Trisomy 13 is characterized by mental disability, cardiac defects, deafness, labial and palatine clefts, and malformations of the eyes. Most children affected by this condition die shortly after birth. In trisomy 18 mental disability, cardiac defects, bent fingers and hands, syndactyly and renal anomalies may occur. The children often die shortly after birth. Trisomy 21 (Down's syndrome) is the most common condition. The children suffer from retarded growth. They have varying degrees of mental disability, oblique palpebral fissures with epicantus (additional skin fold on the inner eye angle), small ears, cardiac defects, and hypotonia. This condition is also often associated with atresia of the duodenum, the anorectal canal and the oesophagus. Most fetuses with this condition spontaneously abort. Ninety-five percent of cases with trisomy 21 are due to non-disjunction during meiosis. In a few cases with less pronounced symptoms, both normal and aneuploid cells are present (mosaicism).
In triploidy, the set of chromosomes is present three times (3n or 69 chromosomes). The sex chromosomes can be XXX or XXY. It occurs when two sperm cells penetrate an oocyte or if a diploid oocyte becomes fertilized. Spontaneous abortion of the fetus usually occurs. Tetraploidy (4n or 92 chromosomes) is a rare condition. This usually gives rise to an intact yet empty chorionic cavity.
Chromosomal mosaicism refers to a condition where a single embryo has two or more cell populations with different karyotypes (phenotype of the chromosome set in the metaphase). It is caused by errors in mitosis during cell cleavage. A chimera is composed of tissues of different genetic origins. Such individuals may result from a fusion of dizygotic twins. Chimera can be produced experimentally in mice when embryonic stem cells are transplanted into a blastocyst.