Sex Linkage Genes linked to the sex chromosomes

Sex Linkage Genes linked to the sex chromosomes © 2016 Paul Billiet ODWS

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

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

Sex linkage explained http: //nobelprize. org/nobel_prizes/medicine/articles/lewis/index. html Thomas Hunt Morgan in The Fly Room! (Columbia University 1910) © 2016 Paul Billiet ODWS

Sex linkage explained Fruit Flies (Drosophila melanogaster) © 2016 Paul Billiet ODWS

Drosophila characteristics sex comb on the tarsus of a fore leg Female wild type © 2016 Paul Billiet ODWS Male vestigial wing

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 F 1 Phenotypes All red-eyed Red eye is dominant to white eye. © 2016 Paul Billiet ODWS

Hypothesis A cross between the F 1 flies should give us: 3 red eye : 1 white eye F 2 Phenotypes Numbers So far so good… © 2016 Paul Billiet ODWS Red eye White eye 3470 82% 782 18%

An interesting observation F 2 Phenotypes Redeyed males Redeyed females Whiteeyed males White-eyed females Numbers 1011 2459 782 0 24% 58% 18% 0% © 2016 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. © 2016 Paul Billiet ODWS

A test cross Phenotypes F 1 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

Genetic diagram for sex linked genes Character Trait Eye colour Genotypes X RX R X RX r X r X RY X r. Y © 2016 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 X RX R X RX r X r X RY X r. Y Red-eyed female White-eyed female Red-eyed male White-eyed male © 2016 Paul Billiet ODWS

Genetic diagrams for sex linked genes P Phenotypes Wild type (red-eyed) female Genotypes Gametes Fertilisation x White-eyed male X RX R XR X r. Y XR Xr Y XR X RX r X RY © 2016 Paul Billiet ODWS

Genetic diagrams for sex linked genes F 1 Phenotypes Red-eyed female Genotypes Gametes Fertilisation x Red-eyed male X RX r XR X RY Xr XR Y XR X RX R X RY Xr X RX r X r. Y © 2016 Paul Billiet ODWS

F 2 Phenotypes Females Males Redeyed Whiteeyed Expected All None 50% Observed 2459 0 1011 782 This gene has its LOCUS on the X-chromosome It is said to be SEX-LINKED. © 2016 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. © 2016 Paul Billiet ODWS

Tortioseshell Cats are Female © 2016 Paul Billiet ODWS

Daltonism = Red-Green Colourblindness Normal vision Colour blind simulation www. onset. unsw. edu. au/. . . /colourblindness. htm © 2016 Paul Billiet ODWS

LIGHT The retina Optic nerve fibres Ganglion layer Bipolar cells (neurones) Rod cell Synapse layer Nuclear layer Cone cell © 2016 Paul Billiet ODWS Inner segments packed with mitochondria Rod and cone outer segments

PHOTORECEPTION VISION COLOUR MONOCHROME PHOTORECEPTOR CONES: red sensitive 560 nm green sensitive 530 nm blue sensitive 420 nm RODS: max. sensitivity 505 nm 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! © 2016 Paul Billiet ODWS

Blood Clotting and Hemophilia A simplified scheme of the important steps Damaged blood vessels Thrombin Active enzyme Prothrombin Inactive enzyme Fibrinogen Globular protein © 2016 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) Antihemophilic factor B Factor VIII Antihemophilic factor A Factor X (inactive) Factor X (active) Ca 2+ ions and blood platelets Vitamin K precursor © 2016 Paul Billiet ODWS Factor II (inactive) Factor II (active) Prothrombin Thrombin Factor I (inactive) Factor I (active) Fibrinogen Fibrin

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

Hemophilia ¢ ¢ ¢ About 85% of hemophiliacs suffer from classic hemophilia (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 hemophilia are sex linked Hemophiliacs do clot their blood slowly because there is an alternative pathway via thromboplastin. © 2016 Paul Billiet ODWS

European Royals & Hemophilia Royal family pedigree © 2016 Paul Billiet ODWS