Sex determination and sex linkage

Question 1

what are non- sex chromosomes called?

Answer 1

autosomes

Question 2

approx how many Mb are each sex chromosome

Answer 2

X = ~160Mb

Y= ~ 60Mb

Question 3

approx how many genes on each ‘specific region’ on the 2 sex chromosomes

Answer 3

X specific = >1000genes

Y (or male) specific region (MSY)= 80 genes

both of these have no sequence similarity between them unlike normal autosomes

Question 4

what is the pseudoautosomal region? (PAR)

Answer 4

regions with sequence similarity on both X and Y

on the tips of each chromosome
~20genes in humans shared

Question 5

why are pseudoautosomal genes important

Answer 5

required for X-Y pairing in male meiosis

Question 6

why are males called hetereogametic and females called homogametic

Answer 6

males can produce gametes with either X or Y
females can only do gametes with X

Question 7

how does aneuploidy give us evidence that Y = maleness

Answer 7

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

Question 8

is ALL of the Y chromosome required to confer maleness?

Answer 8

lol no
only one gene needed = SRY
stands for sex determining region on the Y

Question 9

how do we know that SRY = determining gene?

Answer 9

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

Question 10

how does SRY gene cause maleness

Answer 10

it acts as a transcription factor
so is needed to regulate expression of the genes necassary for testis formation

Question 11

explain the development of the ovaries and testis and how SRY is the key determinent

Answer 11

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

Question 12

what is the genetic sex determination in birds?

Answer 12

ZW system

males = ZZ
females = ZW

Question 13

what is a gynandromorph

Answer 13

a sexual mosaic

e.g. that chicken thats half female ZW cells and half male ZZ cells

Question 14

what is cell autonomous sex identity

Answer 14

in birds
each cell determines its sex depedning on its chromosomes

in mammals, sex hormones will determine whether it’s male or female tissue

Question 15

what type of chromosome were the X and Y originally and how did this evolve into our current X and Y chromosomes

Answer 15

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)

Question 16

what were the 2 phenotypes of the Drosophila used in Morgan’s experiments

Answer 16

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

Question 17

what would a reciprocal cross usually show acording to mendel

Answer 17

reciprocal for example is 2 crosses
1 cross = white male x red female
2 cross = red female x white male

should give same results

Question 17

what would a reciprocal cross usually show acording to mendel

Answer 17

reciprocal for example is 2 crosses
1 cross = white male x red female
2 cross = red female x white male

should give same results

Question 18

see onenote for diagram on Morgan’s results

Answer 18

:)

Question 19

what could we conclude from morgans results in terms of where the mutation lies for the white eye

Answer 19

white is a recessive mutation that lies on the X chromosome

Question 20

which chromosome does sex linked inheritance usually occur on

Answer 20

X chromosome - (X-linkage)

Question 21

what could Y-linked genes be related to

Answer 21

usually have male specific functions (e.g. spermatogenesis)
but this is a vry small % of sex linkage

Question 22

what is hemizygous

Answer 22

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

Question 23

why are males more affected by X linked disease

Answer 23

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)

Question 24

approx how many known X linked disease genes are there and give examples

Answer 24

~300
Haemophilia A and B (affects blood clotting factors)

Duchenne muscular dystrophy

Red-green colour vision deficiency (8% males have it)

Question 25

what is the name of the genes that control photoreceptors

Answer 25

opsin genes

Answer 26

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

Question 27

what’s the positioning of the opsin genes on the X chromosome

Answer 27

right next to each other
red perceptor gene (L)
green perceptor gene (M)

Question 28

how many nm of difference is there between the perception of red wavelength and the green wavelength usually

Answer 28

30nm

Question 29

describe the sequence similarity between the L and M gene

Answer 29

very similar
2 were originally 1 single gene
and at some point in evo, they split and diverged to be sensitive to different wavelength

Question 30

how is deuteranomaly (can’t differentiate between red and green) occur?

Answer 30

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

Question 31

what is dosage compensation

Answer 31

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)

Question 32

what happens to the inactivated X chromosome

Answer 32

becomes highly condensed
doesn’t express genes
visible in interphase (usually shouldn’t see chromosomes in interphase) as Barr body

Question 33

when does inactivation start in devleopment and what’s the process of switching off the X chromosome?

Answer 33

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)

Question 34

why are female mammals described as mosaics

Answer 34

~50% of cells will only express the paternal X
~50% of cells express maternal X

(example of EPIGENETIC control)

Question 35

how can we see X chromosome inactivation in tortoiseshell cats

Answer 35

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

Question 36

how could it be possible to have a male tortoishell cat?

Answer 36

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

Question 37

what happens in individuals that have mulitiple X chromosomes

Answer 37

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