1. PowerPoint Presentation Materials to accompany
Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 8
VARIATION IN CHROMOSOME STRUCTURE AND NUMBER
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2. INTRODUCTION
Genetic variation refers to differences between members of the same species or those of different species
Allelic variations are due to mutations in particular genes
Chromosomal aberrations are substantial changes in chromosome structure
These typically affect more than one gene
They are also called chromosomal mutations
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3. INTRODUCTION A change in chromosome number is called a genome mutation
It is the result of changes in the number of
Sets of chromosomes OR
Numbers of individual chromosomes in a set
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4. 8.1 VARIATION IN CHROMOSOME STRUCTURE
The study of chromosomal variation is important for several reasons
1. They can have major effects on the phenotype of an organism
2. They can have major effects on the phenotype of the offspring of an organism
3. They have been an important force in the evolution of species
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5. Cytogenetics
The field of genetics that involves the microscopic examination of chromosomes
A cytogeneticist typically examines the chromosomal composition of a particular cell or organism
This allows the detection of individuals with abnormal chromosome number or structure
This also provides a way to distinguish between species
Refer to Figure 8.1a
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6. Cytogenetics
Cytogeneticists use three main features to identify and classify chromosomes
1. Size
2. Location of the centromere
3. Banding patterns
These features are all seen in a Karyotype
Figure 8.1c
The procedure for making a karyotype was discussed in Chapter 3 (See Figure 3.2)
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7. 8-7 Figure 8.1 Short arm; For the French, petite Long arm
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8. Cytogenetics
For detailed identification, chromosomes are treated with stains to produce characteristic banding patterns
Example: G-banding
Chromosomes are exposed to the dye Giemsa
Some regions bind the dye heavily
Dark bands
Some regions do not bind the stain well
Light bands
In humans
300 G bands are seen in metaphase
2,000 G bands in prophase
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9. 8-9 Figure 8.1 Banding pattern during metaphase
Banding pattern during prophase
Figure 8.1
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10. Cytogenetics The banding pattern is useful in several ways:
1. It distinguishes Individual chromosomes from each other
2. It detects changes in chromosome structure
3. It reveals evolutionary relationships among the chromosomes of closely-related species
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11. Mutations Can Alter Chromosome Structure
There are two primary ways in which the structure of chromosomes can be altered
1. The total amount of genetic information in the chromosome can change
Deficiencies/Deletions
Duplications
2. The genetic material remains the same, but is rearranged
Inversions
Translocations
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12. Deficiency (or deletion) Duplication
The loss of a chromosomal segment
Duplication
The repetition of a chromosomal segment compared to the normal parent chromosome
Inversion
A change in the direction of the genetic material along a single chromosome
Translocation
A segment of one chromosome becomes attached to a different chromosome
Simple translocations
One way transfer
Reciprocal translocations
Two way transfer
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13. Human chromosome 1
Human chromosome 21
Figure 8.2
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14. Deficiencies
A chromosomal deficiency occurs when a chromosome breaks and a fragment is lost
Figure 8.3
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15. Deficiencies
The phenotypic consequences of deficiencies depends on the
1. Size of the deletion
2. Chromosomal material deleted
Are the lost genes vital to the organism?
When deletions have a phenotypic effect, they are usually detrimental
For example, the disease cri-du-chat syndrome in humans
Caused by a deletion in the short arm of chromosome 5
Refer to Figure 8.4
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16. Chromosomal deletions can be detected by a variety of experimental techniques
Cytological (ie. Microscopic)
Used to detect large deletions
Molecular
Genetic
If a mutant population cannot revert back to the wild-type phenotype
This is a good indication that the mutation is due to a deletion
Deletions can also be revealed by a phenomenon known as pseudodominance
One copy of a gene is deleted
So the recessive allele on the other chromosome is now expressed
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17. Duplications
A chromosomal duplication is usually caused by abnormal events during recombination
Figure 8.5
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18. Duplications
Like deletions, the phenotypic consequences of duplications tend to be correlated to size
Duplications are more likely to have phenotypic effects if they involve a large piece of the chromosome
However, duplications tend to have less harmful effects than deletions of comparable size
In humans, relatively few well-defined syndromes are caused by small chromosomal duplications
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