental impairment and heart problems

32. Down Syndrome

33. Down Syndrome Figure 14.13 Down syndrome, genotype and phenotype.
Fig a, p. 213

34. Down Syndrome Figure 14.13 Down syndrome, genotype and phenotype.
Fig b, p. 213

35. Change in Sex Chromosome Number
A change in the number of sex chromosomes usually results in some degree of impairment in learning and motor skills
In individual with trisomy (XXY, XXX, and XYY) these problems can be subtle and the cause may never be diagnosed

36. Female 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 mature
In XXX syndrome, having extra X chromosomes usually does not result in physical or medical problems

37. Male Sex Chromosome Abnormalities
Males with Klinefelter syndrome (XXY ) tend to be overweight, tall, and within normal range of intelligence
They make more estrogen and less testosterone than normal males, which has feminizing effects
XYY males tend to be taller than average and have mild mental impairment, but are otherwise normal

38. Key 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 change
In humans, such changes usually result in a genetic disorder

39. Animation: Amniocentesis
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40. ABC Video: Genetic Testing: Screening Embryos for Disease

41. 14.7 Genetic Screening
Prospective 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 counselor
Some disorders can be detected early enough to start countermeasures before symptoms develop

42. Newborn 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 levels
The resulting imbalance inhibits protein synthesis in the brain, which results in severe neurological symptoms characteristic of phenylketonuria (PKU)

43. Prenatal Diagnosis
Prenatal genetic testing of an embryo or fetus can reveal genetic abnormalities or disorders before birth
Obstetric sonography
Fetoscopy
Amniocentesis
Chorionic villus sampling (CVS)
An invasive procedure often carries a risk to the fetus

44. Imaging a Fetus in the Uterus
Obstetric sonography (ultrasound) forms images of the fetus’s developing limbs and internal organs
Fetoscopy yields higher-resolution images

45. Imaging a Fetus in the Uterus
Figure Imaging a fetus developing in the uterus.
A An ultrasound image.
Fig a, p. 214

46. Imaging a Fetus in the Uterus
Figure Imaging a fetus developing in the uterus.
B A fetoscopy image.
Fig b, p. 214

47. Tests for Genetic Disorders
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

48. Tests 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 sac
placenta
Fig , p. 215

49. Preimplantation Diagnosis
Clump of cells formed by three mitotic divisions after in vitro fertilization
One cell can be removed for genetic analysis to determine whether the embryo carries any genetic defects
Figure 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.

50. Key Concepts Genetic Testing
Genetic testing provides information about the risk of passing a harmful allele to one’s offspring
After conception, various methods of prenatal testing can reveal a genetic abnormality or disorder in a fetus or embryo

51. Shades of Skin (revisited)
People of Chinese descent carry an allele of the DCT gene which results in conversion of tyrosine to melanin
Distribution 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