MATTERS OF SEX
 Anueploidy

having too many or too few chromosomes
compared to a normal genotype
 Monosomy

Possessing only 1 copy of any particular
chromosomes
 Monosomy,
with the exception of the X
chromosome is incompatable with life
X chromosome dosage
Sex Chromosomes:
Genes on X:

females XX, males XY
females 2 , males 1copy
How to create equal amount of X
chromosome gene products in males and
females?
decrease X gene products by half in
females
(e.g. humans called X-inactivation)
X CHROMOSOMES

This was shown by culturing cells with
different karyotypes
 XY
XO both with 1 X chromosome
 XX
XXY both with 2 X chromosomes
 XXX
XXXX
both with 3 or more X
chromosomes

Looked at levels of enzymes encoded by the
X chromosome
X LINKED GENE
 G6PD,
glucose 6 phosphate
dehydrogenase, gene is carried on the
X chromosome
 This gene codes for an enzyme that
breaks down sugar
 Females produce the same amount of
G6PD enzyme as males
 XXY and XXX individuals produce the
same about of G6PD as anyone else
G6PD GENE
 There
are variant alleles of the G6PD
gene

G6PD A
 Produce
G6PD B
different variants of the
enzyme, but both break down sugar
 Female could be heterozygous for
G6PD A and G6PD B
 Each cell only produces 1 of the 2
forms of the enzyme
X CHROMOSOME
 Only
1 X chromosome is active in any
given cell. The other is inactive
 In some cells the paternal allele is
expressed
 In other cells the maternal allele is
expressed
 In XXX and XXXX females and XXY
males only 1 X is activated in any given
cell the rest are inactivated
X CHROMOSOME
 XXX
embryo survives because it
inactivates 2 X chromosomes and has
only 1 functioning X chromosome in
any given cell
 Trisomy 21 can not inactivate the extra
copy of chromosome 21. So you have
Down syndrome
 The only chromosome we can
inactivate is the X chromosome
Table 12.3
3 TYPES OF CHROMATIN

Euchromatin


Chromosomal regions that possess active genes
Heterochromatin




true chromatin
These regions stain darker than euchromatin
Highly repetitive DNA with very few active genes
Usually found around the centromere and near
the tips of chromosomes (telomeres)
Facultative Heterochromatin

Active like euchromatin in some cells and inactive
like heterochromatin in other cells
Facultative Heterochromatin

Serves as a mechanism for a cell to shut off
a portion or an entire chromosome to
prevent gene expression
 X chromosome is made up of facultative
heterochromatin
 Active X chromosome behaves like
euchromatin, with active genes that are
transcribed
 Inactive X chromosome behaves like
heterochromatin
Facultative Heterochromatin
 The
cell inactivates one X chromosome
by converting the entire chromosome
to heterochromatin or inactive DNA
 This inactive DNA makes up the dark
staining Barr body
 This process occurs in females or any
individual with more than one X
chromosome
LYON HYPOTHESIS
 1961
English geneticist Mary Lyon
proposed this hypothesis to describe X
inactivation
 Consists of 5 tenants


1. Condensed X chromosome is
genetically inactive
2. X inactivation in humans occurs early in
development when embryo consists of
about 32 cells. 1 or 2 days following
fertilization
5 TENANTS OF LYON HYPOTHESIS
 3.
At this stage in each of the 32 cells
one of the X chromosomes is randomly
inactivated
 4. Inactivation is mitotically stable
 5. Net effect of this is to equalize
phenotypes in males and females for
genes that are carried on the X
chromosome
Human Chromosomes
 One
X chromosome in females is
inactivated early in embryonic
development.
PAR REGION NOT INACTIVATED
 RSP4
gene in this region present on
both X and Y chromosomes
 Encodes a protein that makes up part
of the ribosome
 If this gene were inactivated it would
reduce by half the number of
ribosomes made and reduce the
protein synthesized capacity of that cell
by half
X REACTIVATION IN FEMALES

In the female fetus future germ cells undergo
Lyonization along with somatic cells at the 32
cell stage

Following differentiation of female fetus, the
inactivated X chromosomes are reactivated
during female gametogenesis

When germ cells develop into oocytes and
enter meiosis their inactivated X
chromosomes become reactivated so that
every egg produced has an activated X
chromosome prior to fertilization
X REACTIVATION IN MALES

XXY Klinefelter males also reactivate the
second X chromosome during
gametogenisis

The presence of an extra X chromosome
during early puberty causes death of male
germ cells and testicular atrophy

This leads to low levels of testosterone
NONDISJUNCTION OF SEX
CHROMOSOMES
• Extra copies of the X and Y chromosomes
do not cause the severe problems that
extra autosomes do
• Nondisjunction in mother would produce
eggs that are XX or O with no X
chromosome
• If XX egg fertilized with X sperm get an
individual who is XXX.
• Individual will be sterile and have 2 Barr
bodies
NONDISJUNCTION OF SEX
CHROMOSOMES
• If XX egg fertilized with Y sperm get an individual
who is XXY.
• Individual will be sterile male with many female
body characteristics.
• Known as Klinefelter syndrome
• Occurs 1/500 male births
• If O egg fertilized by Y sperm the zygote is nonviable
• If O egg fertilized by X sperm get and individual
who is XO. Turner syndrome
NONDISJUNCTION OF SEX
CHROMOSOMES
•
•
•
•
•
•
•
•
Turner syndrome
Occurs 1/2000 live births
Sterile
Can also have non-disjunction of the Y
chromosome in males
Produce sperm with 2 Y chromosomes
Fertilize an X egg develop into XYY male
Fertile males
Occurs 1/1000 males
MOSAICISM
 Is
due to a mitotic loss of 1 X
chromosome in a cell early in zygotic
development
 This produces a combination of both
XX and XO cells
 The more XO cells an individual has the
more severely she will be affected
 Some estimates put mosaic Turner
females as high as 60% to 80%
TURNER SYNDROME
 Newborns
may not be affected
 Lag behind classmates in sexual
development
 Reach puberty they fail to menstruate
 Small uterus
 Rudimentary ovaries

Remains a primitive streak gonad
 Sterile

W/O an ovary they can not produce eggs
or estrogen
TURNER SYNDROME

Can lead fairly normal lives if they receive
hormone supplements

Hormones promote breast development and
other secondary sex characteristics if
administered during puberty

Growth hormone adds up to 3 inches of
height. Can mean the difference between an
adult height of 4’11” to 5’2”