Sex chromosomes are those specialized for determining sex phenotype of an organism, males and females have different chromosomes. In contrary, autosomal chromosomes are not sex-determining, males and females share same autosomes.
Sex determination systems
Sex determination can be caused by genetics or by environment. There are three genetic sex determination systems: XX-XY, XX-XO and ZW-ZZ. In some insects, sex can be determined by chromosome numbers, for example, honey bees, 2N individuals are females and 1N are males. In plants, there is no particular sex chromosome, and sex is determined by genes scattered within the whole genome.
Inheritance of sex-linked traits
Inheritance of sex-linked traits exhibit special patterns. The inheritance for X-linked recessive trait exhibit crisscross pattern, meaning the recessive trait from male individual will not show in next generation, but will show on the third generation males. When X chromosome nondisjunction occurs during gamete formation, the sex chromosome may change and cause recessive trait show up where it is not expected, it also cause aneuploidy.
Aneuploidy includes nullisomy, monosomy, trisomy and tetrasomy. When this occurs on sex chromosomes, it may cause serious genetic disorder. For exaple, the Klinefelter's Syndrome is caused by extra X chromosome(s).
Human sex-linked traits
The x-linked recessive traits show crisscross inheritance pattern. An example is hemophilia in royal family of England. X-linked dominant traits occur in both male and female, no skipping generation, for example, vitamin D-resistant rickets. Y-linked traits only occur in males and pass from father to son. All these inheritance patterns follow Mendelian genetics rules.
Twin studies are more usefull in determining which factor plays more important role in development of individuals, genetics or environment.