Members of a pair have the same size, shape, and the same banding pattern. Chromosomes in a karyotype are aligned from largest to smallest. A karyotype can be used to diagnose chromosomal abnormalities. All chromosomes besides X and Y are autosomes. Nondisjunction during meiosis causes an abnormal chromosome number in gametes produced.
Nondisjunction is a failure of one or more chromosomes to separate.
Study of spontaneous abortions suggests many trisomies and nearly all monosomies are fatal. Down syndrome is most common autosomal trisomy, involves chromosome 21. Most often, Down syndrome is due to nondisjunction during gametogenesis. Chances of a woman having a Down syndrome child increase with age. Chorionic villi sampling testing or amniocentesis and karyotyping detects a Down syndrome child.