1. GENETICS A Conceptual Approach
Benjamin A. Pierce
GENETICS
A Conceptual Approach
FIFTH EDITION
CHAPTER 8
Chromosome Variation
© 2014 W. H. Freeman and Company

2. Bananas have multiple sets of chromosomes.

3. Chromosome Morphology (position of the centromere on the chromosome):
8.1 Chromosome Mutations Include Rearrangements, Aneuploids, and Polyploids
Chromosome Morphology (position of the centromere on the chromosome):
Metacentric
Submetacentric
Acrocentric
Telocentric

4. 8.1 Chromosome Mutations Include Rearrangements, Aneuploids, and Polyploids
Karyotyping
Chromosomes prepared from actively dividing cells
Halted in metaphase
Chromosomes arranged according to size
Banding
G bands: Giemsa stain
Q bands: quinacrine mustard
C bands: centromeric heterochromatin
R bands: rich in cytosine-guanine base pairs

7. Types of Chromosome Mutations:
8.1 Chromosome Mutations Include Rearrangements, Aneuploids, and Polyploids
Types of Chromosome Mutations:
Rearrangements:
Aneuploidy
Polyploidy

8. Figure 8.3 Chromosome mutations consist of chromosome rearrangements, aneuploids, and polyploids. Duplications, trisomy, and autotriploids are examples of each category of mutation.

9. 8.2 Chromosome Rearrangements Alter Chromosome Structure
Four types of chromosomal rearrangements: Fig. 8.4
Duplication: Fig. 8.5, Fig. 8.6, Fig. 8.7
Unbalanced gene dosage: Fig. 8.8

10. Figure 8.4 In an individual heterozygous for a duplication, the duplicated chromosome loops out during pairing in prophase I.

11. Figure 8.5 In an individual heterozygous for a duplication, the duplicated chromosome loops out during pairing in prophase I.

12. Figure 8.6 The Bar phenotype in Drosophila melanogaster results from an X-linked duplication. (a) Wild-type fruit flies have normal-size eyes. (b) Flies heterozygous and (c) homozygous for the bar mutation have smaller, bar-shaped eyes. (d) Flies with double Bar have three copies of the duplication and much smaller bar-shaped eyes.

13. Figure 8.8 Unbalanced gene dosage leads to developmental abnormalities.

14. 8.2 Chromosome Rearrangements Alter Chromosome Structure
Deletions: loss of a chromosomal segment
Large deletions easily detected; during pairing, normal chromosome loops out (Fig. 8.9)
Effects of deletions
Imbalances in gene product
Expression of a normally recessive gene (pseudodominance)
Haploinsufficiency

17. 8.2 Chromosome Rearrangements Alter Chromosome Structure
Inversion (depends on the involvement of the centromere in the inversion):
Paracentric inversion
Pericentric inversion

18. 8.2 Chromosome Rearrangements Alter Chromosome Structure
Inversions in meiosis:
Individuals homozygous: no problems arise during meiosis
Individuals heterozygous:
homologous sequences align only if the two chromosomes form an inversion loop
Demonstrate reduced recombination in a paracentric inversion, as gametes formed result in nonviable offspring
Have abnormal gametes formed in a pericentric inversion

19. Figure 8.11 Chromosome 4 differs in humans and chimpanzees in a pericentric inversion.19

20. Figure In an individual heterozygous for a paracentric inversion, the chromosomes form an inversion loop during pairing in prophase I.

21. Figure In a heterozygous individual, a single crossover within a paracentric inversion leads to abnormal gametes.

22. Figure In a heterozygous individual, a single crossover within a pericentric inversion leads to abnormal gametes.

23. Concept Check 1
A dicentric chromosome is produced when crossing over takes place in an individual heterozygous for which type of chromosome rearrangement?
Duplication
Deletion
Paracentric inversion
Pericentric inversion

24. Concept Check 1
A dicentric chromosome is produced when crossing over takes place in an individual heterozygous for which type of chromosome rearrangement?
Duplication
Deletion
Paracentric inversion
Pericentric inversion

25. Translocations Nonreciprocal translocation Reciprocal translocation
Robertsonian translocation

26. Figure 8.15 In a Robertsonian translocation, the short arm of one acrocentric chromosome is exchanged with the long arm of another.

27. Translocations in Meiosis

28. Translocations in Evolution

29. Concept Check 3 What is the outcome of a Robertsonian translocation?
Two acrocentric chromosomes
One metacentric chromosome and one chromosome with two very short arms
One metacentric and one acrocentric chromosome
Two metacentric chromosomes

30. Concept Check 3 What is the outcome of a Robertsonian translocation?
Two acrocentric chromosomes
One metacentric chromosome and one chromosome with two very short arms
One metacentric and one acrocentric chromosome
Two metacentric chromosomes

31. Variations in copy number: aneuploidy and polyploidy
8.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes
Variations in copy number: aneuploidy and polyploidy
Causes of aneuploidy:
Deletion of centromere during mitosis and meiosis
Robertsonian translocation
Nondisjunction during meiosis and mitosis

32. Figure 8.19 Aneuploids can be produced through nondisjunction in meiosis I, meiosis II, and mitosis. The gametes that result from meioses with nondisjunction combine with a gamete (with blue chromosome) that results from normal meiosis to produce the zygotes.

