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CBG Genetics > Genetics Lecture 16: Sex determination > Flashcards

Genetics Lecture 16: Sex determination Flashcards

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Biology 101 flashcards
1
Q

what are simultaneous hermaphrodites

give example

A

can self fertilise
eg. penis fencing in marine flatworms
first individual to pierce other deposit sperm in the other

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2
Q

what are the advantages of sexual reproduction

A

variable progeny

by random segregation of chromosomes, we have 8 mill gametes produced

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3
Q

what are the sex determination mechanisms

A

genotypic

environmental

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4
Q

what is genotypic sex determination

A
  1. hormones - sex determining genes initiate sex determination and are controlled by the gonadal hormones
  2. ploidy level - eg. bees - haplodiploidy –> diploid females, haploid males
  3. chromosomal determination : xx/xo , xx/xy , zz/zw, x:A
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5
Q

what is environmental sex determination

A

sex determined after fertilization due to enviro facto

  1. temperature in critical period in juvenile development
  2. stressful condns eg. Daphnia
  3. behaviour eg. lsrval location/ settling behaviour. social structure change (sex ratio) in adults prompting sex changes
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6
Q

discuss haplodiploidy

  • > how is sex determines
  • > what is csd? what does it target
A

2n (female) n (male)
in honey bees - which lack sex chromosome, a single complementary sex determination gene (csd) targets the feminizer (fem) gene needed for female development. the fem product then acts on the double sex gene
haploids are hemizygous for csd and become males as they dont produce fem gene product

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7
Q

why do haploid honey bees become males

A

they dont produce a fem gene product as alternate splicing in males adds a stop codon into the fem coding sequence

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8
Q

discuss sex determination in Drosophila

A

Drosophila have XXX (metafemales), ratio of 0.67 –> most of them are sterile or die off
have a sex lethal gene which starts the cascade of mRNA alternate splicing to produce female specific proteins
female doublesex represses male genes and activates female genes. Male doublesex represses female genes

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9
Q

what is intersex

A

have one ovary and one testes = resemble the phenotypes of both males and females, often infertile

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10
Q

whta results from a X:A 1:1 ratio

A

the sex lethal gene becomes activated and produces a female specific protein which works on the transformer gene which produces another protein that works on double sex gene, which produces a protein that works on the female phenotype

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11
Q

what results from an unbalanced X:A ratio

A

the sex lethal gene produces no functional product and no functional transformer protein, so double sex gene produces male proteins, resulting in male phenotype and represses the female genes

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12
Q

what is the default sex in Drosophila, why?

A

the default sex in Drosophila is MALE: sex lethal needs to be switched ON, if not, the male is default

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13
Q

how to know if someone is male or female

A
  1. anatomy
  2. endocrinology
  3. chromosomes
  4. genes
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14
Q

what are genital ridges

A

all humans have them as fetuses: bipotential gonads that can develop into male or female gonads, depending on what happens after fertilisation

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15
Q

what does the PAR do in Y chromosome

A

pseudoautosomal region right at top.
connects the X and Y chromosomes together and forms a chiasma, needed for correct segregation
also one at bottom

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16
Q

what main gene on Y chrom is involved in sex determination in humans

A

SRY gene - determines male sex
v. close to PAR
it i the male determining region of Y chrom

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17
Q

name a transcription factor in humans required for male gonadal function

A

DMRT1 - double sex mab 3 related transcription factor 1

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18
Q

discuss what effect SRY gene has on sex determining

A

at week 8 after fertilisation, SRY gene gets switched ON (if it is present). it produces gonads which differentiate into testes, then the hormones produced by the testes give the male phenotype

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19
Q

what sex is human with SRY gene

A

male

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20
Q

what happens to human SRY-

A

if SRY gene not switched on, timeline of embryonic development carries until week 13, when the Od (ovary developmental gene) is switched on.
therefore if SRY is on, it inhibits Od gene
if SRY off, you get activation Od gene which results in ovary formation and embryo development into gemale

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21
Q

what relationship do SRY and Od genes have

A

If SRY present/on, it inhibits Od –>MALE

If SRY absent/off, Od gene transcribed = FEMALE

22
Q

what is the default sex in humans

A

females, due to need for SRY to be present to turn OFF Od gene

23
Q

what can uncoupling of chromosomal and phenotypic sex lead to

A

intersex -> gender identity -> DSD disorders of sex development

24
Q

what would XYSRY+ be?

