This keeps popping up in my conversations, so I need to talk about this. Humans are XX/XY sex determining based on the last pair of chromosomes at the most basic level. This is one of four known mechanisms for chromosomal sexual determination. There are multiple non-chromosomal mechanisms, as well as a number of non genetic sex determinations even in things with chromosomal determination.
We learn in grade school, at least I did, that XY is male, and XX is female. And that's that. But on the genetic level, its not that simple.
Base Male - 46,XY
Base Female - 46,XX
Klinefelter's syndrome (47,XXY) - Male
Klinefelter's syndrome is caused by the presence of one or more extra copies of the X chromosome in a male's cells. Most identified cases of Klinefelter's are individuals with only one extra copy of the X, however, there are some cases with 4+ duplicates (48,XXXY or 49,XXXXY). These variations are found throughout all cells in the body.
Very rarely, there are some individuals who are 48,XXYY.
Affected individuals are physiologically male, but have decreased testosterone production, non functioning or poorly functioning testicles, and other sexual developmental problems.
47, XYY syndrome - Male
Most often, the extra Y chromosome causes no unusual physical features or medical problems. 47,XYY boys have an increased growth velocity during earliest childhood, with an average final height approximately 7 cm above expected final height.47,XYY boys have an increased risk of learning difficulties (in up to 50%) and delayed speech and language skills. Testosterone levels (prenatally and postnatally) are normal in 47,XYY males, and individuals usually have normal sexual development and normal fertility.
Trisomy X (47,XXX) - Female
Affects 1 out of every 1000 female births. Due to inactivation and formation of a Barr body in all female cells, only one X chromosome is active at any time in a female cell. Thus, triple X syndrome most often causes no unusual physical features or medical problems. Females with the condition are usually taller than average, and their weight may be low in comparison to their height. They may have menstrual irregularities, and, although rarely exhibiting severe mental impairments, have an increased risk of learning disabilities, delayed speech, and language skills.
Monosomy (45,X) - Female
Also known as Turner Syndrome. Affects 1 out of every 2500 female births. Associated symptoms include short stature, broad chest, low set ears, sterility, under developed sexual organs, increased weight, shortened metacarpal IV (hand), horseshoe kidneys, and visual impairments. approximately 98% of all Turner Syndrome pregnancies will spontaneously abort, and accounts for 10% of all spontaneous abortions in the US. Cardiovascular malformations are the primary health concern for surviving births.
XX Male (46,XX) - Male
Also called de la Chapelle syndrome. Symptoms include small testes, gynecomastia and sterility. Many males with this condition also have effeminate characteristics. This variation is very poorly studied.
Rare XXX+ - Female
Possibly the rarest variation, there are 25 females currently identified with this variation. XXXXX syndrome is associated with microcephaly, micrognathia, and round face. The ears are generally low-set and malformed. Eyes are upslanting and show palpebral fissures, hypertelorism, and strabismus.The hands and feet are small with overlapping toes, camptodactyly, clinodactyly, talipes equinovarus, and metatarsus varus. Heart deformities are common. XXXXX syndrome affects mental, growth, and motor retardation. There is occasional delayed puberty. Behavior and performance is affected by Opisthotonoid posture. The 'Rule of thumb' states that there will be a 10 to 15 IQ point decrease for each extra X chromosome. Thus, the range of IQ points would be 70 to 55.
What this shows, I think, is that we can realistically name four to five normal human genetic sexes: xx, xy, xyy, xxx, and xx (male). The remaining variations are clearly unstable variants, especially xxy and xxx+. However, increasingly Turner Syndrome females are more fertile, and able to produce children, as well as surviving and living close to normal lives. This alone shows that the X variation is possibly stable.
