Triploidy

Triploidy refers to a deviation from the normal
number of chromosomes in which each chromosome
is present threefold instead of twofold.
With tetraploidy, four copies of each chromosome
are present. Triploidy arises when an abnormal
oocyte with a double (46,XX) chromosome
complement instead of a haploid complement
(23,X) is formed. After fertilization by a
normal spermatocyte, triploidy (69,XXX or
69,XXY) of maternal origin arises. In this case,
two of the three complete sets of chromosomes
arematernal. Triploidy may also arise as a result
of abnormalities during spermatogenesis, resulting
in an abnormal spermatozoon that does
not contain the normal haploid chromosome
complement, but rather the diploid (46,XY). In
this case, the triploidy (69,XXY) is of paternal
origin (see p. 402). A further cause of triploidy is
dispermy, or fertilization of a normal egg by two
normal sperm.