Welcome to AJ’s Bio Academy. Today’s lesson
is a continuation from last week’s Introduction to Genetics, so make sure to check that out
before watching this one. Last week, we learned about meiosis; the process of creating haploid
gametes from a diploid cell. However, though the body tries to do its best to perform its
functions properly, there are always mistakes that can happen. The cells in our bodies each have 46 chromosomes.
The egg and sperm cells each have 23 chromosomes. However, there are 2 conditions that can occur
due to a mistake in the production of gametes. Monosomy is the condition of having only 1
copy of a chromosome, and Trisomy is the condition of having 3 copies of a chromosome. These
conditions are the cause of many different syndromes. Today, we’ll be looking at one syndrome
in particular, The Triple X Syndrome. Triple X Syndrome, also known as 47, XXX Syndrome,
is a genetic disorder in which a woman carries an extra X sex chromosome in each of her cells.
Normally, women have two X chromosomes per cell, but in this case, they would have three.
This disorder can only occur in women and is not inherited. However, there is evidence
that suggests that the incidence of triple X increases with women who are older when
giving birth. The triple X syndrome, like other genetic
disorders, occurs due to a mistake in meiosis. The triple X syndrome is rare. It can happen
through nondisjunction or mosaicism. Lets look nondisjunction. Nondisjunction happens
during anaphase of meiosis 1 or meiosis 2, where chromosomes will fail to split properly.
If nondisjunction occurs during anaphase 1 of meiosis, it means that at least one pair
of homologous chromosomes did not separate. Using an example with 4 duplicated chromosomes
(our cells have 46), they would meet in the middle during metaphase 1, however, during
anaphase, one pair of homologous chromosomes (2 sister chromatids), are not separated and
instead, are carried into the other cell. At the end of meiosis 1, instead of having
4 chromosomes in each cell (46 in our cells), you end up with 6 in one cell and 2 in the
other (48 in one and 44 in another). The cells go through normal meiosis 2, splitting apart
the 3 pairs of 6 chromosomes (24 pairs of 48) and 1 pair of two chromosomes (22 pairs
of 44). The end result would be two cells that have 3 chromosomes (24 in our cells)
and two cells that have 1 chromosome (22 in our cells). That makes 2 cells that have an
extra copy of that chromosome (n+1) and 2 cells that are missing a copy of that chromosome
(n-1). In the case of triple X syndrome, the extra chromosome is the X sex chromosome. The separation failure can also occur during
anaphase 2 of meiosis 2. The cell will start out with 4 duplicated chromosomes (46), normally
separate in anaphase 1 to have 2 cells, each with 4 chromosomes (46), but then during anaphase
2, the same problem occurs where one of the 2 cells would fail to separate one of the
chromosomes and the final result would be 1 cell with 3 chromosomes (24), 1 cell with
1 chromosome (22), and 2 cells with the normal 2 chromosomes (23). So, in the end, you have
1 cell with an extra copy of that chromosome (n+1), 1 cell with a missing copy of that
chromosome (n-1) and 2 normal cells with the expected 23 chromosomes. The difference between nondisjunction occurring
in anaphase 2 rather than earlier in anaphase 1, is that you have less cells with an abnormal
amount of chromosomes and still end up with 2 normal cells. Triple X can also occur due to a condition
called mosaicism. Mosaicism can result from trisomy, when there is an extra copy of a
chromosome in a cell, but in most cases happens because of a mutation at some point after
the zygote is created. Mosaicism in genetics is used to describe cell populations with
two different genotypes. Some cells in the body are genetically different from the rest,
even though they are in the same organism. The mosaic triple X form occurs because there
are only a certain percentage of cells in the body that contain the extra copy of the
X chromosome and some that have the normal pair of X chromosomes.
So what does the triple x syndrome do? First of all it’s a rare disorder, only affecting
1 out of 1000 babies born worldwide with five to ten being born each day in the US. This
syndrome is only present in females, and is equal among all races and ethnic groups. The symptoms can sometimes consist of the
child growing taller than expected, having epicanthal folds, hypertelorism (an increased
width between the eyes), a smaller head circumference, or hypertonia (a reduced ability for muscles
to stretch). In 10% of cases the patient may have kidney abnormalities and seizures. There
may also be a delayed development of motor and speech skills, an increased chance of
having reading deficiencies like dyslexia, as well as increased anxiety, depression and
ADHD. Fertility issues are rare but possible, resulting in early menopause or infertility. Fortunately, in most cases the symptoms associated
with this syndrome are mild enough that the majority of carriers never find out they have
this syndrome unless they undergo genetic testing. Most medical professionals do not
consider the triple x syndrome to be a disability since the majority are able to live normal
lives, and is thus non life threatening. This condition is usually diagnosed when searching
for other diseases. Before the baby is born a CVS (chorionic villus sampling) may be done
between the 10th and 13th week of pregnancy to check for chromosome abnormalities in the
fetus. It involves obtaining tissue from the villi of the placenta which would be sent
to a lab for genetic testing. Sometimes an amniocentesis may be done instead between
the 16th and 20th week of pregnancy, for a more complete test result. In this case the
amoniac fluid surrounding the embryo is drawn out with a needle and sent to a lab for testing.
This syndrome can be diagnosed after birth with a blood test that will also be sent to
a lab for a chromosome analysis, which done when delays in development are found. There is no complete cure for triple X syndrome,
however there are treatment options available to reduce the severity of the symptoms and
is most effective when initiated as soon as possible. Since there is such a wide range
of different symptoms that may occur, it is recommended that the child has periodic screening
to assess what kind of treatments may be necessary. During childhood, speech and physical therapy
or behavior counselling can help with delays in development in speech and motor skills.
In adolescence and throughout adulthood, fertility and menstrual issues should be noted and checked
with a doctor since ovarian abnormalities are also linked with the triple x syndrome.
All patients should also undergo kidney and heart assessments to check for abnormalities
in these organs, as well as genetic counseling to better understand the nature of the syndrome
and learn about treatments available to help. In all, triple x syndrome usually isn’t
a very serious disorder to have with the majority of the population showing very few symptoms.
Join us next week to see how the addition of a different chromosome can result in a
totally different set of complications.