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A second practice problem set (with answers) is on the course website. The review session for the second midterm is on Thursday evening, April 10, from 7-9pm in ROOM 141 GIANNINI HALL. determines . G enotype of mother. progeny phenotype mutant.

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A second practice problem set

(with answers)

is on the course website.

The review session for the second midterm

is on Thursday evening, April 10, from 7-9pm

in ROOM 141 GIANNINI HALL


determines

Genotype of mother

progeny

phenotype

mutant

determines (or at least influences) the

m/m

vs.

m/+

progeny: m/m, m/+, or +/+

mother:

Phenotype of the progeny

progeny

phenotype

wildtype

progeny: m/m, m/+, or +/+

Strict maternal effect (only mother can supply the needed + gene product)

(for a blowfly)

mentioned GSD via a Maternal Effect system


influences

Genotype of mother

progeny

phenotype

mutant

determines (or at least influences) the

m/m

vs.

m/+

m/m

vs.

m/+

progeny: m/m, m/+, or +/+

mother:

mother:

Phenotype of the progeny

progeny

phenotype

wildtype

progeny

phenotype

wildtype

progeny: m/m, m/+, or +/+

Strict maternal effect (only mother can supply the needed + gene product)

progeny phenotypemutant

m/m

or

m/+, or +/+

progeny:

progeny phenotypewildtype

progeny: m/m, m/+, or +/+

Rescuable maternal effect (either mother or progeny can supply + gene product)

(for a blowfly)

mentioned GSD via a Maternal Effect system


X chromosome

2800 genes

X-chromosome dosage compensation in Drosophila:

Are male X-linked genes turned UP

or are

female X-linked genes turned DOWN?

Giant Polytene Salivary-Gland Chromosomes

measure rate of RNA precursor incorporation

into “nascent” transcripts (during interphase)

…average transcription rates (per unit DNA)


average transcription rates (per unit DNA):

female X = femaleautosomes = maleautosomes< male X

X chromosome

2800 genes

transcription rate for Male X-linked genes are turned UP

relative to autosomal or female X-linked genes


Female: XX

Male: XY

noX hyperactivation

X hyperactivation

Phenotypic consequences

of loss by mutation:

Normal gene function:

What phenotype would one expect for mutations

that disrupted genes that encode the machinery

for X-chromosome dosage compensation?

needed (only) for hyper

male (X:A=0.5)-specific lethal

needed (only) to preventhyper

female (X:A=1)-specific lethal

That is how the relevant genes are recognized

(MSLs encode protein complex on male X)


Fig. 18.23 (p675)

Sxlnullis female-specific lethal

SxlConstitutiveis male-specific lethal

also dosage

compensation

Among the genetic pathways that control development,

those controlling sexual development are perhaps the best understood.

Sxl controls sex determination; dsx controls sexual dimorphism


XX AA

X AA

hermaphrodites

(females that make sperm)

males

What about worms? (C. elegans)

(1) both hermaphrodite X’s active

(like the fly)

(2) male X twice as active as each hermaph. X

(like the fly)

(3) at the transcriptional level

(like the fly)

(4) hermaph.-specific lethal genes encode protein complex

on hermaphrodite X’s that turns transcription down

fly male-specific lethal genes encode protein complex on male X that turns it up


XY

AA

XX

AA

First clue:

“sex chromatin”

females

males

Barr Body rule:

One

Barr Body

No

Barr Body

#BB = #X-1

XO

AA

Turner females

XXY

AA

Kleinfelter males

XXXX

AA

(mentally retarded) females

How do we mammals dosage compensate?

NoBarr Body

OneBarr Body

ThreeBarr Bodies


G6PD+/G6PD-:

heterozygote

Another clue:

Odd behavior of an X-linked mammalian gene:

Individual blood cells are

phenotypically either

G6PD+or G6PD-

only one or the other X-linked allele

seems to be active

in any given blood cell

not what we saw with the eye of the w+/w-fly


XY

AA

Xx

AA

females

males

mosaic c+ expression whenc+ on X

(translocation of autosomal coat color gene c to X)

(1) Barr Body = inactivated X chromosome

Xxxx

AA

x

females

Barr Body

Geneticist Mary Lyon:

#BarrBodies= #X-1

mosaic expression of G6PD+(on X)

Observations:

Hypothesis:

(2) Dosage compensation by inactivation

of all but one X chromosome


X-chromosome inactivation:

XmaternalXpaternal

(1) initiated very early in development

(at ~500 cell stage in humans)

(2) generally random in embryo proper

(paternal = maternal)

(often paternal in extra-embryonic)

(3) once initiated, stably inherited

an epigenetic phenominon

(4) reactivation of inactivated X

occurs in germ cells during oogenesis


hemizygous males (EDA-/Y) & homozygous females (EDA-/EDA-)

no sweat glands (incl. breasts)

missing & abnormal teeth/hair

EDA+/EDA-

PHENOTYPIC

MOSAICS

Identical twins:

Patchiness signifies

little skin cell mixing during development

Striking human example of X inactivation in action:

Anhidrotic Ectodermal Dysplasia (EDA):

cell autonomous trait


Need to know for gene a:

how is a phenotype related to a+ gene expression?

(2) perhaps cell autonomous, but deleterious early

--- abnormal cells selected against

(they may be outcompeted by normal cells)

For X-linked genes:

If a+/a-mammals are functional mosaics of a+ & a- cells

…are all non-functional X-linked alleles (a-) semi-dominant?

(dominance depends on how phenotype is operationally defined)

NO

(1) perhaps not cell autonomous

(and 50% a+ function is sufficient for normal phenotype)

consider hemophilias

Most animals compensate well for cells lost during development


X1matX1pat

50:50mat vs. pat active

the genetics of the X controlling element

X2matX2pat50:50 mat vs. pat active

65:35mat(1) vs. pat(2) active

X1matX2pat

X2matX1pat

35:65mat(2) vs. pat(1) active

Mapping the source of the inactivation bias definedXce


c+

almost all

genes in cis

shut off

X

Study of variations in “X inactivation strength” (in whole mice)

defined the

Xce (controlling element)

(determines chromosomal inactivation competativeness)

Study of T(X,A)s in mouse tissue-culture cells defined the

Xic (inactivation center)

(“source” of inactivation in cis)

Xic = Xce


(s)

(number of)

Xist

X-inactivation-specific-transcript:

X

X

X

X

X

X

Xic = Xce

The source of a very odd RNA :

…a non-coding RNA from the inactive X that coats

the inactive X chromosome in cis

one of the first examples of a regulatory RNA


X

X

Xist RNA

X

Xist RNA

X

Consequences of deleting Xic (source of Xist):

always the active X


Xist transgene (inducible) on autosome

will coat autsome with Xist and silence it

…but only during an early window of time


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