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Chromosome composition, convention is to list the total number of chromosomes, then the sex chromosomes, and then anything out of the ordinary.
- The most common chromosome number abnormality, trisomy-21. It is present in about 1 in 800 births. Infants with this condition have 3 copies of chromosome 21 rather than the normal 2.
- In general, people with Down syndrome are 47,sex,+21
- Trisomy-21 may arise from a nondisjunction event during meiosis in either parent or during mitosis very early during embryogenesis. However, most cases are due to a first division non-disjunction event occurring in the female parent
Having 3 copies of one chromosome, NOT triploidy, which is have three entire chromosome sets (3x)
People with Down syndrome have mental retardation and often have other health problems such as heart defects. It was first described by John Down in 1866, but chromosomal basis wasn't revealed until 1959. It seems that three copies of the DYRK genes on chromosome 21 cause some of the mental problems. This is active in the brain and it seems that neurons are damaged if there are too many DYRK proteins synthesized
XXY and XXX
Approximately 1/1000 males have an extra Y chromosome and similar for females, without ever knowing it! The extra Y doesn't cause a problem as they have relatively few genes and with the X, they are often inactivated, so again no serious consequences
- (2n-1) for autosomal chromosomes does occur at conception but these embryos never survive.
- Neither can 45,Y because they lack the essential genes on the X chromosome.
- However, 45,X is viable, known as the Turner syndrome
- 45,X. These people are female because they do not have a Y chromosome. Viable as only one X chromosome is typically active.
- Does have health concerns though. They are shorter than average, they have an elevated risk of heart defect, and they are infertile.
- The health problems is due to a few genes that are found on both X and Y chromosome, in the pseudoautosomal region. This region escapes inactivation, but in Turner syndrome as only one X chromosome is present, the lack is apparent...
A region of similar genes on the X and Y chromosomes that escapes inactivation