The<unk>  was identified as a sex-determining chromosome by Nettie Stevens at Bryn Mawr College in 1905 during a study of the mealworm Tenebrio molitor. Edmund Beecher Wilson independently discovered the same mechanisms the same year. Stevens proposed that chromosomes always existed in pairs and that the<unk>  was the pair of the<unk>  discovered in 1890 by Hermann Henking. He realized that the previous idea of Clarence Erwin McClung, that the<unk>  determines sex, was wrong and that sex determination is, in fact, due to the presence or absence of the<unk> . Stevens named the chromosome "Y" simply to follow on from Henking's "X" alphabetically.
The idea that the<unk>  was named after its similarity in appearance to the letter "Y" is mistaken.  All chromosomes normally appear as an amorphous blob under the microscope and only take on a well-defined shape during mitosis. This shape is vaguely X-shaped for all chromosomes.  It is entirely coincidental that the<unk> , during mitosis, has two very short branches which can look merged under the microscope and appear as the descender of a Y-shape.
Most therian mammals have only one pair of sex chromosomes in each cell. Males have one<unk>  and one<unk> , while females have two<unk> s. In mammals, the<unk>  contains a gene, SRY, which triggers embryonic development as a male. The<unk> s of humans and other mammals also contain other genes needed for normal sperm production.
There are exceptions, however. For example, the platypus relies on an XY sex-determination system based on five pairs of chromosomes. Platypus sex chromosomes have strong sequence similarity with the avian<unk> , (indicating close homology), and the SRY gene so central to sex-determination in most other mammals is apparently not involved in platypus sex-determination. Among humans, some men have two Xs and a Y ("XXY", see Klinefelter syndrome), or one X and two Ys (see XYY syndrome), and some women have three Xs or a single X instead of a double X ("X0", see Turner syndrome).  There are other exceptions in which SRY is damaged (leading to an XY female), or copied to the X (leading to an XX male).
Y chromosome
Bacterial artificial chromosome
Cenegermin-bkbj
Tris
SRY
Short tandem repeat
Segrosome
Partial monosomy 13q