[ **up: [[Animals]] | [[Mammals]]** ] # Therians | Theria (Subclass) Therians are a [[Mammals|mammal]] subclass. They include [[Placentals|placentals]] and [[Marsupials|marsupials]].[^1] --- ## Taxonomy - **[[Life (Biology)]]** - **[[Eukaryotes|Eukaryota (Domain)]]** - **[[Animals|Animalia (Kingdom)]]** - [[Chordates|Chordata (Phylum)]] - [[Mammals|Mammalia (Class)]] - [[Therians|Theria (Subclass)]] <small>THERIANS</small> --- ## Reproduction ### Sex determination Most therian mammals “have an extremely conserved XX/XY sex determination system.”[^2] The general standard, in XY organisms, is two copies of the X chromosome in females (XX), and one X and one Y (XY) in males.[^1] [[2025-0323. 'Therian sex determination has a unique origin and is syngamic, meaning that sex is fixed by the presence of the sex chromosome complement at fertilization ...'|'Therian sex determination has a unique origin and is syngamic, meaning that sex is fixed by the presence of the sex chromosome complement at fertilization, and not influenced by environmental factors (e.g. temperature, hormones, social structure).']][^3] [[2025-0313. 'Eutherian mammals have an extremely conserved sex determining system controlled by highly differentiated sex chromosomes.'|'Eutherian mammals have an extremely conserved sex determining system controlled by highly differentiated sex chromosomes. Females are XX and males are XY, and any deviation generally leads to infertility/sterility, particularly due to meiosis disruption.']][^3] [[2025-0313. 'With two copies of the X chromosome in females versus one X and a Y chromosome in males, male heterogamety is the norm in mammals.'|'With two copies of the X chromosome in females versus one X and a Y chromosome in males, male heterogamety is the norm in mammals.']][^1] [[2025-0313. 'The X and the Y are very different both in size and gene content as the result of a long differentiation from an ancient autosomal pair.'|'The X and the Y are very different both in size and gene content as the result of a long differentiation from an ancient autosomal pair.']][^2] - *See:* [[Intersexuality|Intersexuality]] [^1]: Frederic Veyrunes, et al., ‘[[Veyrunes, et al. 'A novel sex determination system in a close relative of the house mouse', 2010.|A novel sex determination system in a close relative of the house mouse]]’, *Proceedings of the Royal Society B: Biological Sciences*, vol. 277, issue 1684 (7 April 2010), p. 1049. [^2]: Frederic Veyrunes, et al., ‘[[Veyrunes, et al. 'A novel sex determination system in a close relative of the house mouse', 2010.|A novel sex determination system in a close relative of the house mouse]]’, *Proceedings of the Royal Society B: Biological Sciences*, vol. 277, issue 1684 (7 April 2010), p. 1049. [^3]: Frederic Veyrunes, et al., ‘[[Veyrunes, et al. 'A novel sex determination system in a close relative of the house mouse', 2010.|A novel sex determination system in a close relative of the house mouse]]’, *Proceedings of the Royal Society B: Biological Sciences*, vol. 277, issue 1684 (7 April 2010), p. 1049.