Trisomies for chromosomes 13 and 18 may also survive to birth but are more severely affected than individuals with Down Syndrome. Another general rule is that loss or gain of an autosome has more severe consequences than loss or gain of a sex chromosome. Although there is substantial variation within each syndrome, affected individuals often lead fairly normal lives. Occasionally an individual carries an extra chromosome which can't be identified by it's banding pattern, these are called marker chromosomes. The introduction of FISH techniques has been a valuable tool in the identification of marker chromosomes.
Structural abnormalities involve changes in the structure of one or more chromosomes.
The effects are typically severe since there is a loss of genetic material. Although an inversion carrier may be completely normal, they are at a slightly increased risk for producing a chromosomally unbalanced embryo. This is because an inverted chromosome has difficulty pairing with it's normal homolog during meiosis, which can result in gametes containing unbalanced derivative chromosomes if an unequal cross-over event occurs. Translocations involve exchange of material between two or more chromosomes. Problems arise with translocations when gametes from a balanced parent are formed which do not contain both translocation products.