10. Sex determination

Typical male and female

Typical male and female

X chromosomes are fairly large

typical female has 46 chromosomes including an X andX

Typical male has 46 chromosome including an X and Y chromosome

Y is considerably smaller

Aneuploidy

A condition where the chromosome number is not an exact multiple of the haploid set e.g. 2n + 1, 2n-1

As opposed to euploidy (e.g., diploidy) where there is an exact multiple of the haploid set of chromosomes, GET REST FROM MARK OR LIA

Klinrfelter syndrome

Klinefelter syndrome: [47, XXY]

Usually, have male genitalia and internal ducts Testes rudimentary, fail to produce sperm

Feminine sexual dev. not entirely suppressed

Often abnormal social development Intelligence often below normal

Note that this individual is biologically male, they have the male genitalia. They would score biologically as a male

Turner syndrome

Turner syndrome: [45, X]

Female external genitalia and internal ducts

Ovaries rudimentary

phenotypically you are as female

What does the Klinefelter and Turner syndromes allow us to conclude

What does the Klinefelter and Turner syndromes allow us to conclude as far as sex determination is concerned? • If Y is present- male. • If Y is absent- female.

The Y chromosome really determines if person is male or female

Human Y chromosome

NRY: non-recombining region

PAR: pseudoautosomal region

SRY: Sex-determining region Y (critical gene in SRY for sex determination)

MSY: Male-specific region of the Y

euchromatin (light staining) correlates with transcriptionally active DNA

and heterochromatin represent regions of the chromosome that are not AS transcriptionally active

heterochromatin and euchromatin should be mixed in together not two completely distinct bands, more like E,H EHH E H, multiple bands

regions on chromosome where there are fewer genes and regions where there are lots of genes

Role of PAR

Small terminal sections needed for synapsis (pairing) and recombination with X during meiosis (without this synapsis you wouldn’t get crossing over or proper alignment of tetrads)

• Critical to proper segregation of the X and Y chromosomes during male gametogenesis (to be able to produce gametes that have the X chromosome or gametes that have the Y chromosome)

• Different from remaining NRY = non-recombining region of Y

What does euchromatin in MSY suggest?

Presence of genes!

Confirmed by Human Genome Project

Y chromosome has at least 75 genes [X has 900-1400 genes]

Some genes on Y chromosome actually have homologous counterparts on the X chromosome, others do not.

we think these genes used to be autosomes were one allele had X and one allele was for Y (like one would indicate female one indicates male)

But overtime ....

what happened to the X and Y chromosome overtime when do certain parts develop

Y chromosome clearly carries genetic info absent from the X • Embryological evidence • XY: show testis initiation in week 7 • XX: primary oocytes can be detected in wk 12

accumulate and stop at diplonema of prophase I

Genes found in MSY 1st region

• NRY/MSY can be divided into 3 regions:

• 15% of MSY is “X-transposed region”

– Originally derived from X chromosome

• 99% identical to Xq21 of modern human X chromosome

doesn't necessarily mean it has the same function...

• 2 genes, each with X chromosome homologs are present

2 ND region

, “X-degenerative region” - 20 % of MSY

- 27 single copy genes and a number of pseudogenes

- 20 share homology with counterparts on X chromosome and evolved from genes on X chromosome.

looks like it might have been a functional gene at one time but now has anomalies that leads it to not code for a functional polypeptide

suggesting genes on X and Y chromosomes have similar origins

-one is SRY , others expressed in all tissues

SRY has impact in specific cell types that make male differentiation patterns.

3 RD region

, “ampliconic region”

-30 % of MSY

-most of genes closely associated with development of testes

-60 transcription units [9 gene families]

within that 60 transcription units you can break them into 9 groups that are similar but not identical...

-spread in seven segments of euchromatic regions in both short and long arms

-thought male infertility linked to mutations in these genes.

dont determine if they are male, determine whether they are fertile or not

between these segments is heterochromatin (not blocks like shown in diagram, heterochromatin and euchromatin mixed in together

TDF

factor is generalized term for a protein that is involved in some sort of process

TDF = testis-determining factor Gene location on Y known as SRY = sex determining region on Y; p arm

How does SRY product trigger gonadal tissue to differentiate into testis rather than ovaries? • Autosomal genes are believed to be involved.

TDF function

Testis-determining factor [TDF] functions as a transcriptional factor. DNA-binding protein -interacts directly with regulatory sequences of other genes -stimulates expression of these genes

-”master switch” controlling other genes downstream in process of sexual (turns off female development, turns on male development)

differentiation. -these target have been difficult to identify

One possible target of TDF

-gene for “Mullerian inhibiting substance” [MIS] or “Mullerian inhibiting hormone” [MIH] or or “anti-Mullerian hormone”.

-cells of developing testes secrete MIS

-causes regression [atrophy] of cells in Mullerian duct -prevents formation of female reproductive tract

Other autosomal genes that are part of cascade

Other autosomal genes that are part of cascade of gene expression initiated by TDF Human SOX9 gene and mouse homolog

-when activated leads to differentiation of cells that form seminiferous tubules [contains male germ cells] where sperm is produced and stored

X chromosome and dosage compensation ]

• XX vs XY • Imbalance for X-linked gene products? • Does dosage disparity exist?

