Sex linkage is the phenomenon in which the expression of genes depends on the gender of the organisms because genes for specific traits are located on the sex chromosomes of organisms. If repeated crosses of the fish indicate the pattern that long tail fins appear only in males and not in females, it means the trait is linked to the male sex.
What are sex chromosomes:
They are usually a pair of chromosomes present in higher organisms like humans and other mammals that combine during fertilization of male and female gametes and determine the sex of offspring. In humans they are X and Y chromosomes and when there is an XX offspring, she will e female and when there will be XY offspring , he will be male.
Significance:
Now there are some genes that are present only on sex chromosomes of the organisms like genes for beard are present in human Y chromosome of men, However,genes found on the X chromosome can be found in either males or females.
Sex linked genetic disorders are very common in human males because males receive only one copy of X chromosome while females receive two copies. So only one copy of defective gene is enough for the expression of disease in males while in women two copies are needed if the trait is recessive.
For example: There is a disease called hemophilia in which the blood is unable to clot even after minor injuries and can be life threatening.
Hemophilia is an X linked recessive disorder and is an example of sex linked traits controlled by sex linked genes.
