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Unformatted text preview: Bizarre Deviants! Non-Mendelian inheritance Pt. 1 1 Last Time
Breaking the Law of Independent Assortment What is linkage? Creating maps Frequency of recombination Determining the order of genes on chromosome 2 Bizarre deviants!
The Maternal Effect Genetic examples Molecular basis for inheritance Maternal effect in ﬂies Epigenetic Inheritance Dosage compensation Molecular basis for dosage compensation
3 Maternal Effect
Mom’s genes count, dad’s and offspring’s genes don’t Classic example: direction of coiling of snail shells http://imagecache2.allposters.com/images/ pic/HAD/5705~Maternal-AffectionPosters.jpg 4 Shells can be left or right coiled
P F1 5 Reciprocal cross
P F1 http://ﬂoridasoftwater.com/imgs/services/Confused_Guy.jpg 6 “Is coiling direction due to a maternal effect?”
Hypothesis: If Mom is D, then offspring are dextral If Mom is d, then offspring are sinistral Dad’s genes have no effect on phenotype Alfred Sturtevant http://www.nature.com/ng/journal/v34/n3/images/ng0703-242-I1.jpg 7 Is the hypothesis supported? YES!
F1 Dd F2 Dd F3 3:1 dextral: sinistral
8 The maternal effect mechanism 9 But how does this affect body plan?
D protein product - controls orientation of mitotic spindle at 2 cell stage. Dextral Sinistral
10 Maternal effect genes: bicoid
Bicoid - Drosophila gene that speciﬁes “front” (anterior) from back (posterior) anteroposterior axis dorsoventral axis 11 Fly development
Morphological changes during ﬂy development Egg Larvae Adult
12 Bicoid expression pattern RNA localization Protein localization 13 bicoid mutants
Lack of bicoid - no anterior speciﬁed 14 Tips for detecting the maternal effect
Do reciprocal crosses! Examine phenotype of offspring relative to the genotype of the mother dd DD DD dd
15 Epigenetic inheritance
Genes are turned ‘off’ in a way that permanently alters their expression in an individual Two kinds of epigenetic inheritance: 1. dosage compensation (today) 2. genomic imprinting (next time) 16 How is this possible? www.pbs.org 17 Dosage compensation
Mammals: XX XY Flies: XX XY
http://loeppert.net/images/ﬂy_icon.jpg http://www.serendipitykids.ie/shop/images/girl%20icon.jpg 18 random X inactivation paternal X inactivation http://www.alerttoday.com/wp-content/uploads/2007/12/wonder-woman.jpg http://imagecache2.allposters.com/images/pic/PTGPOD/541447~Australian-Kangaroo-Posters.jpg 19 X-inactivation Barr body = condensed X chromosome inactive active 20 Female mammals are functionally mosaic
One of two X chromosomes becomes randomly and irreversibly inactivated during early development and is stably inherited by daughter cells 21 Both X chromosomes are active early in development (8 cell stage) One X is inactivated
22 How is X inactivated?
Normal female Normal male Turner Syndrome (female) Triple X Syndrome (female) Klinefelter Syndrome (male) Chromosomes XX XY X0 # X’s 2 1 1 # Barr bodies 1 0 0 XXX XXY 2 2 2 2
23 X-inactivation center (XIC) XIST RNA http://spot.colorado.edu/~strauswm/images/Xist_Bodies.gif 24 X-inactivation center (XIC) Tsix is only expressed from active X chromosome Xist is only expressed from inactive X chromosome
25 X-inactivation center (XIC)
TF? X active (strong) X inactive (weak) 26 Genes on inactive X are not completely ‘off’ pseudoautosomal genes: Located on both Y and X Regions of homology Region of pairing during meiosis 27 Summary
The Maternal Effect - offspring express phenotype of mother regardless of offspring’s genotype Result of diploid nurse cells, which load protein and RNA into the egg cell Maternally loaded bicoid RNA speciﬁes anterior end of developing ﬂy embryo Epigenetic Inheritance - modiﬁed genes change expression (and phenotype) X-inactivation (dosage compensation) results in phenotypically mosaic females
28 Ungraded problems
Go through Chapter 7 S1 Try Chapter 7 C2, C4 Try Chapter 7 E1, E6 29 Next Time
Bizarre Deviants Pt2: More epigenetic inheritance Extranuclear inheritance http://www.willowaynurseries.com/Images/EuonymusGoldSplash.jpg 30 ...
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