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Lissencephaly (/ ˌ l ɪ s. ɛ n ˈ s ɛ f. ə l. i /, meaning 'smooth brain') [1] is a set of rare brain disorders whereby the whole or parts of the surface of the brain appear smooth. [2] It is caused by defective neuronal migration during the 12th to 24th weeks of gestation, resulting in a lack of development of brain folds and grooves . [3]
The extra chromosome content can arise through several different ways. The most common cause (about 92–95% of cases) is a complete extra copy of chromosome 21, resulting in trisomy 21. [92] [97] In 1–2.5% of cases, some of the cells in the body are normal and others have trisomy 21, known as mosaic Down syndrome.
Chromosome 21 from Human Genome Program Normal human Karyotype. Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on chromosome 21, either in whole (trisomy 21) or part (such as due to translocations). The effects of the extra copy varies greatly from individual to individual ...
Mosaicism for aneuploid chromosome content may be part of the constitutional make-up of the mammalian brain. [12] [13] In the normal human brain, brain samples from six individuals ranging from 2–86 years of age had mosaicism for chromosome 21 aneuploidy (average of 4% of neurons analyzed). [14]
XYY syndrome, also known as Jacobs syndrome, is an aneuploid genetic condition in which a male has an extra Y chromosome. [1] There are usually few symptoms. [2] These may include being taller than average and an increased risk of learning disabilities. [1] [2] The person is generally otherwise normal, including typical rates of fertility. [1]
A 7-month-old baby born in New Jersey is being considered the first to ever survive his rare condition past birth.
The number of chromosomes in the cell where trisomy occurs is represented as, for example, 2n+1 if one chromosome shows trisomy, 2n+1+1 if two show trisomy, etc. [2] "Full trisomy", also called "primary trisomy", [2] means that an entire extra chromosome has been copied. "Partial trisomy" means that there is an extra copy of part of a chromosome.
Many organ systems are affected by triploidy, but the central nervous system and skeleton are the most severely affected: . Common central nervous system defects seen in triploidy include holoprosencephaly, hydrocephalus (increased amount of cerebrospinal fluid within the brain), ventriculomegaly, Arnold–Chiari malformation, agenesis of the corpus callosum and neural tube defects.