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Spring 2009: Section 3 – lecture 5
Reading – Chapters 8
Chromosomes with specific functions
Example: sex chromosomes
There are two types of chromosomes in
a cell when considering sex
determination:
sex chromosomes - carry genes that
determine the sex of an individual.
autosomes - chromosomes that are not
involved in sex determination.
Example – humans
2N – 46
autosomes – 44
sex chromosomes - 2
In humans have X and Y chromosomes
XX = female = homogametic sex
XY = male = heterogametic sex
Because the female is the
homogametic sex, all of the female’s
gametes will be X while the male’s
gametes are 50% X and 50% Y. So the
male is the one who determines the sex
of the progeny.
In some species the male is
homogametic and the female is
heterogametic (turkeys, chickens)
Could this help explain how a female
turkey could produce male progeny
even when there are no male turkeys
around to provide male gametes?
What happens to the extra X
chromosome in females?
Good example of facultative
heterochromatin
In most cells one X is extremely
condensed. This can be seen at
interphase and is called a Barr body
In humans:
Females will have one Barr body
Males will not have a Barr body
In cats it is possible to see that
condensation of the X in females is
random. The way to do this is to have
variation in a gene on the X
chromosome.
Example - Coat color in tortoiseshell or
calico cats.
1 X carries a gene for red coat color
1 X carries a gene for non-red color
The color of a section of fur will depend
on which X chromosome condensed
during development. It also indicates
that once an X is designated as a Barr
body it remains a Barr body in that cell
line.
Wright and Walters, 1980
The result of random condensing of the
X chromosomes is a female cat with a
patchwork (red and non-red) coat.
Why would you be surprised to see a
male tortoiseshell or calico cat?
Cloning a calico cat
Would you expect the clone of a calico
cat to have the same appearance as the
nuclear donor?
‘mom’, nuclear
donor
clone
Chromosome modifications
Euploidy - Changes in the number of
sets of chromosomes.
Aneuploidy - changes in the number of
chromosomes in a set.
Changes within a chromosome
Changes within a chromosome
a) deletion
b) duplication
c) inversion
d) translocation
Euploidy - change in the number of
sets of chromosomes.
Most species have two sets, one
paternal and the other maternal. This is
the source of homologous pairs of
chromosomes
If a species has more than two sets of
chromosomes the organism is said to
be a polyploid.
2 sets
3 sets
4 sets
6 sets
diploid
triploid
tetraploid
hexaploid
To describe the number of
chromosome sets in a species you use
X to equal or represent one
chromosome set.
2 sets
3 sets
4 sets
6 sets
diploid
triploid
tetraploid
hexaploid
= 2X
= 3X
= 4X
= 6X
So for each species you will have a
somatic chromosome number, a
gametic chromosome number and a
monoploid chromosome number
that indicates the number of
chromosomes in one set.
Examples:
Humans - diploid
2N = 46 chromosomes
N = 23 chromosomes
2X = 46 chromosomes
X = 23 chromosomes - number of
chromosomes in one set
Wheat – hexaploid
2N = 42 chromosomes
N = 21 chromosomes
6X = 42 chromosomes
X = 7 chromosomes
Potatoes: tetraploid with 48 chromosomes
Rapeseed: tetraploid with 38 chromosomes
If all the sets are homologous, i.e.
originating from the same species, then
the species is called an autopolyploid.
Examples of autopolyploids - potatoes,
strawberries, bananas, aspen
Advantages of an autopolyploid:
- plant parts and fruits are larger
- plant may be more vigorous
Disadvantages of an autopolyploid:
- problems in meiosis with
chromosome pairing and even
distribution of chromosomes.
- low or no seed set
• Why is poor or no seed set not a
problem with potatoes, strawberries
and bananas?
Another way to have no seed set is to
have an uneven number of
chromosome sets in a polyploid.
Example: banana, an autotriploid
Possible origin:
1. Multiple fertilization (not likely)
2. Fertilization of an unreduced (2N)
gamete
3. Hybridization between a tetraploid
and a diploid.
4X
x

3X
2X
Problems in gamete formation and
seed set are due to uneven
chromosome pairing and division in the
reduction division of meiosis.
(i.e. how do you divided a trivalent
equally?)
Advantages of autotriploids
plants - larger fruit
no seeds
more vigorous growth
oysters - no egg or sperm
production
fish – more rapid growth
Disadvantages of autotriploids?