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GGE > Gametes - Sex Determination > Flashcards

Gametes - Sex Determination Flashcards

1
Q

3 levels of sexual development

A

1. Chromosomal sex

XY or XX

2. Gonadal sex

Development of gonads

genetically determined

3. Phenotypic sex

internal and external structures

determined by gonadal hormonal secretion

2 distinct systems - internal ducts and external genitalia

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2
Q

period all embryos go through

A

hermaphroditic period

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3
Q

what happens @ 5th week of gestation

A

Gonadal primordia arise

Primordial germ cells become cortex/inner medulla

Cortex → develops into ovary

Medulla → develops into testis

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4
Q

what happens @ 7th week of gestation

A

If XY chromosomes are present, medulla → testis

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5
Q

cortex (of primordial germ cells) becomes

A

ovary

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6
Q

medulla (of primordial germ cells) becomes

A

testis

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7
Q

what happens at conception

A

chromosomal sex is determined

XX or XY

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8
Q

what happens at week 9

A

cortex develops into ovary in females

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9
Q

describe the 7th week embryo

A

undifferentiated gonads can develop into phenotypic male or female gonads

⇒ sexually bi-potential

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10
Q

where is the gene for the differentiation of testis from the bipotential gonad

A

on the Y chromosome

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11
Q

XX and XY - passing on to offspring

A
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12
Q

Karyotyping

A

analysis of chromosomes

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13
Q

karyotype

A

a pictorial display of metaphase chromosomes from a mitotic cell

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14
Q

cytogenetics

A

the study of chromosome

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15
Q

what tissues are appropriate for chromosome study (preparing a karyotype)

A

cells that can be stimulated to undergo cell division in vitro

chromosome only visible with light microscope during mitosis

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16
Q

how is the karyotype then prepared

A
  1. treated with colchicine which arrests the cells in metaphase (no longer need cells to be in metaphase)
  2. stain to observe the chromosomes - Giemsa Stains (bound to certain parts of the chromosomes)
  3. photograph or visualise using a computer
  4. analysis of chromosomes

size, position of centromere, banding and staining regions determined

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17
Q

22 pairs of autosomes + female sex chromosomes

A
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18
Q

22 pairs of autosomes + male sex chromosomes

A
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19
Q

Turner’s Syndrome

A
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20
Q

Kleinefelter Syndrome

A
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21
Q

Jacob’s Syndrome

A
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22
Q

what does the use of fluorescent dyes as a new karyotyping method allow for

A

dye can bind to specific regions of chromosomes

variations in colour can be detected, resulting in a digital image

pairing of chromosomes becomes easier - homologous pairs show identical colours

aberrations and crossovers are easily recognised

detect translocations not previously recognisable

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23
Q

karyotypes may be prepared using

A

lymphocytes

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24
Q

metaphase - FISH

A

housekeeping genes - ones we know will be expressed

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25
Q

interphase FISH

A

⇒ don’t have to induce metaphase

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26
Q

Smith Magenis Syndrome

A

microdeletion

green control probe is present in both paternal and maternal Chr 17

red SMS probe only fluoresces on a single chromosome

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27
Q

how to identify all chromosomes at once

A

use probes of different colours

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28
Q

describe the genome of cancer cells

A

cancer cells are characterised by aberrations in chromosome size, number, banding patterns

chaotic genome

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29
Q

genetic content of X vs Y chromosome

A

Y chromosome contains far fewer genes than the X chromosome

30
Q

size difference between X and Y chromosome

A

Y is much smaller than X even though it’s thought to have originated from a common progenitor

31
Q

structure of Y chromosome

A
32
Q

PseudoAutosomal Region (PAR) function

A

necessary for tetrad formation so there is crossover during meiosis - synapses and recombines with X chromosome during meiosis

region on Y that shares homology with regions on X

presence of such a pairing region is critical to segregation of X and Y chromosomes during male gametogenesis

33
Q

role of SRY in XY female

A

deletion on Y chromosome

mapped deletion - SRY gene (sex determining region of Y chromosome)

34
Q

role of SRY in XX males

A

translocation of SRY region of Y → X chromosome

35
Q

SRY gene in mice studies

A

converts chromosomal female mouse to phenotypically male

36
Q

mutant mouse lacking SRY

A

XY female

37
Q

expression of SRY is seen when and where

A

sertoli cells at time of testis development - SRY upregulated at this time

transcripts in hypothalamus, midbrain of adult male mice - male specific neural properties

38
Q

transgenic SRY experiment

A

nuclei of fertilised XX eggs were injected with SRY gene, then the eggs were transplanted to surrogate mothers

SRY genes then randomly incorporated into a chromosome and was inherited in subsequent cell divisions

animals were karyotyped after development to adult

of 3 XX transgenic mice that were born, 2 were female and 1 was a sex reversed male

39
Q

genes involved in male testis determination pathway

A

SRY

Sox9

Amh

Fgf9

Dmrt1

40
Q

SRY

what is it

associated syndromes

A

transcription factor

Turner’s Syndrome

Kleinefelter Syndrome

XY sex reversal - loss of function (LOF)

