XY = male; XX = female. But no such thing as a "YY" super-uber male. Just saying
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During Metaphase I of meiosis, the sperm or unfertilized egg can incorrectly separate the duplicated chromosomes and have additional copies of sex chromosomes in one egg or sperm, and then no copies in another egg or sperm. Most of the time it causes a spontaneous abortion (miscarriage), or the sperm/egg will not continue with fertilization (undergo apoptosis), but rarely the fertilization will occur with a defective gene count, and fertilized fetus will be viable. Here's a brief but not all-encompassing list of the conditions:
XY male, normal (included as a reference) XX female, normal (included as a reference)
X0 Turner Syndrome: Affected persons are phenotypically female, but smaller and may or may not undergo sexual maturation; even at sexual maturity sterility is common. Cognitive ability and other capabilities usually are normal, but some mental impairment is possible.
XYY: Double Y karyotype, usually taller than normal, may have slight cognitive impairment, but are otherwise normal, sterility is common. Behavior, mood, and criminal tendencies are not higher than others in the population.
XXY: Klinefelter syndrome, pheontypically male, usually tall, but do not undergo sexual maturity, however may have large breasts, nearly always sterile. Also usually have mild-moderate cognitive impairment.
XXXY: Normally not viable, but in extremely rare cases when an individual survives pregnancy they are similar to Kilnefelter syndrome.
XXX: Trisomy X, phenotypically female, symptoms vary from completely normal to moderate behavioral and cognitive dysfunction.
XY (Female): Defective SRY gene, so genetically male but a defective SRY gene in fetal development blocks development of the testes (and male physical features). Otherwise normal.
YY: Not viable, the X chromosome is required. If an ovum were to somehow still survive the mitotic [meiotic] checkpoints and be present for fertilization, the fetus would abort spontaneously.
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http://www.reddit.com/r/askscience/comments/1wi4wz/if_men_have_xy_and_women_have_xx_what_would/
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technically, humans are female by default. Not the other way around.