A

male with normal gene arrangement

25
Q

XXSRY+

A

male, produced by translocation of SRY gene from Y to X

26
Q

XYSRY-

A

female: deletion or mutation of SRY gene

27
Q

XXSRY-

A

female: normal gene arrangement

28
Q

give examples of uncoupling of human chromosomal and phenotypic sex

A

EXAMPLE ONE: Y chromosome SRY gene: translocation: 1 in 20000 phenotypic males are 46 XX SRY+
Deleteion/mutation: 1 in 100000 phenotypic females are 46 XY SRY-
EXAMPLE TWO: X chromosome AR (androgen receptor gene)
recessive mutation causes 46XY SRY+ chromosomal males to be insensitive to androgens ie. androgen insensitivity syndrome, thus are phenotypic females
Compkete AIS: genitalia female, no internal female organs
Partial AIS: genitalia can be male, female or inbetween

29
Q

what does DAX gene do

A

if youve got duplication of the DAX1 gene, which is on the X chrom, it acts as an anti-testes gene -> not androgen insensitive but can stop testes developing

30
Q

why does presence/absence of SRY alone not determine sex in humans

A

because sex determination is an active process
In presence of SRY: 46XY SRY+ individuals can be phenotypically efemale if DAX1 gene is duplicated on X chromosome and it can act as an anti-testis gene
In absence of SRY (ie. Y)
2 X chromosomes are required for functional ovaries, but XO females have ovarian dysfunction

31
Q

why is a mouse a good model for human sex determination

A

share many similar/identical pathways (in humans SOX 3 regulates SOX 9)
both have bipotential gonad at beginning

32
Q

discuss the cascade of the current mouse model for sex determination in MALES

A

in males: SRY+ = cascade of genes involving SOX 9 which will form male testes
also get a block of FOXL2 and b-catenin (female genes)
however in adult testes you still require female genes to be switched off and thats done by DMRT1 and SOX9

33
Q

discuss the cascade of the current mouse model for sex determination in MALES

A

FOXL2 switches off Sox9 and that allows formation of female ovaries
male specific genes like Sox 9 and DMRT1 are blocked and ongoing presence of FOXL2 in ovaries is required to prevent trans-differentiation into testes

34
Q

name some genes involved in sex determination and function

A

SOX9 : form male testes, blocks female genes
DMRT1: maintains teste function in adults
FOXL2 and beta catenin female genes
ongoing presence of FOXL2 in ovaries is required to prevent trans differentiation into testes

35
Q

discuss temperature dependent environmental sex determination

A

REPTILES eg. American alligator

egg incubation temp critical @ certain time in embryonic development specifies temp

36
Q

give a gene involved in temperature dependent environmental sex determination

A

CYP191A: converts testosterone to estradiol in gonads

37
Q

give example of organism involved in in temperature dependent environmental sex determination

A

REPTILES: egg incubation temperature is critical at certain time in embryonic development
eg. CYP191A involved
low activity @25degree = MALE production
high activity @30degree=FEMALE production
in the Europeam POND TURTLE: Emys obicularis

38
Q

outline model of sex determination in red eared slider turtle

A

two candidate sex determining genes: FOXL2:forkhead box protein L2 and doublesex mab3 related transcription factor 1 (Dmrt1) implicated

39
Q

give an example of when Low incubation temperatures determines sex

A

Australian skink

some females change to phenotypic males ie. XX females and XX ales

40
Q

give an example of when high incubation temperatures determines sex

A

Australian dragon lizard

some ZZ males change to phenotypic females ie. ZZ females

41
Q

example of Environmental sex determination (ESD) due to stressful condns
what constitutes stressful condns
what main gene involved

A

Daphnia magna - stressful conds: lack resources.food
parthenogenetic females produce clonal females, but also clonal males in stressful condns
clonal males and females then reproduce sexually
doublesex homolog (DSX1) switching on though to be producing the males

42
Q

example of Environmental sex determination (ESD) due to larval settling behaviour

A

greem spoon worm:
if the larvae settle on sea bed: will grow into 10-20cm females
if they settle on female proboscis: develop into male

43
Q

example of Environmental sex determination (ESD) due to tropical reef dwelling social dominance

A

social dominance in tropical reef dwelling teleost fish
can have a sex change: sequential hermaphrodites.
develop into dominant males first, however dominance in fish is determined by size, so most dominant fish in hierarchy turns into female and second fish in heirarachy remains as male.
usually only have one dominant fertile male

44
Q

what is protoandry/protogyny

A

protoandry: switch from male to female
protogyny: switch from female to male

45
Q

give example of protoandy

A

in teleost: ORANGE CLOWN FISH

46
Q

give example to protogyny

A

WRASSE FISH

47
Q

what genes are involved in sequential hermaphroditism

A

Aromatase A
gonad specific gene-> only expressed in radial glial cells
down regulation of aromatase A associated with testicular differentiation
upregulation associated with ovarian differentiation and reproductive functions

BUT NEED SEXUALISATION OF THE BRAIN to connect gonadal sex and brain sex

48
Q

how does aromatase activity in brain of mammals compare with that of teleosts

A

in mammals, v. high activityy in brain till birth, then brain is sexualised
in teleosts, which can sex change, activity of aromatase stays high, so you have reversible brain sexualisation

49
Q

what are the primary sex determining mechanisms; give examples

A

Drosophila - X:A ratios regulated by mRNA splicing
Caenorhabditis - X:A ratio regulated by intercellular ligand receptor signalling
the Box turtle (Terapene corolina) - temp sensitive mechanism
Humans - SRY Y specific transcription factor

50
Q

discuss convergence of sex determining mechanisms

A

there are diverse Genetic and Enviro SD mechanisms - suggests weve had multiple origins BUT they all converge to a conserved DM (doublesex) regulator therefore know there was one origin which has diverged

CBG Genetics (4 decks)

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