The various mechanisms illustrated for determination of sex fall under three classes: i) environmental ii) chromosomal and iii) genie. Understanding sex-linked inheritance of characters is simple or clearer in organisms having sex chromosomes. Many dioecious plants do not show sex chromosomes.
But others distinctly show the sex chromosomes. Mammals and many other animals also have characteristic sex chromosomes. Generally there are two types of sex chromosomes: X and Y. X chromosome is present both in male and female where as Y chromosome is present only in male. The number of autosomes and sex chromosomes for some species is outlined.
It should be noted that there are different mechanisms of chromosomal sex determina:’ besides above mentioned XX and XY mechanisms. Other mechanisms are I) XX-female, X male ii) XO female, XX-male iii) XX-male, XY-female etc.
Sex-linked inheritance can be studied easily in animals than plants because morphological characters relating to sex is well exhibited in the young animal. Among the anil sex-linked inheritance is elaborately studied in Drosophila melanogaster (fruitfly). There many well defined sex linked characters in human, which are also studied in detail.
The autosomes of Drosophila are six and the sex chromosomes are two with XY type the sex chromosomes are distinct from autosomes. In human there are 22 pairs, autosomes with one pair of X are one X and another Y chromosome.