By the end of this session, you should be able to:
Define the terms: Euploid, Aneuploid, Polypoid,
Diploid, Monosomy, Trisomy
Define nondisjunction & explain its consequences
Explain mosaicism and compare it with aneuploidy
Explain numerical chromosomal anomalies regarding
their incidence, embryological basis, characteristis &
No & characteristics of chromosomes encountered in an
Refers to any exact multiple of haploid no (n) 23 e.g., diploid
Is any deviation from human diploid no of 46 chromosomes.
An aneuploid is an individual who has a chomosome no that
is not an exact multiple of haploid no (n) of 23 (e.g. 45 or 47)
Principal cause of aneuploidy is nondisjunction
Nondisjunction occurs during either 1st or 2nd meiotic
division of germ cells, may involve autosomes or sex
Nondisjunction may occur during maternal or paternal
Occasionally nondisjunction occurs during mitosis in
an embryonic cell during earliest cell divisions. Such
conditions produce MOSAICISM, with some cells
having abnormal chromosome no & others being
A person who has at least two cell lines with two
or more different genotypes (genetic constitutions)
is a mosaic.
Either the autosomes or sex chromosomes may be
Usually the anomalies are less serious than in persons with
monosomy or trisomy, e.g., the features of Turner
syndrome are not as evident in 45, X/46, XX mosaic
females as in the usual 45, X females.
o Nondisjunction during early cleavage of the zygote.
o Loss of a chromosome by anaphase lagging (the
chromosomes separate normally but one of them is delayed
in its migration and is eventually lost)
Types of Chromosomal Abnormalities
NUMERICAL CHROMOSOMAL ABNORMALITIES
Numerical aberrations are represented as:
Any deviation from the human diploid number of
An aneuploid is an individual who has a chromosome
number that is not an exact multiple of the haploid number
of 23 (e.g. 45 or 47)
A form of aneuploidy in which there is absence of one
chromosome of a homologous pair in the complement of
an otherwise diploid cell
The few that survive are unmistakably female in appearance.
Trisomy of Autosomes
The presence of three chromosome copies in a
given chromosome pair (extra copy of a
chromosome) is called trisomy
Down syndrome & maternal age
Edwards Syndrome Trisomy 18
Patau Syndrome Trisomy 13
Trisomy of sex chromosomes
Chromosome compliment 47xxy
incidence 1 to 1000
Origins of triploid and tetraploid
Tetrasomy and Pentasomy of sex chromosomes
Persons with these abnormalities have four or five sex
The following chromosome complexes have been
reported in females:
48, XXXX and 49, XXXXX;
& males: 48, XXXY, 48, XXYY, 49, XXXYY, and 49, XXXXY.
The extra sex chromosomes do not accentuate sexual
characteristics; however, usually the greater the number
of sex chromosomes present, the greater the severity of
mental retardation and physical impairment.
The most common type of polyploidy is triploidy (69
Triploid fetuses have severe IUGR with a disproportionately
small trunk. Several other anomalies are common.
Could result from the second polar body failing to separate
from the oocyte during the second meiotic division
But more likely triploidy results when an oocyte is fertilized
by two sperms (dispermy) almost simultaneously.
Triploidy occurs in approximately 2% of embryos, but most of them
abort spontaneously. Triploid fetuses account for approximately 20%
of chromosomally abnormal miscarriages. Although triploid fetuses
have been born alive, this is exceptional. These infants all died within
a few days because of multiple anomalies and low birth weight.
Doubling the diploid chromosome number to 92
(tetraploidy) probably occurs during the first cleavage
Division of this abnormal zygote would subsequently
result in an embryo with cells containing 92
Tetraploid embryos abort very early, and often all that is
recovered is an empty chorionic sac, which used to be
referred to as a “blighted embryo.”