Download Chromosomal Polymorphism

Chromosomal Polymorphism
Polymorphism: a more than one version of
a trait being actively present in a population, a
heritable difference between individuals in the
same species.
For example:
ABO blood groups
Immunoglobulins
Minor variants in chromosome structure
DNA sequence polymorphism
Exactly we should call these
chromosome heteromorphism.
variants
Common positions & forms of chromosome heteromorphism
 Size of chromosome
About 10% of clinically normal males have a
longer or short Y.
 Satellite
—presence or absence, size
mainly on 13,14,15,21 and 22
Secondary constriction of chromosome 1, 9 and 16
Presence or absence, length
Banding pattern polymorphism
Fluorescence intensity of chromosome 3, 4
Chromosome heteromorphism
Characteristics
Having no remarkable heritable effect and cannot
result in pathologic reaction
According to Mendelian inheritance
Often happening in constitutive heterochromatin
regions
Application
Individual Identification
Detecting the origin of extra chromosomes
Detecting the origin of cells
Gene mapping
Gene mapping--Duffy blood group

Donahue observed a peculiar microscopicallyvisible stretch of chromatin on his own largest
chromosome (Chromosome #1).
Ⅰ
Ⅱ
ab
bb
bb
ab
ab
bb
ab
ab
bb
Ⅲ
ab
ab
bb
bb
aa
ab aa
bb
Ⅳ
ab
aa
Presence of the
heteromorphism on
chromosome 1
a and b
ab
ab
The Duffy blood group genotypes
Sex Chromosomes
Human
males are the
heterogametic sex with
two different sex
chromosomes, (XY).
22 pairs of auto
1 pair of sex chro.
XX or XY
Human
females are
the homogametic sex
(XX).
DNA content of human chromosomes
Chromosome
Amount of DNA
(Mb)
Chromosome
Amount of DNA
(Mb)
1
2
3
4
5
6
7
8
9
10
11
12
263
255
214
203
194
183
171
155
145
144
144
143
13
14
15
16
17
18
19
20
21
22
X
Y
114
109
106
98
92
85
67
72
50
56
164
59
X chromosome dosage
Sex Chromosomes:
Genes on X:
females XX, males XY
females 2 , males 1

How to create equal amount of X chromosome
gene products in males and females?

Shouldn’t XX females produce twice the amount of
X-linked gene products (proteins) as XY males?
Dosage Compensation
decrease X gene products by half in females
(e.g. humans called X-inactivation)

XX females “compensate” by inactivating
one of their X chromosomes to make a
single “dosage” of X-linked genes.
X-inactivation reveals alleles in cats
heterozygous for the fur color gene
Genotype is Xyellow/Xblack
Yellow patches: black allele is inactive Xyellow/Xblack
Black patches: yellow allele is inactive Xyellow/Xblack
The Lyon Hypothesis of X Inactivation


In normal females, only one of the two X
chromosomes is genetically active
X chromosome inactivation occurs early
in development (late blastocyst stage of
embryogenesis, 32-64 cell stage).
Proposed by Mary Lyon and Liane Russell (1961)

X inactivation is random.
The inactive X can either be maternal or
paternal in origin and the choice is random in
each cell and independent of the choice in
other embryonic cells.

X inactivation is irreversible in somatic
cells - the inactive X in a particular cell
remains inactive in all descendents of
that cell.
X-inactivation
Barr Bodies


Murray Barr (1949)
chromatin (inactive X) appears as a dense
object in the nucleus
XX with one Barr body
 XY chromatin negative (no Barr body)

Barr Bodies are Inactivated X
Chromosomes in Females
Normal male,
Turner female
Normal female,
Klinefelter male
0
1
2
3
# Barr bodies=
N-1 rule
Inactive X chromosome is visible as Barr body
XXX female
XXXX female
imprinted inactivation
Reversed (early blastocyst)
Extraembryonic
tissue
Inconsistencies between syndromes and
X inactivation
If normal XX female has one X inactivated,
why is a X Turner female not normal?
Similarly, if XXY male has one X inactivated,
why does he have Klinefelter syndrome?
Exceptions to Lyon hypothesis
•although X inactivation is usually random, a
structurally abnormal X, e.g., an X chromosome
bearing a deletion, is preferentially inactivated;
•in individuals with X-autosome translocations, it is
usually the normal X chromosome that is
preferentially inactivated;


although X inactivation is extensive, it is not
complete, some genes are known to escape
inactivation;
while x inactivation is permanent in most
somatic cells, it must be reversible in the
development of germ cells.
Some genes on X are not
inactivated.
Genes in pseudo-autosomal regions
PAR1 and PAR2.
XIST(X-inactive specific transcript ),
active only on the Inactive X.
Many Genes Escape X-Inactivation
In a survey of 224
human X-linked
genes, 34 (15%)
escaped X
inactivation.
The genes escaping
X-inactivation
occur primarily
on Xp.
From: Carrel et al. Proc. Natl. Acad. Sci. 96(1999)14440-14444.
Nonrandom X Chromosome
Inactivation


