BIOLOGY 207 - Dr. McDermid
Lecture#25
Changes in chromosome number
Readings: Griffiths et al 7th Edition: Ch. 18 pp 555-570
Problems: Ch. 18, Tier 1:# 3, 4, 5, 6, 12, 15, 18 (parts 1-6)
Tier 2: #2, 7, 9, 10a, 14, 19 (a-c), 20
Concepts:
How do changes in chromosome number affect organisms?
1. Some mutations are the result of a change in chromosome number.
2. Aneuploidy involves a gain or loss of a single chromosome (of the set) and usually
produces physical defects.
3. Polyploidy involves changes in whole chromosome sets and is much less destabilizing in
plants and some animals.
Aneuploidy
Changes in chromosome number
Normally:
No errors during meiosis
Chromosome set = 1 N
2N individuals
Example:
humans
Meiosis acts to produce gametes with
appropriate type and number of
chromosomes.
Abnormal meiosis
Errors during meiosis can result in abnormal
gametes that have extra or missing
chromosomes.
Disjunction:
Nondisjunction –
Figure 18-16
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In a human gamete:
Such gametes with lost or extra chromosomes are often referred to as unbalanced.
Fertilization
Aneuploid
2N =
Extra chromosome =
Absent chromosome =
Changes in single chromosomes show deleterious effects.
Example: human aneuploidy
eg) Down syndrome
Aneuploidy of sex chromosomes are viable
Turner's Syndrome Klinefelter's Syndrome XXX and XYY
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Polyploidy: Changes in whole chromosome sets.
In a diploid, a single chromosome set is 1N.
Diploid (2N) = 2 sets of chromososmes.
Polyploids (>2N) have multiples of the monoploid number (x)
N usually = x, but not always
Example: modern wheat: Fig. 18-12
N and x are different
Monoploids
They have only one chromosome set
In most species monoploids are abnormal and die.
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Diploids
Polyploids
There are two types of polyploids:
Autopolyploids
Allopolyploids
Triploids are usually autopolyploids.
They are usually derived from crosses between:
tetraploid (4X) X diploid (2X)
P1
!!!!!!!!!2X !!!!!!!!!!!!!!!!!!!!!!!!!1X !!!!!!!!!!!! Gametes
Triploids are usually sterile.
Why seedless?
Banana has 11 chromosomes x 3 sets
At meiosis I each chromosome has 2 homologues that try to "pair up".
Autotetraploids
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Lecture notes: Copyright © 2002 Heather McDermid and the Department of Biological Sciences, University of Alberta
Images are Copyright©2000 by W.H. Freeman & Co. in Griffiths et al, Introduction to Genetic Analysi
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