How does Down syndrome affect phenotype?
Down syndrome is characterized by extensive phenotypic variability; while cognitive impairment, muscle hypotonia at birth, and dysmorphic features occur to some extent in all affected individuals, most associated traits occur in only a fraction of affected individuals.
Is Down syndrome genotype or phenotype?
Down syndrome (DS) is one of the most frequent congenital birth defects, and the most common genetic cause of mental retardation. In most cases, DS results from the presence of an extra copy of chromosome 21. DS has a complex phenotype, and a major goal of DS research is to identify genotype–phenotype correlations.
What is the genotype of someone with Down syndrome?
Regular trisomy 21 – also known as standard or free trisomy 21 – in which all the cells have an extra chromosome 21. Around 94% of people with Down’s syndrome have this type.
What are the phenotypic characteristics of Down syndrome?
People with Down syndrome often have a characteristic facial appearance that includes a flattened appearance to the face, outside corners of the eyes that point upward (upslanting palpebral fissures ), small ears, a short neck , and a tongue that tends to stick out of the mouth.
What is the basic pathophysiological basis for Down syndrome?
About 95 percent of the time, Down syndrome is caused by trisomy 21 — the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.
Is Down syndrome caused by nondisjunction?
Down syndrome is caused by a random error in cell division that results in the presence of an extra copy of chromosome 21. The type of error is called nondisjunction (pronounced non-dis-JUHNGK-shuhn).
Are there genotype-phenotype correlations in Down syndrome?
DS has a complex phenotype, and a major goal of DS research is to identify genotype-phenotype correlations. Cases of partial trisomy 21 and other HSA21 rearrangements associated with DS features could identify genomic regions associated with specific phenotypes.
What is the genetic architecture of Down syndrome?
Although several hypotheses have been put forward, it is unclear as to whether particular gene loci on chromosome 21 (HSA21) are sufficient to cause DS and its associated features. Here we present a high-resolution genetic map of DS phenotypes based on an analysis of 30 subjects carrying rare segmental trisomies of various regions of HSA21.
How are phenotypes and genotypes related in DS?
The rationale for the first approach is that cases of partial trisomy 21 associated with DS features could identify genomic regions associated with specific phenotypes. Because of variable penetrance, only the presence of a particular phenotypic trait is informative for mapping.
Which is more common trisomy 21 or Down syndrome?
The frequency of Down syndrome (DS) is much higher at conception, given that up to 75% and 50% of DS fetuses identified during the first and second trimester, respectively, are lost before term [ 1, 2 ]. Trisomy for some other autosomes occurs more frequently than trisomy 21, nearly always resulting in prenatal loss [ 3 ].
