Sex determination and sex linkage Flashcards
what are non- sex chromosomes called?
autosomes
approx how many Mb are each sex chromosome
X = ~160Mb
Y= ~ 60Mb
approx how many genes on each ‘specific region’ on the 2 sex chromosomes
X specific = >1000genes
Y (or male) specific region (MSY)= 80 genes
both of these have no sequence similarity between them unlike normal autosomes
what is the pseudoautosomal region? (PAR)
regions with sequence similarity on both X and Y
on the tips of each chromosome
~20genes in humans shared
why are pseudoautosomal genes important
required for X-Y pairing in male meiosis
why are males called hetereogametic and females called homogametic
males can produce gametes with either X or Y
females can only do gametes with X
how does aneuploidy give us evidence that Y = maleness
aneuploidy = incorrect no. chromosomes
so if individual only has one X = female
tells us u dont need 2 X chromosomes to be female
if indidual has XXY = male
so a single Y chromosome confers maleness
is ALL of the Y chromosome required to confer maleness?
lol no
only one gene needed = SRY
stands for sex determining region on the Y
how do we know that SRY = determining gene?
rare XX males
where the SRY gene has translocated to the X chromosome
and causes sex reversal
also
rare XY females
where the SRY gene has LOF mutation
and
they experimented on mice
where giving them the single SRY gene
and no other part of the Y chromosome
would make them male
how does SRY gene cause maleness
it acts as a transcription factor
so is needed to regulate expression of the genes necassary for testis formation
explain the development of the ovaries and testis and how SRY is the key determinent
at week 4 = a structure called the genital ridge
- only has somatic cells
- not differentiated yet
at week 6 = turns into indifferent gonad
- germ cells (undifferentiated reproductive cells) migrate and join the somatic cells
- still not differentiated
at week 7 = SRY gene expressed (or not expressed in females)
at week 8 = will see indiff gonad develop into testis if SRY was expressed
or defualt as ovaries
what is the genetic sex determination in birds?
ZW system
males = ZZ
females = ZW
what is a gynandromorph
a sexual mosaic
e.g. that chicken thats half female ZW cells and half male ZZ cells
what is cell autonomous sex identity
in birds
each cell determines its sex depedning on its chromosomes
in mammals, sex hormones will determine whether it’s male or female tissue
what type of chromosome were the X and Y originally and how did this evolve into our current X and Y chromosomes
autosomes
at some point in evolution, Y acquired SRY gene
Y started to accumulate genes that were advatageous to males
and selection prevented recombination of the X and Y
end up with 2 vry diff chromosomes and only a short bit of sequence similarity between them
btw birds and mammal sex genes have evolved from 2 diff sets of chromosomes (see onenote for diagrams)
what were the 2 phenotypes of the Drosophila used in Morgan’s experiments
wild type (w+ or +) = had red eyes
white (w or w-) mutant = had white eyes
Morgan isolated the white mutants that didnt give Menedelian phenotypic ratios
what would a reciprocal cross usually show acording to mendel
reciprocal for example is 2 crosses
1 cross = white male x red female
2 cross = red female x white male
should give same results
what would a reciprocal cross usually show acording to mendel
reciprocal for example is 2 crosses
1 cross = white male x red female
2 cross = red female x white male
should give same results
see onenote for diagram on Morgan’s results
:)
what could we conclude from morgans results in terms of where the mutation lies for the white eye
white is a recessive mutation that lies on the X chromosome
which chromosome does sex linked inheritance usually occur on
X chromosome - (X-linkage)
what could Y-linked genes be related to
usually have male specific functions (e.g. spermatogenesis)
but this is a vry small % of sex linkage
what is hemizygous
males will be hemizygous for genes on the X chromosome
so only one mutant copy, cuz Y chromosome doesnt have corresponding gene related to the one on the X
why are males more affected by X linked disease
a female would carry the mutation in only one chromosome but a normal gene on the other
so usually is unaffaced
but male has only one X chromosome, so if a gene mutates on it then he’ll most liekly show it (hemizygous)
approx how many known X linked disease genes are there and give examples
~300
Haemophilia A and B (affects blood clotting factors)
Duchenne muscular dystrophy
Red-green colour vision deficiency (8% males have it)
what is the name of the genes that control photoreceptors
opsin genes
the opsin genes that encode for red and green are right next to each other on the X chromosome
~30nm of difference between wavelgth that =red and the wavelength = green
these 2 were originally 1 single gene
and at some point in evo, they split and diverged to be sensitive to different wavelength
but still significant DNA sequence similarity
so is vry possible that in females during meiosis, there’s abnormal recombination between the 2 genes
this leads to chimeric genes
= 2 regions of red replace regions on the green gene
this means the difference in sensitivity is 12nm
so they cant distinguish easily
what’s the positioning of the opsin genes on the X chromosome
right next to each other
red perceptor gene (L)
green perceptor gene (M)
how many nm of difference is there between the perception of red wavelength and the green wavelength usually
30nm
describe the sequence similarity between the L and M gene
very similar
2 were originally 1 single gene
and at some point in evo, they split and diverged to be sensitive to different wavelength
how is deuteranomaly (can’t differentiate between red and green) occur?
due to sequence similarity, its vry possible that in females during meiosis, there’s abnormal recombination between the 2 genes
this leads to chimeric genes
= 2 regions of red replace regions on the green gene (see onenote for diagrams)
this means the difference in sensitivity is reduced to 12nm
so they cant distinguish easily
what is dosage compensation
because females have 2 X and males only have 1
so females would have double the gene dosage
one of the 2 X chromosomes in female cells becomes inactivated (so the amount of genes become the same as a man with only 1)
what happens to the inactivated X chromosome
becomes highly condensed
doesn’t express genes
visible in interphase (usually shouldn’t see chromosomes in interphase) as Barr body
when does inactivation start in devleopment and what’s the process of switching off the X chromosome?
starts very early on
at beginning, both copies of X chromosome are expressed,
cells will decide randomly whether to switch off the paternal X or the maternal X
but once this is decided, then all subsequent daughter cells will switch off the same X
(so this happens with each cell)
why are female mammals described as mosaics
~50% of cells will only express the paternal X
~50% of cells express maternal X
(example of EPIGENETIC control)
how can we see X chromosome inactivation in tortoiseshell cats
only in females duh
theyre heterzygous for O (orange) o (black) alleles and these exist on the X chromosome
if chromosome with the O becomes inactivated, then only o allele is expressed (black fur)
and then vice versa
how could it be possible to have a male tortoishell cat?
if male cat has more than one X chromosome
(1/3000 males will have that)
= XXY male
if one of these X chromosomes is inactivated for either O or o then leads to colour difference
what happens in individuals that have mulitiple X chromosomes
each cell will only ever express one X chromosome
so no matter if its XX female, XXX female or even XXY male
only one X is expressed and the other X is condensed and forms Barr bodies
