Chromosomal aberrations

Mutations that includes fragments of DNA long enough to be seen with light microscope are called chromosomal aberrations. They can affect autosomes or sex chromosomes. These are structural aberrations (abnormal chromosome) and abnormal numbers of normal chromosomes. Ususally they are very serious and often result in prenatal death or stillbirth. Structural aberrations can occur when more than one chromosome breaks. When chromosome is broken there are certain mechanisms that unite the fragments but no distinguish is made between chromosomes and thus they can be incorrectly joint. Abnormal number of chromosomes have usually devastating consequences. Multiplying the whole set leads to prenatal death or stillbirth. It can be the result of fertilization by two spermatozoons, lack of division in egg cell or sperm, inappropriate division of zygote. Triploid has the karyotype 69,XXY, 69,XXX or 69,XYY. Additional copy of an autosome can produce a live birth for trisomies of 21(Down’s syndrome), 18 or 13 chromosome.  Down’s syndrome the most frequently observed trisomy. It is higher for older mothers being about 100 times greater for mothers over 45 than for those between 20 and 30. Imbalances involving sex chromosomes have relatively mild developmental defects. These are boys with extra X chromosome (karyotype 47,XXY), with extra Y chromosome (karyotype 47,XYY), with two X chromosomes (karyotype 46,XX), girls with extra X chromosome (karyotype 47,XXX), lack of one X chromosome (45,X). These chromosomal aberrations are results of abnormalities in meiosis.