How is the dosage of X-linked gene expression compensated/reduced? • Cytological evidence • Experiments by Barr et al. on mammals and subsequent interpretation by Ohno • “Barr bodies”

Barr bodies

Darkly staining • Present in females not in [normal] males • 1 micron in diameter • Lies against the nuclear envelope of interphase cells • Stains for DNA • N- 1 rule; N = total number of X chromosomes

where the number of bar body is the number of X chromosomes (N) -1

staining darkly is not transcriptionally active, even in females there is only one X chromosome that is functionally active

Complications:

• If X chromosome inactivation results in dosage compensation then: • [1] Why is Turner [45,X] individual not entirely normal? [2] In Klinefelter [47,XXY] individual, why doesn’t X chromosome inactivation make them effectively [46,XY]?

anomolies still with having an extra or lacking an X chromosome

Barr body Possiby inactivated

Possibly inactivation delayed during embryogenesis

-excessive expression of certain X-linked genes might occur at critical [early] times during development despite apparent inactivation of “superfluous” X chromosomes

• Not entire Barr body (i.e. X chromosome) DNA inactivated

– recent evidence suggests that 15 % of human X chromosomal genes actually escape inactivation

for some key embryonical event the extra chromosome is still active?

Which X is inactivated? Paternal or maternal?

• 1961 Mary Lyon proposed hypothesis

• Inactivation occurs at random early in development

• Thus entire mitotic cell progeny/lineages have same X inactivated

• Based on mice heterozygotes for X-linked coat colour, also in cats; human cell clones; mosaic retina of female heterozygotes for red-green blindness

when there is relatively few cells some of the cells will have the paternal a X chromosome deactivated and some have maternal X chromosome inactivated

this would make them chiameric

What is the mechanism by which almost an entire chromosome is being inactivated?

XIC = X-inactivation center on proximal p arm Only expressed in X that are inactivated (paradoxical) 1996 gene manipulation experiment Targeted deletion prevented inactivation

they deleted the XIC from one of the X chromosome, they found when you deleted it that chromosome no longer condenses and forms the Barr body.

Xic

Xic is about 1 Mb in size (1 million base pairs)

-several putative regulatory sequences and 4 genes.

-one gene, X-inactive specific transcript [XIST]

-long noncoding RNA [lncRNA] (exerts its funcion as an RNA transcript)

-recruit protein complex that silences transcription -Xist lncRNAs spread over and coat X chromosome that produced it - prevents RNA polymerase II from binding transcription complexes.

Drosophila

sex determination and compensation • XX and XY long known • Bridges, Morgan, Sturtevant early 1900s • Aneuploids XXY = female; XO = male!!!

• Conclusion?

Y chromosome not essential for maleness in fruit flies • “The presence of the Y chromosome in the XXY did not cause maleness and its absence in XO (zero) did not produce femaleness” • Y lacks male-determining factors. - Has genes that affects male fertility.

Factors determining maleness in drosophila

In Drosophila factors determining maleness not on sex chromosomes but on autosomes • The critical factor: ratio of X chromosomes to haploid set of autosomes • 1.0 = normal female • 1.5 = (weak) meta females • 0.5 = normal male • 0.33 = (weak) meta male

between 1 and 0.5 is where intersex occurs

Other chromosome based mechanisms for determining sex

Other chromosome based mechanisms for determining sex:

ZZ/ZW: Comparable to XX/XY except Homogametic individuals [ZZ], individuals with homomorphic sex chromosomes are male

ZZ male, ZW female

Reptiles evolved multiple times during the development of vertebrates...

Common in birds Many but not all snakes Many lizards [but others are XX/XY]

XX/XO

XX/XO:

XX-

XO- only has 1 sex chromosome

Interesting example: Caenorhabditis elegans

XX- Hermaphrodite

XO- Male (no individual strictly female)

Believed ratio of X chromosomes to number of sets of autosomes

The underlying mechanism may be the ratio thing with drosophila

No Sex Chromosomes- Mating Types [AA/Aa]

evolutionary origins of heteromorphic X and Y chromosomes due to mutations such as inversions. No way to get aa

[AA/Aa] -no morphologically distinct sex chromosomes -single gene with two alleles [call A and a]

-females: AA, males: Aa

-inheritance resembles XX/XY system but no morphologically distinct sex chromosomes

-seen in many different vertebrates some reptiles, fish and amphibians -seen in a few plants, e.g. gingko trees

-postulated that mating types may be ancestral condition for chromosomal sex determination.

Environmental sex determination

• Temperature-dependent sex determination

• Most turtles, all crocodiles and some lizards • Incubation temperature during critical phase of embryogenesis (embryonic development)

Tp= temperature of parity: temp at chich there is roughly a 1:1 ratio of males to females

Three cases: case 1: high temp more males, low temp more females cse 2: high temp more females, low temp more males case 3: two temps of parity high and low temps encourage females

Temperature influences

Temperature influences synthesis of steroids (mostly estrogen) • Aromatase converts androgens [male hormones] to estrogens [female hormones] High in developing ovaries, low in testes. -proposal of thermosensitive factor to mediate transcription of reptilian aromatase gene

a transcriptional factor

Whatever temperature this enzyme is active, that temperature will favour the development of females

….implications of global warming