Sex-linked characteristics in humans and Drosophila insect
Sex-linked characteristics are the characteristics that their genes are located on sex chromosomes X and Y and their appearance isn’t affected by sex hormones .
During studying the eye colour character in Drosophila insect , Thomas Morgan scientist discovered that some genes of body characters are located on sex chromosomes X and Y , So , he named them sex-linked characteristics .
Sex-linked characteristics in Drosophila insect
Eye colour characteristics in Drosophila insect :
Morgan crossed a white eyed male ( Xr Y) with a red-eyed female ( XR XR ) , All insects of first generation were red-eyed , This indicates that the red eyes characteristic is dominant over the white-eyes characteristic .
When members of the first generation are crossed with each other , the members of the 2nd generation were red-eyed and white-eyed in ratio 3 : 1 , respectively , He noticed that all white-eyed individuals were males .
Morgan could consider the eye colour characteristic in Drosophila insect as a Mendelian characteristic where the dominant characteristic appeared in 1st generation in ratio 100 % but he noticed that ¼ 2nd generation ( 25 % ) which carries the recessive characteristics ( white eyes ) are males .
Therefore , Morgan considered the eye colour characteristic is a sex-linked characteristic where its genes are carried on the sex chromosomes ( X ) , where the sex-chromosome ( Y ) carries few genes of them only .
Sex-linked characteristics in humans
In humans , the chromosome ( X ) carries genes responsible for some body characteristics , Such as Colour blindness , Haemophilia , Short-sightedness and Muscle atrophy .
Sex chromosome ( Y ) in male human carries some genes without the presence of corresponding genes to them on the sex chromosome ( X ) , such as the gene responsible for appearance of hair on ear margins , So , these traits are restricted only to males .
The case of colour blindness is caused by a recessive gene carried on the sec chromosome ( X ) , this gene causes the inability to distinguish the colours especially the red and green colours .
Colour blindness is genetic case cause the inability of distinguishing the colours especially the red and green ones .
Haemophilia ( Blood liquidity )
The case of haemophilia is caused by a recessive gene carried on the chromosome ( X ) , This gene causes a liquidity in blood due to lack of some substances necessary for blood clotting , Haemophilia disease may cause death especially in childhood stage .
Haemophilia is genetic case causes blood liquidity due to the lack of some substances necessary for blood clotting .
When a healthy man married a haemophilic carrier woman , the produced generation includes healthy and sick individuals .
The case of muscle atrophy is caused by a sex-linked lethal gene carried on the chromosome ( X ) , It is restricted only on males and its symptoms appear at the age of twelve years where it causes gradual atrophy of muscles , It ends by death .
The sex-linked characteristics ( in Drosophila or human ) , In males : They are represented by one gene only because the chromosome ( Y ) does not carry sex-linked genes .
In females : They represented by a pair of genes because female cells contain a pair of sex chromosomes ( XX ) , So , Sex-linked characteristics are more popular among males than females .
The male passes the trait to his daughters and does not pass it to his sons because he passes the chromosome ( Y ) to the males ( sons ) and the chromosome ( X ) which carries the gene of trait to the females ( the daughters ) .
The male passes the passes the trait to his grandsons through his daughters , The mother passes the trait to her sons and daughters , Male sons continually inherit the gene of trait from both their father and mother .