The only viable monosomy is known as Turner syndrome
and it occurs in females who instead of two X chromosomes, have one X chromosome.
It can occur if portions of one X chromosome are significantly deleted
so that only one functional one remains.
The vast majority of major chromosomal abnormalities do not make it to term
and as I mentioned before,
Turner syndrome is the only monosomy with any degree of survival after birth.
Individuals with Turner syndrome
typically have short stature and a webbed neck
extending from the base of the skull and ears
down to points of the shoulders,
lymphedema in the limbs,
swelling and then a wide chest with widely spaced nipples.
There are also absences of ovaries and frequently you see amenorrhea
or failure of the menstrual periods to occur.
The ovaries are often fibrous called streak ovaries
that are present or all together absent ovaries.
Typically, these people have no major intellectual disability
and hormonal treatments
or surgical treatments can deal with the other sequelae of Turner syndrome,
giving normalcy and functionality to people with this condition.
Klinefelter syndrome is a trisomy wherein the affected individual is XXY.
Because a Y chromosome is present, these individuals develop as male
and typically they only see problems develop around puberty
as masculine secondary sexual characteristics
do not develop to the degree
that would normally be expected.
People with XXY are typically not mentally retarded
or having any developmental delays
or associated with it but the more copies of an X chromosome someone has
due to very early nondisjunction events,
the more likely developmental delays are going to be encountered.
People with Klinefelter syndrome will be sterile
because the testes and the seminiferous tubules
within them will atrophy
and fail to get a lumen that can transport spermatozoa.
They will frequently have gynecomastia or breast tissue develops in males,
less body hair, decreased muscle growth and increased height.
Cognitive impairment may occur, but as I mentioned before,
doesn't intend to be strictly associated with Klinefelter syndrome.
Trisomy X or women who are born with three copies of an X chromosome
tend to develop very typically of females and may be completely asymptomatic.
Only karyotyping will eventually show
that these people have three copies of the X chromosome.
In terms of commonalities, they often are taller than average,
have wide set eyes and epicanthal folds,
fold over on the top of the eyelid,
and there are usually no intellectual or developmental deficits
associated with this condition
and may be clinically unknown unless the person has genetic testing done.
Jacob syndrome is another trisomy wherein the affected person is XYY.
Because the Y chromosome is present,
these people develop in a typically male pattern
and they're often asymptomatic,
much as people with triple X syndrome.
These boys will eventually develop increased height,
but otherwise will develop normally
and there may be minor cognitive delays
but there's no consensus
on whether that is linked to a Jacob syndrome or not.
XYY people have traditionally been described as aggressive,
developmentally delayed and more likely to wind up in jail than others,
but subsequent research are shown that this is really not a compelling link
and there may be not be any sort of aggressive behavior caused by XYY trisomy.
In addition to nondisjunction events during meiosis
that cause problems in the germ cells,
there can sometimes be nondisjunction events during mitosis.
If this occurs while an embryo is growing,
we can wind up with nondisjunction event
creating two separate genetic lineages within a single embryo,
meaning part of the body will develop
in one way with a separate genetic blueprint than another part of the body.
This is known as mosaicisim.
It's very different from something called chimerism,
although the two may manifest similarly with genetically distinct lineages
appearing in different parts of the body.
A chimera is when two blastocysts that would otherwise develop
as completely distinct individuals
fuse during very early development to create a single organism
but with cells from two separate sources.
In addition to nondisjunction during mitosis or meiosis,
portions of chromosomes can also be damaged,
they can be struck off or otherwise badly damaged
or fragile leading to spontaneous abortion
and occasionally to a viable offspring.
In the case of Cri du chat or cry of the cat syndrome,
there's a deletion on chromosome 5.
This is sometimes been described as a partial monosomy
because it leaves a single, truly functional version of chromosome 5.
Children with this condition tend to have smaller facial characteristics
and you can see in the image that's associated with this,
he has a very small mouth so microstomia is present.
And the reason this syndrome has the name that it does
is that children with this condition, even immediately after birth,
tend to cry in the way that is reminiscent of a kitten,
so cry of the cat syndrome is because of this distinctive yell
that these children give very soon after they're born.
Fragile X syndrome occurs because of genetic problem within the X chromosome.
An excessive number of nucleotide repeats CGG over and over and over
creates a very fragile portion of the X chromosome.
The gene which this occurs is one that's involved with allowing neurons
to connect with other neurons effectively and therefore,
the problems that manifest generally manifest to in their neurologic ways.
Affects males more often than females because males
have one and only one copy of the X chromosome available,
although females can be carriers
and occasionally symptomatic for fragile X syndrome.
These children typically have protruding ears,
a relatively elongated face, hypermobile digits and cognitively,
there's gonna be some issues involved with processing of information
and sometimes these are going to appear autistic
and these children may need to have
very specific testing done to distinguish whether or not it's true autism
or a result of a defect in the X chromosome
that's causing these connections in the brain
to be a little bit nonfunctional.
Early intervention can help get these children more functional
and lead to better outcomes
throughout the rest of their life.
Rett syndrome is in fact very similar in effect to fragile X syndrome,
but it's due to a mutation of the MECP2 gene that is also in the X chromosome.
In this case, only females are affected
because this mutation is invariably fatal in males
who have one copy of X chromosome and cannot live without a functional copy.
It's not typically inherited but it's going to occur as an acquired mutation
after the child has already been conceived or in the germ cell line.
Affected infants are gonna develop normally
but regress at roughly 6 months of age
and in fact sometimes it become microcephalic as the brain fails to grow
and continue pushing the skull outward.
Symptoms are gonna manifest again at 6 months of age,
language and motor delays are most commonly described,
as well as seizures, breathing abnormalities
and sometimes autistic-like behavior
occurring in children with Rett syndrome.
Thank you very much and I'll see you for our next talk.