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The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births. [5] The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births, [6] [7] [8] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may ...
Identical twin studies are a useful mechanism for assessing the role of genes and environment. Twin studies have also found that among twins with differing sexual orientations, homosexual twins were significantly more gender nonconforming than their heterosexual co-twin, and that this was noticeable from a young age. [29] Bailey states:
Intersex people have been treated in different ways by different cultures. Whether or not they were socially tolerated or accepted by any particular culture, the existence of intersex people was known to many ancient and pre-modern cultures and legal systems, and numerous historical accounts exist.
Androgyny is the possession of both masculine and feminine characteristics. [1] Androgyny may be expressed with regard to biological sex or gender expression.. When androgyny refers to mixed biological sex characteristics in humans, it often refers to conditions in which characteristics of both sexes are expressed in a single individual.
Intersex is a general term for an organism that has sex characteristics that are between male and female. [1] It typically applies to a minority of members of gonochoric animal species such as mammals (as opposed to hermaphroditic species in which the majority of members can have both male and female sex characteristics). [2]
Heteropaternal superfecundation is common in animals such as cats and dogs. Stray dogs can produce litters in which every puppy has a different sire. Though rare in humans, cases have been documented. In one study on humans, the frequency was 2.4% among dizygotic twins whose parents had been involved in paternity suits. [6]
In humans and many other species of animals, the father determines the sex of the child. In the XY sex-determination system, the female-provided ovum contributes an X chromosome and the male-provided sperm contributes either an X chromosome or a Y chromosome, resulting in female (XX) or male (XY) offspring, respectively.
Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are: Thoracopagus (28% of cases): [7] Two bodies fused from the upper chest to the lower chest. These twins usually share a heart and may also share the liver or part of the digestive system. [13] Survival rate is ...