XX sex reversal - gain of function (GOF)

41
Q

Amh

what is it

associated syndromes

A

hormone

XY sex reversal (LOF)

codes for Anti-Mullerian Hormone - responsible for regression of female ductal system

42
Q

Sox9

what is it

associated syndromes

A

transcription factor

campomelic dysplasia XY sex reversal (LOF)

abnormal sertoli cell differentiation

XY sex reversal (LOF)

XX sex reversal (GOF)

43
Q

Fgf9

what is it

associated syndromes

A

growth factor

XY sex reversal (LOF)

44
Q

Dmrt1

what is it

associated syndromes

A

transcription factor

XY gonadal dysgenesis

XY sex reversal (LOF)

post-natal feminisation in XY mice, defective seminiferous tubule (LOF)

45
Q

Sox9 and sertoli cells

A

Sox9 is upregulated in sertoli cells just after SRY expression

phenotypically female with XY genotypes

altered expression/coding of Sox9

46
Q

genes involved in the initial development of the bipotential gonad

A

Emx2

Gata4

Wt1

Lhx9

Sf1

47
Q

Emx2

protein function

mouse models

A

aberrant tight junction assembly

failure in genital ridge formation (LOF)

48
Q

Gata4

protein function

human syndrome

mouse models

A
  • transcription factor
  • ambiguous external genitalia, congenital heart disease (LOF)
  • failure in thickening of coelomic epithelium, defective initial formation of genital ridge (LOF)
49
Q

Wt1

protein function

human syndrome

mouse models

A
  • transcription factor
  • Denys-Drash, Frasier syndrome (LOF)
  • disruption of seminiferous tubule and somatic cell apoptosis, XY sex reversal (LOF)
50
Q

Lhx9

protein function

mouse models

A

transcription factor

failure in genital ridge formation (LOF) - no gonads forming at all

51
Q

Sf1

protein function

human syndrome

mouse models

A
  • nuclear receptor
  • embryonic testicular regression syndrome, gonadal dysgenesis
  • delayed organisation of male testis cord, failure in genital ridge formation (LOF)

initially expressed in genital ridges of both sexes but remains solely in developing testes

52
Q

what allows for the expression of Sox9

A

SF1

53
Q

emerging genetic pathways involved in initial expression and maintenance of Sox9 in sertoli cells

A
54
Q

absence of gonads

A

can have individuals without gonads so female not default state

55
Q

what is on the Y chromosome of XY females

A

XY females with Sry - extra region on Y chromosome - includes region for DAX1 gene

56
Q

gene needed for female gonadal formation

A

DAX gene

57
Q

other evidence for role of DAX1 (over-riden SRY in this case)

A

expressed in developing gonads at critical time - repressed by SRY

58
Q

balance between male and female sex-determining pathways

A
59
Q

dosage compensation

A

female (XX) should have a genetic dosage problem for all X linked genes

potential for females to produce twice as much X linked gene products

60
Q

X chromosome dosage compensation?

A

inactivation of 1 X chromosome

61
Q

what is present in females only

A

female cats - single condensed body (black dot)

sex chromatin - inactive X chromosome

⇒ Barr body

(happens at day 12)

62
Q

what does the Barr body represent

A

inactivated X chromosome

63
Q

what is present in XXY males

A

Barr body

64
Q

when does inactivation of X chromosome (Barr body) occur

A

outside of ovary at day 12 in utero

(happens outside of ovary because both X chromosomes are required for the initial oogonia - without 2 X chromosomes there would be rudimentary oogonia development)

65
Q

what would be absent in Turner’s Syndrome

A

Barr body

66
Q

X-inactive Specific Transcript (XIST)

A
  • product of an X-linked gene
  • initially XIST is transcribed from both X chromosomes
  • when transcribed the mRNA binds to the X chromosome
  • coats the 2nd X chromosome - condensation
  • X inactivation centre (Xic) is active on the inactive X
  • only 1 X will continue to produce XIST - inactive

(X inactivated centre is only active in X chromosome that is about to be inactivated)

67
Q

inactive X - what does it look like

A

highly condensed

can be observed in stained interphase cells

BARR BODIES

68
Q

overview of X inactivation

A
69
Q

anhidrotic ectodermal dysplasia

A

heterozygous for the X-linked condition

depiction of the absence of sweat glands (shaded regions) in a female

locations vary from female to female based on the random pattern of X chromosome inactivation during early development, resulting in unique mosaic distributions of sweat glands in heterozygotes

70
Q

androgen insensitivity syndrome

what does it confirm

how is it caused

A

confirms importance of sex hormones in sex differentiation

caused by a mutation in the gene encoding the androgen receptor

→ coded for on X chromosome gene

71
Q

symptoms of males who inherit this condition

A

produce testosterone and DHT

are unable to respond to either hormone

hence they develop as phenotypically females but have normal male chromosomes

the low levels of oestrogen that is produced by adrenal glands is enough to stimulate female secondary characteristics

72
Q

how do individuals with androgen insensitivity syndrome develop female secondary sex characteristics

A

low levels of oestrogen produced by adrenal glands

GGE (33 decks)

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