SEX LINKAGE Chapter 7

Characters which are associate more with one gender Characters associated with gender, ex : 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)

Sex linkage explained Thomas Hunt Morgan in The Fly Room! (Columbia University 1910) Fruit Flies (Drosophila melanogaster)

The case of the white-eyed mutant CharacterTraits Eye colourRed eye (wild type) White eye (mutant) P Phenotypes Wild type (red-eyed) female x White-eyed male F 1 Phenotypes All red-eyed Red eye is dominant to white eye

Hypothesis A cross between the F 1 flies should give us: 3 red eye : 1 white eye F2F2 PhenotypesRed eyeWhite eye Numbers % % So far so good

An interesting observation F2F2 PhenotypesRed- eyed males Red- eyed females White- eyed males White- eyed females Numbers %58%18%0%

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

A test cross PhenotypesF1 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

Genetic diagram for sex linked genes CharacterTraitAlleles Eye colourRed eyeR White eyer GenotypesPhenotypes XRXRXRXrXrXrXRXRXRXrXrXr XRYXrYXRYXrY

Genetic diagrams for sex linked genes CharacterTraitAlleles Eye colourRed eyeR White eyer GenotypesPhenotypes XRXRXRXrXrXrXRXRXRXrXrXr Red-eyed female White-eyed female XRYXrYXRYXrY Red-eyed male White-eyed male

PPhenotypesWild type (red-eyed) female xWhite-eyed male GenotypesXRXRXRXR XrYXrY GametesXRXR XRXR XrXr Y FertilisationXrXr Y XRXR XRXrXRXr XRYXRY XRXR XRXrXRXr XRYXRY

F1F1 PhenotypesRed-eyed female xRed-eyed male GenotypesXRXrXRXr XRYXRY GametesXRXR XrXr XRXR Y FertilisationXRXR Y XRXR XRXRXRXR XRYXRY XrXr XRXrXRXr XrYXrY

F2F2 PhenotypesFemalesMales Red- eyed White- eyed Red- eyed White- eyed ExpectedAllNone50% Observed This gene has its LOCUS on the X-chromosome It is said to be SEX-LINKED

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

Tortioseshell Cats are Female

Daltonism = Red-Green Colourblindness Normal vision Colour blind simulation

LIGHTLIGHT 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 The retina

PHOTORECEPTION VISIONCOLOURMONOCHROME PHOTORECEPTORCONES: red sensitive 560nm green sensitive 530nm blue sensitive 420nm RODS: max. sensitivity 505nm DISTRIBUTIONConcentrated in the foveaWidely spread over whole retina, absent from fovea PIGMENTS3 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 BLEACHINGSlowFast (very sensitive) REGENERATIONSlow (after images in bright light, complementary colours) Fast USEDaylight vision Light adaptation 5 min Night vision Dark adaptation 20 min or wear red goggles!

Blood Clotting and Haemophilia A simplified scheme of the important steps Damaged blood vessels Prothrombin Inactive enzyme Thrombin Active enzyme Fibrinogen Globular protein Fibrin = Clot Fibrous protein

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) Antihaemophilic factor B Factor X (inactive)  Factor X (active) Factor II (inactive)  Factor II (active) ProthrombinThrombin Factor I (inactive)  Factor I (active) FibrinogenFibrin Factor III Thromboplastin released from blood vessel walls Factor VIII Antihaemophilic factor A Ca 2+ ions and blood platelets Vitamin K precursor

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

Heamophilia About 85% of haemophiliacs suffer from classic haemophilia (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 haemophilia are sex linked Haemophiliacs do clot their blood slowly because there is an alternative pathway via thromboplastin