The Human Y Chromosome: Genetics, History, and Discovery
The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is used for sex-determining as the presence of the Y chromosome typically causes offspring produced in sexual reproduction to develop phenotypically male. In mammals, the Y chromosome contains the SRY gene, which usually triggers the differentiation of male gonads. The Y chromosome is typically only passed from male parents to male offspring.
Key Features and Characteristics
The human Y chromosome is characterized by specific biological markers as shown below:
- Length: 62,460,029 bp (CHM13)
- No. of genes: 693 (CCDS)
- Type: Allosome
- Centromere position: Acrocentric
History of Discovery
The Y chromosome 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, working with Hemiptera. Stevens proposed that chromosomes always existed in pairs and that the smaller chromosome (now labelled "Y") was the pair of the X chromosome discovered in 1890 by Hermann Henking. She realized that the previous idea of Clarence Erwin McClung, that the X chromosome determines sex, was wrong and that sex determination is, in fact, due to the presence or absence of the Y chromosome. In the early 1920s, Theophilus Painter determined that X and Y chromosomes determined sex in humans (and other mammals).
Naming Conventions and Physical Appearance
The chromosome was given the name "Y" simply to follow on from Henking's "X" alphabetically. The idea that the Y chromosome 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 Y chromosome, during mitosis, has two very short branches which can look merged under the microscope and appear as the descender of a Y-shape.
Chromosomal Variations and Conditions
Most therian mammals have only one pair of sex chromosomes in each cell. Males usually have one Y chromosome and one X chromosome, while females usually have two X chromosomes. Differences in the Y chromosome are associated with both atypical sexual development and fertility conditions. In humans, people with an extra X chromosome often develop Klinefelter syndrome, and people with an extra Y chromosome develop Jacob's Syndrome, as genes on the Y chromosome generally trigger the development of a male phenotype. Other chromosomal variations include three X chromosomes (or Trisomy X), and Monosomy X (or Turner Syndrome), where individuals only have one X chromosome and no Y chromosome. Some individuals with an XY karyotype, develop phenotypically female due to mutation in genes such as the SRY gene or MAP3K1.
Evolutionary and Biological Origins
Many ectothermic vertebrates have no sex chromosomes. If these species have different sexes, sex is determined environmentally rather than genetically. For some species, especially reptiles, sex depends on the incubation temperature. Some vertebrates are hermaphrodites, though hermaphroditic species are most commonly sequential, meaning the organism switches sex, producing male or female gametes at different points in its life, but never producing both at the same time.