Chromosomal Polymorphism

Polymorphism: a more than one version ofa trait being actively present in a population, aheritable difference between individuals in thesame species.

For example:

ABO blood groupsImmunoglobulinsMinor variants in chromosome structureDNA sequence polymorphism

Exactly we should call these variantschromosome heteromorphism.

Common positions & forms of chromosome heteromorphism

Size of chromosome

About 10% of clinically normal males have alonger or short Y.

Satellite—presence or absence, sizemainly on 13,14,15,21 and 22

Secondary constriction of chromosome 1, 9 and 16Presence or absence, length

Banding pattern polymorphismFluorescence intensity of chromosome 3, 4

Chromosome heteromorphism

Having no remarkable heritable effect and cannot result in pathologic reaction

According to Mendelian inheritance

Often happening in constitutive heterochromatin regions

Characteristics

ApplicationIndividual IdentificationDetecting the origin of extra chromosomesDetecting the origin of cellsGene mapping

Donahue observed a peculiar microscopically-visible stretch of chromatin on his own largest chromosome (Chromosome #1).

Gene mapping--Duffy blood group

Ⅰ

Ⅱ

Ⅲ

Ⅳ

ab bb

aa

ab ab

bb ab ab ab

ab

ab abab

aa aa

bb

bbbbbb

bb ab

a and bPresence of the heteromorphism on chromosome 1

The Duffy blood group genotypes

Sex Chromosomes

22 pairs of auto

1 pair of sex chro.

XX or XY

Human males are the heterogametic sex with two different sex chromosomes, (XY).

Human females are the homogametic sex (XX).

Chromosome Amount of DNA (Mb)

Chromosome Amount of DNA (Mb)

1 263 13 1142 255 14 1093 214 15 1064 203 16 985 194 17 926 183 18 857 171 19 678 155 20 729 145 21 50

10 144 22 5611 144 X 16412 143 Y 59

DNA content of human chromosomes

X chromosome dosage

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?

Sex Chromosomes: females XX, males XYGenes on X: females 2 , males 1

Dosage Compensation

XX females “compensate” by inactivating one of their X chromosomes to make a single “dosage” of X-linked genes.

decrease X gene products by half in females(e.g. humans called X-inactivation)

X-inactivation reveals alleles in cats heterozygous for the fur color gene

Genotype is Xyellow/Xblack

Yellow patches: black allele is inactive Xyellow/Xblack

Xyellow/Xblack Black patches: yellow allele is inactive

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).

The Lyon Hypothesis of X Inactivation

Proposed by Mary Lyon and Liane Russell (1961)

X inactivation is random.The inactive X can either be maternal orpaternal in origin and the choice is random ineach cell and independent of the choice inother embryonic cells.

X inactivation is irreversible in somaticcells - the inactive X in a particular cellremains inactive in all descendents ofthat 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

0 1

2 3

Normal male,Turner female

Normal female,Klinefelter male

# Barr bodies=N-1 rule

Inactive X chromosome is visible as Barr body

XXX female XXXX female

imprinted inactivation Reversed (early blastocyst)

Extraembryonic tissue

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?

Inconsistencies between syndromes andX inactivation

•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;

Exceptions to Lyon hypothesis

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.

In a survey of 224 human X-linkedgenes, 34 (15%)escaped X inactivation.

The genes escapingX-inactivationoccur primarilyon Xp.

From: Carrel et al. Proc. Natl. Acad. Sci. 96(1999)14440-14444.

Many Genes Escape X-Inactivation

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

G6PDG6PD

Sex Chromatin

X-chromatin Y-chromatin

Origin Long arm of YInactivated X

Feature Facultative Heterochromatin

ConstitutiveHeterochromatin

Number =No. of Y=No. of X -1

Sex Determination

What determines gender?

Number of X chromosomes, or presence of Y chromosome?

Sex ChromosomesXX (female)

XY (male)

What determines gender?

Individuals with unusual chromosome combinations provide a clue:

XO femaleXXY male

=> Y chromosome determines gender

mostly inert, very few genes, mostly repeat sequence DNA (high and middle).

Y Chromosome

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 chromosomeThere 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

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.

Mapping the gene responsible for maleness

Sex-determining region Y

Sex determining region YGene 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

An embryo develops as a male or female using information from the Y chromosome.

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.

Effect of YChromosome

10 weeks

Y present

Y absent

7 weeks

birth approaching

appearance of structuresthat will give rise to

external genitalia

appearance of “uncommitted” duct system

of embryo at 7 weeks

Y present

Yabsent

testis

ovary

testes ovaries

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 ?

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

Other sex determination genes

The End