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

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

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

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%

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

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

Genetic diagram for sex linked genes

Character Trait Alleles

Eye colour Red eye R

White eye r

Genotypes Phenotypes

XRXR

XRXr

XrXr

XRY

XrY

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

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

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

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

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

http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm

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

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!

Blood Clotting and Haemophilia

A simplified scheme of the important steps

Damaged blood vessels

ProthrombinInactive enzyme

ThrombinActive enzyme

FibrinogenGlobular protein

Fibrin = ClotFibrous 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)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

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 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