Biomedical Engineering Reference
In-Depth Information
No environmental agents have shown a clear effect on nondisjunctional
events in humans as measured by the frequency of trisomic infants born
to mothers exposed to such agents compared to control groups. Agents
that have been studied include low-level radiation, exogenous hormones,
alcohol and cigarette smoke.
There is a maternal age effect in which there is an increased risk for
trisomies with advancing maternal age. The effect was documented for
Down syndrome in the 1930s but to date, the exact mechanism for the
effect is not known and the factors that infl uence chromosome segrega-
tion behavior are yet to be totally understood.
The term
chromosomal mosaicism
is often used to describe a special con-
dition where two or more cell lines are found within an individual. The
phenomenon can occur for all the chromosomes and is caused by a non-
disjunctional event during cleavage division (Fig. 7.12). Because of the
effect of the combined normal and abnormal cells on the overall phenotype,
mosaic individuals will have considerable variability in the presentation of
Primary spermatocyte
First division
non-disjunction
Meiosis I
Secondary
spermatocyte
Second division
non-disjunction
Meiosis II
Sperm
After fertilization of
a normal gamete
Monosomic
Trisomic
Euploid
Monosomic
Trisomic
Zygotes
a. Non-disjunction at meiosis I
b. Non-disjunction at meiosis II
7. 1 2
Non-disjunction during (a) meiosis I and (b) meiosis II. Note the
difference in the number and type of gametes having an abnormality
in the number of chromosomes. (Image created by Patricia Verano,
St Peter's University, Art Department, Graphic Arts, Jersey City, NJ.)
