We will now examine how germ cells develop and can occasionally have problems
resulting from abnormalities present in the chromosomes,
either within the chromosomes
or due to difficulty partitioning the chromosomes
to the appropriate daughter cell.
Now, given the wade and the rapidity with which cells are dividing,
both in mitosis and meiosis,
it?s fairly amazing that we don?t have more errors occur than we do,
but generally the process of mitosis and meiosis is very faithful
it results in a very few errors.
However, errors do occur and meiosis can cause chromosomal abnormalities
in the daughter cells
that are then used to produce new life and that can lead to various problems.
It?s estimated that around 50% of all conceptions are lost
and 50% of those losses,
so 25 over all,
are lost due to chromosomal abnormalities in the germ cells.
Chromosomal defects of some sort account for roughly 7%
of all major birth deficits
with 8% being account for by the mutations of individual genes
within those chromosomes.
Now, any human cell is gonna contain either 46 or 23 chromosomes
depending on whether it?s a cell undergoing mitosis or a germ cell
which is going to be able to combine with another cell
to create a diploid cell.
The normal state of being for any cell is going to be euploidy,
having 46 or 23 chromosomes.
Any variation on that number
creates a state called aneuploidy and that can cause birth defects
or spontaneous abortion if it?s inherited in a germ cell.
Here we can see an example
where a nondisjunction event has occurred during meiosis one
which leaves some daughter cells with no chromosomes
and the resulting daughter cells
on the opposite side having two copies on a single chromosome each.
This aneuploidy can occur during nondisjunction event
at either meiosis one or two
and tends to result in a chromosome
with either too many or too few chromosomes of one type.
In the end, we have either trisomy
or the affected individual have three copies of a chromosome
or monosomy, one copy.
Nondisjunction events, again, can occur meiosis one or two
but when these cells combine with another germ cell,
we can have it proceed to a clinically detectable state
as the embryo and fetus develops.
Trisomy 21, most commonly known as Down syndrome, is the most common trisomy.
If we take a look at the karyotype shown on the screen here,
we can see that there are three copies of chromosome 21 present,
when normally we?d expect there only to be two.
People with this condition have varying degrees of mental capacity,
ranging from profoundly affected and unable to function
to nearly full social function with some degree of accommodation.
People with Down syndrome typically have delays in growth,
some immune system delays and flattened facial features,
as well as what?s known as epicanthal fold over the top of the eye
and the Simian crease that goes all the way across the front of the palm.
The risk of Trisomy 21 and any nondisjunction event for that matter
increases with maternal age.
So here at the bottom of the screen
we can see two children who are born at the same time, twins,
but the one on the right is displaying the typical appearance
of someone with Down syndrome,
particularly the epicanthal folds right over the eyes.
Trisomy 18 is called Edwards syndrome
and it is the second most common trisomy.
Infants with these are going to have several abnormalities.
Heart and kidney problems, omphalocele,
where contents of the gut have herniated into the umbilical cord,
micrognathia, small mouth and microcephalic skulls,
with a skull that?s actually smaller than we?d expect,
sometimes as much as 2 to 3 standard deviations below the expected norm.
The second and fifth fingers tend to overlap
which is very interesting and very distinctive for trisomy 18.
If you look at the picture
you can see how the second and fifth digits are wrapping
over the top of digits three and four.
80% of infants with these are going to pass away within a year,
with 13% living to ten years of age
so it does have a significant morbidity associated with it.
Trisomy 13 or Patau syndrome is another trisomy
that is not as severe as others
in that the affected individual can make it to birth,
but children with this condition
will have a variety of problems including again,
heart and kidney deficiencies, cleft palate, holoprosencephaly
where there?s failure of midline structures of the face
to separate fully sometimes causing cyclopia, single or fused eyes
and a proboscis on the forehead.
They can also experience polydactyly with too many fingers,
muscle weakness and varying degrees of mental disability.