Gametes - Sex Determination Flashcards
3 levels of sexual development
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
period all embryos go through
hermaphroditic period
what happens @ 5th week of gestation
Gonadal primordia arise
Primordial germ cells become cortex/inner medulla
Cortex → develops into ovary
Medulla → develops into testis
what happens @ 7th week of gestation
If XY chromosomes are present, medulla → testis
cortex (of primordial germ cells) becomes
ovary
medulla (of primordial germ cells) becomes
testis
what happens at conception
chromosomal sex is determined
XX or XY
what happens at week 9
cortex develops into ovary in females
describe the 7th week embryo
undifferentiated gonads can develop into phenotypic male or female gonads
⇒ sexually bi-potential
where is the gene for the differentiation of testis from the bipotential gonad
on the Y chromosome
XX and XY - passing on to offspring
Karyotyping
analysis of chromosomes
karyotype
a pictorial display of metaphase chromosomes from a mitotic cell
cytogenetics
the study of chromosome
what tissues are appropriate for chromosome study (preparing a karyotype)
cells that can be stimulated to undergo cell division in vitro
chromosome only visible with light microscope during mitosis
how is the karyotype then prepared
- treated with colchicine which arrests the cells in metaphase (no longer need cells to be in metaphase)
- stain to observe the chromosomes - Giemsa Stains (bound to certain parts of the chromosomes)
- photograph or visualise using a computer
- analysis of chromosomes
size, position of centromere, banding and staining regions determined
22 pairs of autosomes + female sex chromosomes
22 pairs of autosomes + male sex chromosomes
Turner’s Syndrome
Kleinefelter Syndrome
Jacob’s Syndrome
what does the use of fluorescent dyes as a new karyotyping method allow for
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
karyotypes may be prepared using
lymphocytes
metaphase - FISH
housekeeping genes - ones we know will be expressed
interphase FISH
⇒ don’t have to induce metaphase
Smith Magenis Syndrome
microdeletion
green control probe is present in both paternal and maternal Chr 17
red SMS probe only fluoresces on a single chromosome
how to identify all chromosomes at once
use probes of different colours
describe the genome of cancer cells
cancer cells are characterised by aberrations in chromosome size, number, banding patterns
chaotic genome
genetic content of X vs Y chromosome
Y chromosome contains far fewer genes than the X chromosome
size difference between X and Y chromosome
Y is much smaller than X even though it’s thought to have originated from a common progenitor
structure of Y chromosome
PseudoAutosomal Region (PAR) function
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
role of SRY in XY female
deletion on Y chromosome
mapped deletion - SRY gene (sex determining region of Y chromosome)
role of SRY in XX males
translocation of SRY region of Y → X chromosome
SRY gene in mice studies
converts chromosomal female mouse to phenotypically male
mutant mouse lacking SRY
XY female
expression of SRY is seen when and where
sertoli cells at time of testis development - SRY upregulated at this time
transcripts in hypothalamus, midbrain of adult male mice - male specific neural properties
transgenic SRY experiment
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
genes involved in male testis determination pathway
SRY
Sox9
Amh
Fgf9
Dmrt1
SRY
what is it
associated syndromes
transcription factor
Turner’s Syndrome
Kleinefelter Syndrome
XY sex reversal - loss of function (LOF)
XX sex reversal - gain of function (GOF)
Amh
what is it
associated syndromes
hormone
XY sex reversal (LOF)
codes for Anti-Mullerian Hormone - responsible for regression of female ductal system
Sox9
what is it
associated syndromes
transcription factor
campomelic dysplasia XY sex reversal (LOF)
abnormal sertoli cell differentiation
XY sex reversal (LOF)
XX sex reversal (GOF)
Fgf9
what is it
associated syndromes
growth factor
XY sex reversal (LOF)
Dmrt1
what is it
associated syndromes
transcription factor
XY gonadal dysgenesis
XY sex reversal (LOF)
post-natal feminisation in XY mice, defective seminiferous tubule (LOF)
Sox9 and sertoli cells
Sox9 is upregulated in sertoli cells just after SRY expression
phenotypically female with XY genotypes
altered expression/coding of Sox9
genes involved in the initial development of the bipotential gonad
Emx2
Gata4
Wt1
Lhx9
Sf1
Emx2
protein function
mouse models
aberrant tight junction assembly
failure in genital ridge formation (LOF)
Gata4
protein function
human syndrome
mouse models
- transcription factor
- ambiguous external genitalia, congenital heart disease (LOF)
- failure in thickening of coelomic epithelium, defective initial formation of genital ridge (LOF)
Wt1
protein function
human syndrome
mouse models
- transcription factor
- Denys-Drash, Frasier syndrome (LOF)
- disruption of seminiferous tubule and somatic cell apoptosis, XY sex reversal (LOF)
Lhx9
protein function
mouse models
transcription factor
failure in genital ridge formation (LOF) - no gonads forming at all
Sf1
protein function
human syndrome
mouse models
- 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
what allows for the expression of Sox9
SF1
emerging genetic pathways involved in initial expression and maintenance of Sox9 in sertoli cells
absence of gonads
can have individuals without gonads so female not default state
what is on the Y chromosome of XY females
XY females with Sry - extra region on Y chromosome - includes region for DAX1 gene
gene needed for female gonadal formation
DAX gene
other evidence for role of DAX1 (over-riden SRY in this case)
expressed in developing gonads at critical time - repressed by SRY
balance between male and female sex-determining pathways
dosage compensation
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
X chromosome dosage compensation?
inactivation of 1 X chromosome
what is present in females only
female cats - single condensed body (black dot)
sex chromatin - inactive X chromosome
⇒ Barr body
(happens at day 12)
what does the Barr body represent
inactivated X chromosome
what is present in XXY males
Barr body
when does inactivation of X chromosome (Barr body) occur
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)
what would be absent in Turner’s Syndrome
Barr body
X-inactive Specific Transcript (XIST)
- 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)
inactive X - what does it look like
highly condensed
can be observed in stained interphase cells
BARR BODIES
overview of X inactivation
anhidrotic ectodermal dysplasia
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
androgen insensitivity syndrome
what does it confirm
how is it caused
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
symptoms of males who inherit this condition
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
how do individuals with androgen insensitivity syndrome develop female secondary sex characteristics
low levels of oestrogen produced by adrenal glands
