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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
Thomas Hunt Morgan in The Fly Room! (Columbia University 1910)
Fruit Flies (Drosophila melanogaster)
http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html
© 2007 Paul Billiet ODWS
The case of the white-eyed mutant
Character TraitsEye colour Red eye (wild type)
White eye (mutant)
P PhenotypesWild 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 Red eye White eye
Numbers 3470
82%
782
18%
So far so good
© 2007 Paul Billiet ODWS
An interesting observation
F2 Phenotypes Red-eyed males
Red-eyed
females
White-eyed 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 roundwhite-eyed female x red-eyed male
ResultAll red-eyed females and all white-eyed males
This confirmed what Morgan suspectedThe 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 Alleles
Eye colour Red eye R
White eye r
Genotypes Phenotypes
XRXR
XRXr
XrXr
XRY
XrY© 2007 Paul Billiet ODWS
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
Red-eyed female
White-eyed female
XRY
XrY
Red-eyed male
White-eyed male © 2007 Paul Billiet ODWS
P Phenotypes Wild type (red-eyed)
female
x White-eyed male
Genotypes XRXR XrY
Gametes XR XR Xr Y
Fertilisation Xr Y
XR XRXr XRY
XR XRXr XRY
© 2007 Paul Billiet ODWS
F1 Phenotypes Red-eyed female
x Red-eyed male
Genotypes XRXr XRY
Gametes XR Xr XR Y
Fertilisation XR Y
XR XRXR XRY
Xr XRXr XrY
© 2007 Paul Billiet ODWS
F2 Phenotypes Females Males
Red-eyed
White-eyed
Red-eyed
White-eyed
Expected All None 50% 50%
Observed 2459 0 1011 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
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
© 2007 Paul Billiet ODWS
PHOTORECEPTION
VISION COLOUR MONOCHROME
PHOTORECEPTOR CONES:red sensitive 560nmgreen sensitive 530nmblue 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 linkedBlue 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!© 2007 Paul Billiet ODWS
Blood Clotting and Haemophilia
A simplified scheme of the important steps
Damaged blood vessels
ProthrombinInactive enzyme
ThrombinActive enzyme
FibrinogenGlobular protein
Fibrin = ClotFibrous protein
© 2007 Paul Billiet ODWS
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)Prothrombin Thrombin
Factor I (inactive) Factor I (active)Fibrinogen Fibrin
Factor IIIThromboplastin released from blood vessel walls
Factor VIIIAntihaemophilic factor A
Ca2+ ions and blood platelets
Vitamin K precursor
© 2007 Paul Billiet ODWS
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