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

mentioned GSD via a Maternal Effect system (for a blowfly) progeny: m/m, m/+, or +/+ progeny phenotype mutant progeny phenotype wildtype Genotype of mother determines (or at least influences) the Phenotype of the progeny Strict maternal effect (only mother can supply the needed + gene product) mother: m/m vs. m/+ determines

mentioned GSD via a Maternal Effect system (for a blowfly) progeny: m/m, m/+, or +/+ progeny phenotype mutant progeny phenotype wildtype progeny: m/m or m/+, or +/+ progeny phenotype mutantprogeny phenotype wildtype progeny: m/m, m/+, or +/+ progeny phenotype wildtype Genotype of mother determines (or at least influences) the Phenotype 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/m vs. m/+ mother: m/m vs. m/+ influences

Are male X-linked genes turned UP or are female X-linked genes turned DOWN? Giant Polytene Salivary-Gland Chromosomes X chromosome 2800 genes measure rate of RNA precursor incorporation into “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 UP relative 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 mutations that 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 consequences of loss by mutation: Female: XXMale: XY no X hyperactivationX hyperactivation That is how the relevant genes are recognized (MSLs encode protein complex on male X)

Fig (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 dosage compensation Sxl Constitutive is male-specific lethal Sxl null is female-specific lethal

What about worms? (C. elegans) XX AAX 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)

How do we mammals dosage compensate? One Barr Body No Barr Body XO AA Turner females XXY AA Kleinfelter males XXXX AA (mentally retarded) females No Barr Body One Barr Body Three Barr Bodies XY AA femalesmales XX AA “sex chromatin” First clue: #BB = #X-1 Barr Body rule:

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 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 XY AA males females Xx AA females Xxxx AA (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 X maternal X paternal (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 - PHENOTYPIC MOSAICS 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

X 2 mat X 2 pat 50:50 mat vs. pat active X 1 mat X 2 pat X 2 mat X 1 pat Mapping the source of the inactivation bias defined Xce the genetics of the X controlling element X 1 mat X 1 pat 50:50 mat vs. pat active 65:35 mat (1) vs. pat(2) active 35:65 mat (2) vs. pat(1) active

almost all genes in cis shut off Study of T(X,A)s in mouse tissue-culture cells defined the Xic (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 X c+c+

The source of a very odd RNA : X-inactivation-specific-transcript: X ist (number of) (s) …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 X X X X

X X Consequences of deleting Xic (source of Xist): always the active X Xist RNA X X

Xist transgene (inducible) on autosome will coat autsome with Xist and silence it …but only during an early window of time