SEX (Gender) LINKAGE Characters which are associated more with one gender © 2007 Paul Billiet...

SEX (Gender) LINKAGE

Characters which are associated more with one gender

© 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


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


Determining Trait Dominance

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


But wait… 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 color 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

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

Fertilization XR XR

Xr XRXr XRXr

Y XRY XRY© 2007 Paul Billiet ODWS

F1 Phenotypes Red-eyed female

x Red-eyed male

Genotypes XRXr XRY

Gametes XR Xr XR Y

Fertilization XR Xr

XR XRXR XRXr

Y XRY 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


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


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


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


SEX (Gender) LINKAGE Characters which are associated more with one gender © 2007 Paul Billiet...

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