Sex chromatin bodies

Barr body

OrganismOrganism No. chromosomesNo. chromosomes

HumanHuman 4646 ChimpanzeeChimpanzee 4848 DogDog 7878 HorseHorse 6464 ChickenChicken 7878 GoldfishGoldfish 9494 Fruit flyFruit fly 88 MosquitoMosquito 66 NematodeNematode 11(m), 12(f)11(m), 12(f) HorsetailHorsetail 216216 SequoiaSequoia 2222 Round worm Round worm 22

What Exactly is a What Exactly is a chromosomechromosome??

Chromosomes are the Chromosomes are the rod-shapedrod-shaped, , filamentous bodies filamentous bodies present in the present in the nucleusnucleus, which become visible , which become visible during during cell divisioncell division . .

They are the They are the carriers of the gene carriers of the gene or unit of heredityor unit of heredity..

Chromosome are Chromosome are not visible not visible in active nucleus due to in active nucleus due to their their high water contenthigh water content, but are clearly seen during , but are clearly seen during cell divisioncell division . .

Their number can be counted easily only Their number can be counted easily only

during during mitotic metaphase.mitotic metaphase.

The term “Chromosome”, however was first The term “Chromosome”, however was first

used by used by Waldeyer Waldeyer in in 18881888. .

They were given the name chromosome They were given the name chromosome

((ChromoChromo = colour; = colour; Soma Soma = body) due to their = body) due to their

marked marked affinity for basic dyesaffinity for basic dyes. .

This X-Y system of This X-Y system of mammals is not the mammals is not the only chromosomal only chromosomal mechanism of mechanism of determining sex.determining sex.

Other options include Other options include the X-0 system, the Z-the X-0 system, the Z-W system, and the W system, and the haplo-diploid system.haplo-diploid system.

Barr BodiesBarr Bodies 1940’s two Canadian scientists 1940’s two Canadian scientists

noticed a noticed a dark staining massdark staining mass in in the nuclei of cat brain cellsthe nuclei of cat brain cells

Found these dark staining spots Found these dark staining spots in female but not males.in female but not males.

This held for cats and humansThis held for cats and humans They thought the spot was They thought the spot was a a

tightly condensed X tightly condensed X chromosome. chromosome.

X Inactivation of FemalesX Inactivation of Females

During embryonic development During embryonic development one X one X chromosome becomes inactive (Barr chromosome becomes inactive (Barr Body).Body).

All mitotic divisions create cells All mitotic divisions create cells with the same inactive X.with the same inactive X.

the inactive X chromosome and are normally found only in female somatic cells.

Dosage CompensationDosage Compensation

Sex Chromosomes: females XX, males XY Females have two copies of every X-

linked gene; males have only one. How is this difference in gene dosage

compensated for? OR How to create equal amount of X How to create equal amount of X

chromosome gene products in males chromosome gene products in males and females?and females?

(gene exepretion )(gene exepretion ) Levels of Levels of enzymes or proteins encoded by enzymes or proteins encoded by genes on the X chromosome are genes on the X chromosome are the the same in both males and femalessame in both males and females

Even though males have 1 X chromosome and females have 2.

To compensate for females having 2 X chromosomes vs. males having only 1 X

chromosome could do one of two things :

•Double the amount of transcription of X-chromosome genes in males.

•Inactivate one of the X-chromosomes in females.→→→ Barr Bodies

OR

Dosage Compensation

Barr bodies represent the inactive X chromosome and are normally found only in female somatic cells.

Barr Bodies

46, XX 1 Barr body45, X 0 Barr body47, XXY 1 Barr body47, XXX 2 Barr bodies48, XXXX 3 Barr bodies

No. Barr bodies = N-1

(N = # X chromosomes present)

A woman with the chromosome constitution 47, XXX should have 2 Barr bodies in each cell.

XXY individuals are male, but have a Barr body.

XO individuals are female but have no Barr bodies.

Example:-Example:- G6PDG6PD, glucose 6 phosphate dehydrogenase, , glucose 6 phosphate dehydrogenase,

gene is carried on the gene is carried on the X chromosome.X chromosome. This gene This gene codes for an enzyme that breaks codes for an enzyme that breaks

down sugar which helps red blood cells work down sugar which helps red blood cells work

properly.properly. →→→→normal normal FemalesFemales produce produce the same amount the same amount

of G6PD enzymeof G6PD enzyme as as normal normal malesmales

XXYXXY and and XXXXXX individuals produce the same individuals produce the same

about of G6PD as anyone else.about of G6PD as anyone else.

Dosage compensationDosage compensation

In cells with more than two X chromosomes, only one X remains genetically active and all the others become inactivated.

