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Down syndrome (United States) or Down's syndrome (United Kingdom and other English-speaking nations), also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. [3]
Karyotype of a human with Trisomy 21 (Down syndrome). Trisomies can occur with any chromosome, but often result in miscarriage rather than live birth.For example, Trisomy 16 is most common in human pregnancies, occurring in more than 1%, but the only surviving embryos are those having some normal cells in addition to the trisomic cells (mosaic trisomy 16). [3]
Individuals with PKS present prenatally or at birth with multiple birth defects.These defects include: brain atrophy, agenesis of the corpus callosum, polymicrogyria of the brain, and/or spot calcifications in the brain's lateral sulcus; deafness and/or blindness; autonomic nervous system dysfunctions such as anhidrosis, hypohidrosis, and/or episodic spells of hyperventilation interspersed ...
Human genetic disorders can be caused by ring chromosome formation. Although ring chromosomes are very rare, they have been found in all human chromosomes. Symptoms seen in patients carrying ring chromosomes are more likely to be caused by the deletion of genes in the telomeric regions of affected chromosomes, rather than by the formation of a ring structure itself. [5]
Human conditions due to monosomy: Turner syndrome – Females with Turner syndrome typically have one X chromosome instead of the usual two X chromosomes. Turner syndrome is the only full monosomy that is seen in humans — all other cases of full monosomy are lethal and the individual will not survive development.
For example, either isodisomy or heterodisomy can disrupt parent-specific genomic imprinting, resulting in imprinting disorders. Additionally, isodisomy leads to large blocks of homozygosity , which may lead to the uncovering of recessive genes, a similar phenomenon seen in inbred children of consanguineous partners.
This results in an extra chromosome, making the haploid number 24 rather than 23. Fertilization of eggs or insemination by sperm that contain an extra chromosome results in trisomy, or three copies of a chromosome rather than two. [14] Trisomy 18 (47,XX,+18) is caused by a meiotic nondisjunction event.
Cri du chat syndrome is a rare genetic disorder due to a partial chromosome deletion on chromosome 5. [1] Its name is a French term ("cat-cry" or "call of the cat") referring to the characteristic cat-like cry of affected children (sound sample ). [2] It was first described by Jérôme Lejeune in 1963. [3]