upper eyelids that cover the inner corners of the eyes, and varying
degrees of mental retardation are characteristics of people with
Down syndrome.
In mothers younger than 30, Down syndrome occurs in about 1
in 1,500 births. In mothers 37 years old, the incidence doubles to 1
in 290 births. In mothers over 45, the risk is as high as 1 in 46
births. Older mothers are more likely to have a baby with Down
syndrome because all the eggs a female will ever produce are pre-
sent in her ovaries when she is born, unlike males who produce new
sperm throughout adult life. As a female ages, her eggs can accu-
mulate an increasing amount of damage. Because of this risk, a
pregnant woman over the age of 35 may be advised to undergo pre-
natal testing that includes fetal karyotyping.
What events can cause an individual to have an extra copy of a
chromosome? When sperm and egg cells form, each chromosome
and its homologue separate, an event called disjunction (dihs
JUHNK shuhn). If one or more chromosomes fail to separate prop-
erly—an event called nondisjunction—one new gamete ends up
receiving both chromosomes and the other gamete receives none.
Trisomy occurs when the gamete with both chromosomes fuses
with a normal gamete during fertilization, resulting in offspring
with three copies of that chromosome instead of two. In Down syn-
drome, nondisjunction involves chromosome 21.
SECTION 1Chromosomes 123
On the Trail of a Chromosomal Deletion
O
ne of every 4,000 babies is
born with a genetic disorder
called DiGeorge syndrome. This
disorder causes serious heart
defects that must be surgically
corrected within a few days after
birth. Children born with DiGeorge
syndrome can also have blood
ailments, facial abnormalities, a
deficient immune system, and
other problems.
A Faulty Chromosome
Karyotypes of people with Di-
George syndrome show that
they have one normal and one
faulty 22nd chromosome. The
faulty chromosome is missing a
small region that contains 25
genes. To understand how this
chromosomal deletion results inDiGeorge syndrome, researchers
at Baylor College of Medicine
in Houston have been studying
the disorder in mice.
First, the researchers found
that they could produce similar
heart defects in mice by deleting
a part of mouse chromosome 16.
When these mice were bred with
mice that had a duplication of the
same part of chromosome 16,
their offspring had no heart
defects. Because the deleted
part contained only 15 genes, the
search for the cause of DiGeorge
syndrome was narrowed from 25
genes to 15 genes.
Finding the Crucial Gene
Using a technology called chro-
mosome engineering, the Baylorresearchers eventually zeroed in
on a gene called Tbx1. They
showed that deleting Tbx1 on
one chromosome 16 in mice
causes the heart defects of
DiGeorge syndrome. Tbx1is also
required for the development of
other embryonic structures
besides the heart. Thus, the
results of research on DiGeorge
syndrome may provide clues
about the genetic causes of
other birth defects.http://www.scilinks.org
Topic: Genetic Disorders
Research in Texas
Keyword: HXX4008