Transcript Brooker Chapter 8

PowerPoint Presentation Materials
to accompany
Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 8
VARIATION IN
CHROMOSOME STRUCTURE
AND NUMBER
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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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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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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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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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Cytogenetics

Cytogeneticists use three main features to identify
and classify chromosomes



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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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Short arm;
For the French, petite
Long arm
Figure 8.1
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Cytogenetics

For detailed identification, chromosomes are
treated with stains to produce characteristic
banding patterns

Example: G-banding
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
Chromosomes are exposed to the dye Giemsa
Some regions bind the dye heavily


Some regions do not bind the stain well


Dark bands
Light bands
In humans


300 G bands are seen in metaphase
2,000 G bands in prophase
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Banding
pattern
during
metaphase
Figure 8.1
Banding
pattern
during
prophase
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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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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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
Deficiency (or deletion)


Duplication
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
The repetition of a chromosomal segment compared to
the normal parent chromosome
Inversion
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The loss of a chromosomal segment
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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Human
chromosome 1
Human
chromosome 21
Figure 8.2
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Deficiencies

A chromosomal deficiency occurs when a
chromosome breaks and a fragment is lost
Figure 8.3
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Deficiencies

The phenotypic consequences of deficiencies
depends on the


1. Size of the deletion
2. Chromosomal material deleted

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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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
Chromosomal deletions can be detected by a variety
of experimental techniques

Cytological (ie. Microscopic)



Molecular
Genetic
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

Used to detect large deletions
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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Duplications

A chromosomal duplication is usually caused by
abnormal events during recombination
Figure 8.5
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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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