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SEX LINKAGE Characters which are associate more with one gender © 2007 Paul Billiet ODWS Characters associated with gender Anhiorotic ectodermal dysplasia Small teeth, no sweat glands, sparse body hair Occurs primarily in men Never transmitted from father to son Unaffected daughters may pass the condition onto their sons (the grandsons) © 2007 Paul Billiet ODWS Sex linkage explained http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html Thomas Hunt Morgan in The Fly Room! (Columbia University 1910) Fruit Flies (Drosophila melanogaster) © 2007 Paul Billiet ODWS The case of the white-eyed mutant Character Eye colour Traits Red eye (wild type) White eye (mutant) P Phenotypes Wild type (red-eyed) female x White-eyed male F1 Phenotypes All red-eyed Red eye is dominant to white eye © 2007 Paul Billiet ODWS Hypothesis A cross between the F1 flies should give us: 3 red eye : 1 white eye F2 Phenotypes Numbers So far so good © 2007 Paul Billiet ODWS Red eye White eye 3470 82% 782 18% An interesting observation F2 Phenotypes Redeyed males Redeyed females Whiteeyed males White-eyed females Numbers 1011 2459 782 0 24% 58% 18% 0% © 2007 Paul Billiet ODWS A reciprocal cross Morgan tried the cross the other way round white-eyed female x red-eyed male Result All red-eyed females and all white-eyed males This confirmed what Morgan suspected The gene for eye colour is linked to the X chromosome © 2007 Paul Billiet ODWS A test cross Phenotypes F1 Red-eyed female x White-eyed male Expected result 50% red-eyed offspring: 50% white-eyed offspring Regardless of the sex Observed Results Red-eyed Males Red-eyed Females White-eyed Males White-eyed Females 132 129 86 88 © 2007 Paul Billiet ODWS Genetic diagram for sex linked genes Character Trait Eye colour Genotypes XRXR XRXr XrXr XRY XrY © 2007 Paul Billiet ODWS Alleles Red eye R White eye r Phenotypes Genetic diagrams for sex linked genes Character Trait Eye colour Alleles Red eye R White eye r Genotypes Phenotypes XRXR XRXr XrXr XRY XrY Red-eyed female Red-eyed female White-eyed female Red-eyed male White-eyed male © 2007 Paul Billiet ODWS P Phenotypes Wild type (red-eyed) female XRXR Genotypes Gametes XR XR x White-eyed male XrY Xr Y Xr Y XR XRXr XRY XR XRXr XRY Fertilisation © 2007 Paul Billiet ODWS F1 Phenotypes Red-eyed female Red-eyed male XRXr Genotypes Gametes x XR XRY Xr XR Y XR Y XR XRXR XRY Xr XRXr XrY Fertilisation © 2007 Paul Billiet ODWS F2 Phenotypes Expected Observed Females Males Redeyed Whiteeyed Redeyed Whiteeyed All 2459 None 0 50% 1011 50% 782 This gene has its LOCUS on the X-chromosome It is said to be SEX-LINKED © 2007 Paul Billiet ODWS X-linked genes In sex linked characteristics the reciprocal crosses do not give the same results For X-linked genes fathers do not pass the mutant allele onto their sons For X-linked genes fathers pass the mutant allele onto their daughters who are carriers Carrier mothers may pass the allele onto their sons (50% chance) Females showing the trait for an X-linked mutant allele can exist but they are rare Female carriers may show patches of cells with either trait due to X chromosome inactivation © 2007 Paul Billiet ODWS Tortioseshell Cats are Female © 2007 Paul Billiet ODWS Daltonism = Red-Green Colourblindness Normal vision Colour blind simulation http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm © 2007 Paul Billiet ODWS LIGHT The retina Optic nerve fibres Ganglion layer Bipolar cells (neurones) Synapse layer Rod cell Nuclear layer Cone cell © 2007 Paul Billiet ODWS Inner segments packed with mitochondria Rod and cone outer segments PHOTORECEPTION VISION COLOUR MONOCHROME PHOTORECEPTOR CONES: red sensitive 560nm green sensitive 530nm blue sensitive 420nm RODS: max. sensitivity 505nm DISTRIBUTION Concentrated in the fovea Widely spread over whole retina, absent from fovea PIGMENTS 3 proteins controlled by 3 genes. RHODOPSIN = Retinol Red and green pigments sex linked (Vit A) + Opsin (a protein). Blue pigment autosomal (Chr.7) Also called visual purple BLEACHING Slow Fast (very sensitive) REGENERATION Slow (after images in bright light, complementary colours) Fast USE Daylight vision Light adaptation 5 min Night vision Dark adaptation 20 min or wear red goggles! © 2007 Paul Billiet ODWS Blood Clotting and Haemophilia A simplified scheme of the important steps Damaged blood vessels Thrombin Active enzyme Prothrombin Inactive enzyme Fibrinogen Globular protein © 2007 Paul Billiet ODWS Fibrin = Clot Fibrous protein Contact with collagen fibres in blood vessels Factor III Thromboplastin released from blood vessel walls Factor XII (inactive) Factor XII (active) Factor XI (inactive) Factor XI (active) Factor IX (inactive) Factor IX (active) Antihaemophilic factor B Factor VIII Antihaemophilic factor A Factor X (inactive) Factor X (active) Ca2+ ions and blood platelets Vitamin K precursor © 2007 Paul Billiet ODWS Factor II (inactive) Factor II (active) Prothrombin Thrombin Factor I (inactive) Factor I (active) Fibrinogen Fibrin The antihaemophilic factors The blood clotting reaction is an enzyme cascade involving Factors XII, XI, IX, X and II Each of these enzymes are proteases that cut the next protein in line Other factors including proteins like Factor VIII are essential as coenzymes © 2007 Paul Billiet ODWS Heamophilia About 85% of haemophiliacs suffer from classic haemophilia (1 male in 10 000) They cannot produce factor VIII The rest show Christmas disease where they cannot make factor IX The genes for both forms of haemophilia are sex linked Haemophiliacs do clot their blood slowly because there is an alternative pathway via thromboplastin © 2007 Paul Billiet ODWS
