A second practice problem set(with answers)

is on the course website.

The review session for the second midtermis on Thursday evening, April 10, from 7-9pm

in ROOM 141 GIANNINI HALL

mentioned GSD via a Maternal Effect system(for a blowfly)

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

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

progenyphenotype

mutant

progenyphenotypewildtype

Genotype of mother

determines (or at least influences) thePhenotype of the progeny

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

mother: m/mvs.m/+

determines

mentioned GSD via a Maternal Effect system(for a blowfly)

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

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

progenyphenotype

mutant

progenyphenotypewildtype

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

progeny phenotype mutant

progeny phenotype wildtype

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

progenyphenotypewildtype

Genotype of mother

determines (or at least influences) thePhenotype of the progeny

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

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

mother: m/mvs.m/+

mother: m/mvs.m/+

influences

Are male X-linked genes turned UPor are

female X-linked genes turned DOWN?

Giant Polytene Salivary-Gland Chromosomes

X chromosome 2800 genes

measure rate of RNA precursor incorporationinto “nascent” transcripts (during interphase)

…average transcription rates (per unit DNA)

X-chromosome dosage compensation in Drosophila:

transcription rate for Male X-linked genes are turned UPrelative to autosomal or female X-linked genes

X chromosome 2800 genes

average transcription rates (per unit DNA):

female X = female autosomes = male autosomes < male X

What phenotype would one expect for mutationsthat disrupted genes that encode the machinery

for X-chromosome dosage compensation?

needed (only) for hyper

needed (only) to prevent hyper

male (X:A=0.5)-specific lethal

female (X:A=1)-specific lethal

Normal gene function: Phenotypic consequencesof loss by mutation:

Female: XX Male: XY

no X hyperactivation X hyperactivation

That is how the relevant genes are recognized

(MSLs encode protein complex on male X)

Fig. 18.23 (p675)

Among the genetic pathways that control development,those controlling sexual development are perhaps the best understood.

Sxl controls sex determination; dsx controls sexual dimorphism

also dosagecompensation

SxlConstitutive is male-specific lethal

Sxlnull is female-specific lethal

What about worms? (C. elegans)

XX AA X AA

hermaphrodites(females that make sperm)

males

(1) both hermaphrodite X’s active

(3) at the transcriptional level

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

(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

(like the fly)

(like the fly)

(like the fly)

How do we mammals dosage compensate?

OneBarr Body

NoBarr Body

XOAA Turner females

XXYAA

Kleinfelter males

XXXXAA (mentally retarded) females

No Barr Body

One Barr Body

Three Barr Bodies

XYAA

females males

XXAA

“sex chromatin”First clue:

#BB = #X-1

Barr Body rule:

Odd behavior of an X-linked mammalian gene:

Individual blood cells arephenotypically either

G6PD+ or G6PD-

only one or the other X-linked allele seems to be active

in any given blood cell

Another clue:

G6PD+/G6PD-:heterozygote

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

Geneticist Mary Lyon:

#BarrBodies = #X-1

Observations:

Hypothesis:

(2) Dosage compensation by inactivation of all but one X chromosome

XYAA

malesfemales

XxAA

females

XxxxAA

(1) Barr Body = inactivated X chromosome

x

Barr Body

mosaic expression of G6PD+ (on X)

mosaic c+ expression when c+ on X

(translocation of autosomal coat color gene c to X)

X-chromosome inactivation:

(1) initiated very early in development

(2) generally random in embryo proper (paternal = maternal)

(3) once initiated, stably inherited

(4) reactivation of inactivated X occurs in germ cells during oogenesis

Xmaternal Xpaternal

(often paternal in extra-embryonic)

an epigenetic phenominon

(at ~500 cell stage in humans)

Striking human example of X inactivation in action:

Anhidrotic Ectodermal Dysplasia (EDA):

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

no sweat glands (incl. breasts)

missing & abnormal teeth/hair

EDA+/EDA-

PHENOTYPICMOSAICS

cell autonomous trait

Identical twins:

little skin cell mixing during developmentPatchiness signifies

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

For X-linked genes: If a+/a- mammals are functional mosaics of a+ & a- cells

NO

(1) perhaps not cell autonomous (and 50% a+ function is sufficient for normal phenotype)

(2) perhaps cell autonomous, but deleterious early--- abnormal cells selected against (they may be outcompeted by normal cells)

how is a phenotype related to a+ gene expression?Need to know for gene a:

(dominance depends on how phenotype is operationally defined)

consider hemophilias

Most animals compensate well for cells lost during development

X2matX2pat 50:50 mat vs. pat active

X1matX2pat

X2matX1pat

Mapping the source of the inactivation bias defined Xce

the genetics of the X controlling element

X1matX1pat 50:50 mat vs. pat active

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

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

almost allgenes in cis

shut off

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

(“source” of inactivation in cis)

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

Xce (controlling element)(determines chromosomal inactivation competativeness)

Xic = Xce

Xc+

Xic = Xce

The source of a very odd RNA :

X-inactivation-specific-transcript:

Xist

(number of) (s)

…a non-coding RNA from the inactive X that coatsthe inactive X chromosome in cis

one of the first examples of a regulatory RNA

X

X

X

X

X

X

X

X

Consequences of deleting Xic (source of Xist):

always the active X

Xist RNA

X

X

Xist RNA

Xist transgene (inducible) on autosomewill coat autsome with Xist and silence it

…but only during an early window of time