The cells with deleted X:
The cells carry X-autosomal translocation
Nonrandom X Chromosome
Inactivation
Normal X
Deleted X
Nonrandom X Chromosome
Inactivation
Normal X
from X
from auto
Possible


Mechanism
autosomally-encoded 'blocking factor' which
binds to the X chromosome and prevents its
inactivation
Sequences at the X inactivation center (XIC),
binding to the “blocking factor”, control the
silencing of the X chromosome
Xist & Tsix
Application

Research on cancer
Patient
Tumor cells
Normal cells
cell culcure
G6PD
G6PD
Sex Chromatin
X-chromatin
Origin
Inactivated X
Feature
Facultative
Heterochromatin
Number
=No. of X -1
Y-chromatin
Long arm of Y
Constitutive
Heterochromatin
=No. of Y
Sex Determination
What determines gender?
XX (female)
Sex Chromosomes
XY (male)


Number of X chromosomes, or
presence of Y chromosome?
What determines gender?
Individuals with unusual chromosome
combinations provide a clue:
XO
XXY
female
male
=> Y chromosome determines gender
Y Chromosome
mostly inert, very few
genes, mostly repeat
sequence DNA (high
and middle).
PAR: pseudo-autosomal regions on
tips of X and Y: homologous,
contain some genes.
PAR1 has a required cross over for
successful sperm development.
Genes on the Y chromosome
There
are three classes of genes
on the Y.
Genes
shared with X
chromosome define the
pseudoautosomal regions (PAR)
Genes
similar to X chromosome
genes are X-Y homologs
Genes
unique to the Y including
SRY gene
Mapping the gene
responsible for maleness


Experimental paradigm: sex-reversed
individuals
 XY females (missing critical bit of Y)
 XX males (possessing critical bit of Y)
Deletion mapping of Y coupled with analysis
of sex-reversed individuals and “chromosome
walking” to get new sequences
Mapping the gene
responsible for maleness
In 1990, Sinclair and colleagues
narrowed the region to a 35,000 base-pair
domain of the small arm of the Y chromosome.
Sex-determining region Y
Sex determining region Y
Gene symbol : SRY Location : Yp11.3
SRY encodes a 223 amino acid zinc
finger transcription factor that is a
member of the high mobility group (HMG)box family of DNA binding proteins. The
protein is expressed during testis
development for only 2 days.


Sequencing revealed a conserved motif
that could have DNA-binding function
SRY = TDF
XY sex-reversed females have deletions or
mutations of SRY
 transgenic mouse model - XX + Sry leads to
testis development
 SRY expressed in gonad, but only transiently,
at the onset of differentiation


SRY gene on the Y chromosome was
identified as the gene that codes for TDF:
 SRY is translocated to X in rare XX males
 SRY is absent from Y in rare XY females

The “home run” experiment by Koopman et al.
used transgenic mice.
Genotypically Female Mice Transgenic
for SRY are Phenotypically Male
XY male
XX male
Anne McLaren.
What makes a man a man?
Nature. 1990,19;346(6281):216-7.
Sexual development
At the beginning of human
development either male or
female development is
possible.
Unspecialized gonads and
two sets of reproductive
ducts exist until week 6.
An embryo develops as a
male or female using
information from the Y
chromosome.
Effect of Y
Chromosome
appearance of structures
that will give rise to
external genitalia
appearance of
“uncommitted” duct system
of embryo at 7 weeks
7 weeks
Y
present
Y
absent
Y
present
Y
absent
testes
ovaries
10 weeks
ovary
birth approaching
testis
Other sex determination genes


Puzzling XY sex-reversed females without
detectable mutation in SRY
SRY gene acts as a repressor or inhibitor of
another gene, “Z”, that is involved in female
development ?
Other sex determination genes

DAX1 - on X, exerting its effects early on in
development, can suppress testis-formation in
a dosage-sensitive manner

SOX9 - on 17q, required with SRY for normal
testis formation. Mutations in this gene cause
campomelic dysplasia, which occurs in many
sex reversed males
The End