3/6/16
BIOLOGY 207 - Dr. Locke
Lecture#19 - Changes in chromosome number
Required readings and problems:
Reading: Open Genetics, Chapter 9
Problems: Chapter 9
Optional
Griffiths (2008) 9th Ed. Readings: pp 555-572
Problems: 9th Ed. Ch. 16: 32-33, 35-37, 50, 52
Campbell (2008) 8th Ed. Readings: Concept 15.4
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 change (gain or loss) of a whole single chromosome (of the
set) and usually produces physical defects (despite all genes being normal).
3. Polyploidy involves changes in whole chromosome sets and is much less
destabilizing (eg. in Plants).
Biol207 Dr. Locke section
Lecture#19
Fall'11
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3/6/16
Aneuploidy - Changes in chromosome number
Normal Meiosis:
 No errors during meiosis
 2N individuals -> gametes with one chromosome set (1N)
Examples:
Humans = set of chromosomes = set found in gametes (1N)
 - sperm 22 Autosomes + X (or Y) sex chromosome
 - egg 22 Autosomes + X sex chromosome -> total 46 chromosomes
Drosophila = 4 chromosomes in gametes (3 autosomes + 1 sex chromosome)
Abnormal meiosis
Errors occur -> abnormal gametes (extra/missing chromosomes)
Errors in the segregation (disjunction) of the chromosomes
-> result is the non-disjunction of the chromosomes.
They do not segregate properly at either anaphase I, or II.
Biol207 Dr. Locke section
Lecture#19
Fall'11
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3/6/16
Non-disjunction
Non-disjunction of one chromosome pair in
meiosis results in a gametes having either gain
or loss of a chromosome:
Example: A human gamete has
23+1 chromosomes (1N+1) = 24 or
23-1 chromosomes (1N-1) = 22
Such gametes are called unbalanced.
It can occur in males or females.
Fertilization
Abnormal gamete + normal gamete (of the
opposite type)
= a zygote with a gained or lost chromosome - referred to as aneuploid
2N+1 or 2N-1 individual
an-(not-) euploid
Extra chromosome = trisomy Missing a chromosome = monosomy
Changes in single, whole chromosomes show deleterious effects.
Zygotes derived from unbalanced gametes usually show phenotypic abnormalities.
Most show an abnormal or incomplete/ improper development and die as embryos.
They are mutants and will show mutant phenotypes.
Biol207 Dr. Locke section
Lecture#19
Fall'11
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3/6/16
Example: human aneuploidy
Aneuploidy (loss or gain) of
most human autosomes results in death (before or soon after birth)
Classic Exception: Down syndrome
 - where 2N+1, +1 is chromosome 21 (trisomy 21)
- the smallest human chromosome.
Aneuploidy of the sex chromosomes appears more viable but they are still
affected.
Turner's Syndrome - 45, X/O (monosomy of X-chromosome)
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1/2500 to 1/10,000 (98% are aborted - 20% of spont. abortions are Turners)
sterile - 77% have maternal X
short stature, ovaries don't develop, webbed neck
normal IQ
Klinefelter's Syndrome - X/X/Y Trisomy
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1/400 - increases with maternal age
variable male genital development (hypogonadism)
develop breasts after puberty
most/some show mild retardation
Only extra Y-chromosome (XYY)
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1/1000
individuals show marginal differences from normality.
not "criminal behavior"
tall?, but normal, fertile
Triple-X (XXX)
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1/1000
- can pass for normal, need karyotype to tell
- most are fertile and produce normal children
- more than 3 X is rare and abort or are more severe.
Biol207 Dr. Locke section
Lecture#19
Fall'11
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3/6/16
Polyploidy:
Changes in whole chromosome sets.
A single set is 1N. Many organisms are diploid (2N).
Others are 3N, 4N, 5N, 6N, etc.
- triploid, tetraploid, pentaploid, hexaploid.
All have multiples of the monoploid number (x)
The number (x) is similar to the haploid number N or usually x=N in diploid organisms.
But N refers to the number of chromosomes in a normal
gamete.
The N=x situation may not be accurate in polyploids
Example: modern wheat:
N and x are different
Wheat has 42 chromosomes but it is also hexaploid
It has 6 sets of 7 chromosome sets
6x=42 x=7
But the gametes of wheat contain 21 chromosomes
(3 X 7 chromosome sets)
Therefore N=21 2N=42
x is not N
Biol207 Dr. Locke section
Lecture#19
Fall'11
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Monoploids
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They have only one chromosome set (effectively haploid).
In most species monoploids are abnormal and die.
In some species of insects, such as bees, wasps, ants, males are monoploid.
They develop from unfertilized eggs, with only one chromosome set (of maternal
origin).
One chromosome set has consequences for meiosis.
 chromosomes can't pair, thus no meiosis.
 but they still produce gametes - How?
 Males bees, wasps, ants form gametes via mitosis
(all cells are haploid to begin with).
females are diploid and have a typical meiosis.
Haploid-Diploid Sex-determination system
Diploids
 Most of the organisms used in genetic studies are diploid. One thinks of it as the
typical state.
Biol207 Dr. Locke section
Lecture#19
Fall'11
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Triploids: The first of the polyploids.
 There are two types of polyploids:
Autopolyploids are composed of multiple sets of chromosomes from within one
"species"
Allopolyploids are composed of chromosome sets from different "species"
Although the sets are from different "species" they are usually from closely related
species.
e.g. wheat
Triploids are usually autopolyploids.
 They are usually derived from crosses between:

 tetraploid (4X)
cross to a diploid (2X) P1
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2X
1X
 Gametes - fuse to make 3X individual (triploid).
Triploids are usually sterile.
 This is expressed as seedless fruit varieties.
 Commercially exploited as seedless fruit.
Example: banana, watermelon.
http://www.flickr.com/photos/stevendepolo/4673589988/sizes/l/in/photostream/
Biol207 Dr. Locke section
Lecture#19
Fall'11
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3/6/16
Why seedless banana?
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Banana has 11 chromosomes x 3 sets
At meiosis I each chromosome has 2 homologues that try to "pair up".
At anaphase I two chromosomes go to one pole and one to the other.
Each of the chromosome triplets segregate 2:1 independently of each other.
The probability that a gamete will receive only one copy
of each chromosome is 1/211 or 1/2084
 Only gametes with one of each chromosome make good
seeds.
 The result is few if any seeds are made
--> seedless variety
The use of triploids is genetic engineering
--> seedless fruit
http://www.soil-net.com/album/Plants/Fruit_Veg/slides/FruitVeg%20Banana%2002.html
Banana:
See: http://waynesword.palomar.edu/ecoph7.htm#banana
Triploid Salamander:
See: http://en.wikipedia.org/wiki/Jefferson_Salamander
Triploid Salmon:
See:
http://icesjms.oxfordjournals.org/cgi/content/abstract/58/2/525
Biol207 Dr. Locke section
Lecture#19
Fall'11
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