In XXX and XXXX females and XXY males In XXX and XXXX females and XXY males only only 1 X is activated1 X is activated in any given cell the in any given cell the rest are inactivated.rest are inactivated.

In some cells the In some cells the paternal paternal allele is allele is expressedexpressed

In other cells the In other cells the maternalmaternal allele is allele is expressedexpressed

X-inactivationX-inactivation

Which chromosome is inactiveWhich chromosome is inactive is a is a matter of matter of

chance (random inactivation)chance (random inactivation), , but but once an X once an X

has become inactivated , all cells arising has become inactivated , all cells arising

from that cellfrom that cell will will keep the same inactive keep the same inactive

X chromosome. X chromosome.

In human embryos, sex chromatin bodies In human embryos, sex chromatin bodies have been observed by the have been observed by the 1616thth day of day of gestationgestation. .

Mechanism of X-chromosome Mechanism of X-chromosome InactivationInactivation

A region of X chromosome near the centromere called the X-inactivation center (XIC) is the control unit.

This region contains the gene for X-inactive specific transcript (XIST RNA).

The Xist gene is the only gene which is expressed from the X inactivated but not from the X activated.

The silencing of genes along the Xi occurs soon after coating by Xist RNA. (repressive )(repressive )

The Xi has high levels DNA methylation locks the chromosome in the inactive state. (associated with gene silencing ) )

methylation has two of the methylation has two of the requirements for mechanism of requirements for mechanism of determination:determination:

1. It represses gene activity1. It represses gene activity

2. It is permanent.2. It is permanent.

To check this hypothesis To check this hypothesis Peter Jones Peter Jones and Lawrence Shapiro grew cells in the and Lawrence Shapiro grew cells in the presence of drug presence of drug

5-azacytosine5-azacytosine, which , which prevents DNA prevents DNA methylation. methylation.

This reactivated the X chromosome.This reactivated the X chromosome. Furthermore, Shapiro showed these Furthermore, Shapiro showed these

reactivated chromosomes could be reactivated chromosomes could be transferred to other cells and still transferred to other cells and still remain active. remain active.

X-inactivation facts about calico cats

What determines maleness and femaleness in humans?

2 possibilities:

1. Two X chromosomes are needed to make a female. If you have one X chromosome then you default to male. No, not the mechanism.

2. The Y chromosome is needed to make a male. Lack of the Y chromosome and you default to female. Yes, this is the mechanism.

X-chromosome Y-chromosome

22.322.2

22.1

21.321.221.1

11.411.311.2311.2211.2111.111.212

13

21.121.2

21.3

22.122.222.323

24

25

26

27

28

p

q

11.3

11.2

11.111.2111.22111.22211.22311.23

12

p

q

Duchenne Muscular Dystrophy locus

pseudoautosomal regionregion that escapes X-inactivation

X-inactivation center, XIST

critical region for ovarian development

Fragile X (FMR-1) locuspseudoautosomal region

pseudoautosomal regionSRY (testes determining factor)

AZF (azoospermia factor) (sperm development)heterochromatin

-Presence of Barr bodies in a buccal smear

was introduced by the International Olympic

Committee Medical Commission as a sex

determination (gender verification) test in

1968.

-This test, known as the Barr test or buccal

smear sex test, was responsible for excluding

about one female competitor in 400 from

international competition.

-At the 1992 Barcelona Olympics, the Barr

test was replaced by the

polymerase chain reaction test.

Becouse,

*Someone with a mutation in the SRY

gene can develop into a female even

though there are SRY and the Y

chromosome are present.

* In rare cases, the SRY gene can be

transferred to the X chromosome by

chromosomal crossover during the

production of sperm, and the resulting

XX individual would be phenotypically

male.

*Testicular feminization syndrome (TFS),

a genetic condition in which an XY (male) zygote develops as a phenotypically female adult, due to failure of androgen receptors (Androgen

insensitivity syndrome AIS).

Dora Ratjen, November 20, 1918 Bremen, Germany - April 22, 2008) was a German athlete who competed for Germany in the Women's High Jump at the 1936 Summer Olympics at Berlin, finishing fourth, but was later discovered to be male.

Stella Walsh won a silver medal at the 1936 Olympics - but was later found to be a man

Stella Walsh was killed in a random actof violence in 1968.9 As part of a routine autopsy, coroners discovered that Stella was amasculine pseudohermaphrodite (mosaicism)

800m gold medal in Berli 2009

Semenya ( south Africa) told

to take gender test

Reports in two newspapers in September said the

results of the tests showed Semenya has both male

and female characteristics. The IAAF has declined

to confirm those reports.