XX male syndrome , also known as De la Chapelle syndrome , is a rare congenital intersex condition where an individual with a 46 XX karyotype otherwise associated with females has phenotypically male characteristics that can vary among cases. The appearance of XX males can fall into one of three categories: 1 males that have normal internal and external genitalia, 2 males with external ambiguities, and 3 males that have both internal and external genital ambiguities true hermaphrodites. The degree to which individuals with XX male syndrome develop the male phenotype is variable, even among SRY-positive individuals. Masculinization of SRY-negative XX males is dependent upon which genes have mutations and at what point in development these mutations occur. Females typically have two X chromosomes. XX males that are SRY-positive have two X chromosomes, with one of them containing genetic material from the Y chromosome, making them phenotypically male but genetically female. The SRY gene plays an important role in sex determination by initiating testicular development. The tip of the Y chromosome contains the SRY gene and, during recombination , a translocation occurs in which the SRY gene on the Y chromosome is moved to become part of an X chromosome. Hypothesis that XX occurs in males because of the interaction of the testis-determining portion of the Y chromosome and part of the X chromosome, called the Xg gene, is generally supported by various data.
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The XY sex-determination system is the sex-determination system found in humans , most other mammals , some insects Drosophila , some snakes, some fish guppies , and some plants Ginkgo tree. In this system, the sex of an individual is determined by a pair of sex chromosomes. Females typically have two of the same kind of sex chromosome XX , and are called the homogametic sex. Males typically have two different kinds of sex chromosomes XY , and are called the heterogametic sex. In humans, the presence of the Y chromosome is responsible for triggering male development; in the absence of the Y chromosome, the fetus will undergo female development.
This page has been archived and is no longer updated. Published online 15 October Nature doi Kerri Smith. A gene has now been discovered that, when mutated, turns girls into boys. The finding advances, but also complicates, our understanding of how sex is determined by our genes.
Humans are born with 46 chromosomes in 23 pairs. Most women are 46XX and most men are 46XY. Research suggests, however, that in a few births per thousand some individuals will be born with a single sex chromosome 45X or 45Y sex monosomies and some with three or more sex chromosomes 47XXX, 47XYY or 47XXY, etc.