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Genes linked to the sex chromosomes
Sex Linkage Genes linked to the sex chromosomes © 2016 Paul Billiet ODWS
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Linked genes Genes for different characters inherited as if they were a single character These genes are called LINKED GENES That is the LOCI for these genes are linked on the same chromosome. © 2016 Paul Billiet ODWS
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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). © 2016 Paul Billiet ODWS
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The Fly Room! Thomas Hunt Morgan in Sex linkage explained
(Columbia University 1910) Thomas Hunt Morgan in The Fly Room! © 2016 Paul Billiet ODWS
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Sex linkage explained Fruit Flies (Drosophila melanogaster)
© 2016 Paul Billiet ODWS
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Drosophila characteristics
sex comb on the tarsus of a fore leg Female wild type Male vestigial wing © 2016 Paul Billiet ODWS
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The case of the white-eyed mutant
Character Traits Eye colour 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. © 2016 Paul Billiet ODWS
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Hypothesis A cross between the F1 flies should give us: 3 red eye : 1 white eye F2 Phenotypes Red eye White eye Numbers 3470 82% 782 18% So far so good… © 2016 Paul Billiet ODWS
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An interesting observation
F2 Phenotypes Red-eyed males Red-eyed females White-eyed males White-eyed females Numbers 1011 2459 782 24% 58% 18% 0% © 2016 Paul Billiet ODWS
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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. © 2016 Paul Billiet ODWS
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A test cross Red-eyed Males Red-eyed Females White-eyed Males
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 © 2016 Paul Billiet ODWS
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Genetic diagram for sex linked genes
Character Trait Alleles Eye colour Red eye R White eye r Genotypes Phenotypes XRXR XRXr XrXr XRY XrY © 2016 Paul Billiet ODWS
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Genetic diagrams for sex linked genes
Character Trait Alleles Eye colour Red eye R White eye r Genotypes Phenotypes XRXR XRXr XrXr Red-eyed female White-eyed female XRY XrY Red-eyed male White-eyed male © 2016 Paul Billiet ODWS
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Genetic diagrams for sex linked genes
P Phenotypes Wild type (red-eyed) female x White-eyed male Genotypes XRXR XrY Gametes XR Xr Y XR Y Xr Fertilisation XRXr XRY XRXr XRY © 2016 Paul Billiet ODWS
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Genetic diagrams for sex linked genes
Phenotypes Red-eyed female x Red-eyed male Genotypes XRXr XRY Gametes XR Xr Y Xr XR Y Fertilisation XRXR XRY XRXr XrY © 2016 Paul Billiet ODWS
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This gene has its LOCUS on the X-chromosome
F2 Phenotypes Females Males Red-eyed White-eyed Expected All None 50% Observed 2459 1011 782 This gene has its LOCUS on the X-chromosome It is said to be SEX-LINKED. © 2016 Paul Billiet ODWS
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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. © 2016 Paul Billiet ODWS
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Tortioseshell Cats are Female
© 2016 Paul Billiet ODWS
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Daltonism = Red-Green Colourblindness
Normal vision Colour blind simulation © 2016 Paul Billiet ODWS
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The retina LIGHT Optic nerve fibres Ganglion layer
Bipolar cells (neurones) Synapse layer Nuclear layer Inner segments packed with mitochondria Rod and cone outer segments Rod cell Cone cell © 2016 Paul Billiet ODWS
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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. Red and green pigments sex linked Blue pigment autosomal (Chr.7) RHODOPSIN = Retinol (Vit A) + Opsin (a protein). 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! © 2016 Paul Billiet ODWS
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Blood Clotting and Hemophilia
A simplified scheme of the important steps Damaged blood vessels Prothrombin Inactive enzyme Thrombin Active enzyme Fibrinogen Globular protein Fibrin = Clot Fibrous protein © 2016 Paul Billiet ODWS
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Antihemophilic factor B Antihemophilic factor A
Contact with collagen fibres in blood vessels Factor XII (inactive) Factor XII (active) Factor XI (inactive) Factor XI (active) Factor IX (inactive) Factor IX (active) Antihemophilic factor B Factor X (inactive) Factor X (active) Factor II (inactive) Factor II (active) Prothrombin Thrombin Factor I (inactive) Factor I (active) Fibrinogen Fibrin Factor III Thromboplastin released from blood vessel walls Factor VIII Antihemophilic factor A Ca2+ ions and blood platelets Vitamin K precursor © 2016 Paul Billiet ODWS
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The antihemophilic 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. © 2016 Paul Billiet ODWS
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Hemophilia About 85% of hemophiliacs suffer from classic hemophilia (1 male in ) They cannot produce factor VIII The rest show Christmas disease where they cannot make factor IX The genes for both forms of hemophilia are sex linked Hemophiliacs do clot their blood slowly because there is an alternative pathway via thromboplastin. © 2016 Paul Billiet ODWS
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European Royals & Hemophilia
Royal family pedigree © 2016 Paul Billiet ODWS
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