33. Types of Aneuploidy
Nullisomy: loss of both members of a homologous pair of chromosomes. 2n − 2
Monosomy: loss of a single chromosome n − 1
Trisomy: gain of a single chromosome. 2n + 1
Tetrasomy: gain of two homologous chromosomes. 2n + 2

34. Concept Check 3
A diploid organism has 2n = 36 chromosomes. How many chromosomes will be found in a trisomic member of this species?

35. Concept Check 3
A diploid organism has 2n = 36 chromosomes. How many chromosomes will be found in a trisomic member of this species?
2n + 1 = = 37

36. 8.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes
Effects of Aneuploidy:
In plants: Fig
In humans:
Sex-chromosome aneuploids:
Turner syndrome. XO
Klinefelter sydrome. XXY

37. Figure 8.20 Mutant capsules in Jimson weed (Datural stramonium) result from different trisomies. Each type of capsule is a phenotype that is trisomic for a different chromosome.

38. 8.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes
Effects of Aneuploidy:
In plants: mutants could actually be trisomics (Fig. 8.20)
In humans:
Autosomal aneuploids:
Trisomy 21: Down syndrome
Primary Down syndrome, 75% random nondisjunction in egg formation (Fig. 8.21)
Familial Down syndrome, Robertsonian translocation between chromosomes 14 and 21 (Figs and 23)

39. Figure 8.21 Primary Down syndrome is caused by the presence of three copies of chromosome 21. Karyotype of a person who has primary Down syndrome.

40. Figure 8.22 The translocation of chromosome 21 onto another chromosome results in familial Down syndrome. Here, the long arm of chromosome 21 is attached to chromosome 15. This karyotype is from a translocation carrier, who is phenotypically normal but is at increased risk for producing children with Down syndrome.

41. Figure 8.23 Translocation carriers are at increased risk for producing children with Down syndrome.

42. 8.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes
Effects of Aneuploidy:
In humans:
Autosomal aneuploids:
Trisomy 18: Edward syndrome, 1/8000 live births
Trisomy 13: Patau syndrome, 1/15,000 live births
Trisomy 8: 1/25,000 ~ 1/50, 000 live births
Why is there a drastic decrease in frequency of this trisomic syndrome from chromosome 18 to chromosome 8?

43. Concept Check 4
Briefly explain why in humans and mammals, sex-chromosome aneuploids are more common than autosomal aneuploids.

44. Concept Check 4
Briefly explain why in humans and mammals, sex-chromosome aneuploids are more common than autosomal aneuploids.
No mechanism of dosage compensation for autosomal chromatids. Autosomes carry more genes. Most autosomal aneuploids are spontaneously aborted.

45. 8.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes
Effects of Aneuploidy:
In humans:
Autosomal aneuploids:
Aneuploidy and maternal age. Fig. 8.24
Possible interpretation of this connection?
Uniparental disomy: both chromosomes are inherited from the same parent.
Mosaicism and nondisjunction in mitotic division.

46. Figure 8.24 The incidence of primary Down syndrome and other aneuploids increases with maternal age.

47. 8.4 Polyploidy is the Presence of More Than Two Sets of Chromosomes
Autopolyploidy:
From single species
Fig. 8.26
Allopolyploidy
From two species
Fig. 8.28

48. Figure 8.26 Autopolyploidy can arise through nondisjunction in mitosis (a) or meiosis (b).

49. Figure In meiosis of an autotriploid, homologous chromosomes can pair or not pair in three ways.

50. Figure 8.28 Most allopolyploids arise from hybridization between two species followed by chromosome doubling.

51. 8.4 Polyploidy is the Presence of More Than Two Sets of Chromosomes
The significance of polyploidy:
Increase in cell size
Larger plant attributes
Evolution: may give rise to new species

52. Figure Modern bread wheat, Triticum aestivum, is a hexaploid with genes derived from three different species. Two diploid species, T. monococcum (n = 14) and probably T. searsii (n = 14), originally crossed to produce a diploid hybrid (2n = 14) that underwent chromosome doubling to create T. turgidum (4n = 28). A cross between T. trugidum and T. tauschi (2n – 14) produced a triploid hybrid (3n = 21) that then underwent chromosome doubling to produce T. aestivum, which is a hexaploid (6n = 42).

55. Concept Check 5
Species A has 2n = 16 chromosomes and species B has 2n = 14. How many chromosomes would be found in an allotriploid of these two species?
21 or 24
42 or 48
22 or 23 45

56. Concept Check 5
Species A has 2n = 16 chromosomes and species B has 2n = 14. How many chromosomes would be found in an allotriploid of these two species?
21 or 24
42 or 48
22 or 23 45