Sex-linked characteristics in humans and Drosophila insect

Sex-linked traits

Sex-linked characteristics are the characteristics that their genes are located on sex chromosomes X & 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 ( X^r Y) with a red-eyed female ( X^R X^R ) , 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 2^nd 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 1^st generation in ratio 100 % but he noticed that ¼ 2^nd generation ( 25 % ) which carries the recessive characteristics ( white eyes ) are males .

Therefore, Morgan considered the eye color 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

Colour blindness

In humans , the chromosome ( X ) carries genes responsible for somebody 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 the appearance of hair on ear margins, So, these traits are restricted only to males .

Colour blindness

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 .

Sex-influenced traits , Sex-limited traits and Medical examination before the marriage