Transcript Powerpoint Presentation: Sex Linkage

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
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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