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Powerpoint Presentation: Sex Linkage

 

Genetics Index

Chromosomes
Meiosis
Meiosis: Crossing over
Mitosis and Meiosis
Introduction to Mendelian Genetics
Test Cross
Codominance
Multiple Alleles
Pedigree Charts
Twin Studies
The dihybrid cross
Dihybrid Cross : Test Cross
Autosomal Linkage
The Genetic Diagram for Linked Genes
Calculating the cross over value using a test cross
Genetic diagram for sex linked genes
Blood Clotting and Haemophilia
The Retina and Daltonism
Genetic Modification
Cloning Animals
Cloning Plants

Topic Chapters Index

 

1882

Cell division (mitosis) observed for the first time.

1888

Chromosome ("coloured bodies") observed in the nucleus.

1891

Insect sperm cells found to contain an extra structure in half of their nuclei. Called the "X-body".

1902

Female grasshoppers had one chromosome more than the males.

Sex Linkage

1910 Morgan: Working on Fruit Flies (Drosophila melanogaster)

Morgan: Working on Fruit Flies

Morgan: Working on Fruit Flies

This result tells us that red eye is dominant to white eye.

A cross between the F1 flies should give us: 3 red eye : 1 white eye

F1 cross

A test cross was necessary to find out the true nature of the F1 females.

test cross

Expected result: 1 red-eyed offspring: 1 white-eyed offspring regardless of the sex.

Observed Results

observed results

Hypothesis: The gene for eye colour is carried on the X-chromosome only.

 

 

GENETICS

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Sex determination and sex linkage

For many populations to successfully breed it is desirable to have a sex ratio of 1 male birth to 1 female birth.

The sex of many animals is not determined by a single gene but by whole chromosomes called sex chromosomes.

(Note: The other chromosomes are called autosomes.)

  • One sex is homogametic: The gametes of this sex contain similar sets of sex chromosomes.

  • The other sex is heterogametic: The gametes have different numbers of chromosomes.

 

HOMOGAMETIC SEX

HETEROGAMETIC SEX

SEX DETERMINATION

 

Female XX

 

Male XY

Presence of Y-chromosome = maleness (mammals and fish)

Presence of second X-chromosome = femaleness (Drosophila, the fruit fly)

Male ZZ

Female ZW

Birds, amphibians, reptiles, butterflies, moths.

Female XX

Male Xo

Grasshoppers

 

 

For mammals

 

sex determination

 

Thus the expected sex ratio is: 100 males born for every 100 females. The observed sex ratio at birth is: 106 males born for every 100 females born. X carrying sperm are slow but long-lived, Y carrying sperm a fast but short-lived.

The chances of fertilisation by X and Y carrying sperm are not equal. However, the life expectancy of males and females is not equal either. This is partly due to sex linked inheritance (sex linkage).

 

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