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A simplified diagram
of a diploid cell
at metaphase one of meiosis
homologous chromosomes have paired
and they have assorted independently
as they begin
two separate meiosis allows diploid cells
to form haploid gametes or sex cells
if the diploid number is four then
the haploid gametes will each have two chromosomes
examples of gametes include
*** cells egg cells
pollen and ovules upon fertilization
haploid gametes unite to restore the
diploid number in humans
the chromosomes in the diploid cells
can be studied and images
of these cells can be enlarged to produce
the human karyotype here we can see
the 23 pairs that come together
by fusion of haploid gametes
noticed the 22 identical pairs
with her 23rd pair
the XY pair standing out
males are referred to as the hetero
gametic sex and it is the presence
of this tiny Y chromosome which determines the development
of male *** characteristics but sometimes the process
of gamete formation is abnormal and if fertilization is successful
it can lead to more or less chromosomes
in the diploid number this
abnormality may or may not
apply to the sex chromosomes here in this
male karyotype the X chromosome has been moved away from the Y chromosome
to make room for this
abnormality trisomy
21 in this case
chromosomal non destruction
has occurred at metaphase
of meiosis chromosomes lined up
and we're about to separate. but as separation
happened during anaphase one two chromosomes that
should have gone their separate ways ended up staying together
if these two chromosomes
are members of the number 21 set
then what should be a pair
now becomes a trio and the result
is trisomy 21 or downs
syndrome a child with Down's syndrome
faces many challenges and with
modern medicine expectant families can determine the full
karyotype of their fetus
using ultrasound energy techniques like amniocentesis
which takes a sample of
fetal cells from the protective amniotic fluid that surrounds
the fetus as it develops another similar technique
is known as chorionic villus sampling
here another cell sample from the developing fetus is taken
and both techniques are designed to inform
expectant family about the karyotype of their child
there are some risks associated with both amniocentesis
and chorionic villus sampling and there are some small technical differences
between each method but the real question is an
ethical one if a family discovers
that their unborn child suffers from trisomy 21
then what should they do next
here we see another karytype again with
trisomy 21 but this
is the karyotype of a female this can be determined
by examining the pair of X chromosomes
and here we can see an identical pair of X chromosomes
this difference
in the structure these chromosomes
is very significant because
there are certain genes carried on
that part of the X chromosome for which there is no
corresponding section on the Y chromosome
so it means that there might be some traits
which are controlled or governed
by just one allele of a gene
if a male has just a
recessive allele for this trait
then it will be expressed in his phenotype
a widely studied example of this sex-linked
condition is hemophilia
the absence of a blood clotting protein makes
even the tiniest of cuts very serious for the hemophiliac
the allele for the clotting of blood
is dominant and carried on this part the X chromosome
for which there is no corresponding part
on the Y chromosome so
if this X chromosome carried the recessive allele
which allowed for a lack of this blood clotting factor
then this is all that would be required
for a male to express the condition
of hemophilia whereas in females
the recessive allele may be present
but its effect would be masked
by a dominant allele if this is carried
on the corresponding X chromosome
if you can see this image it means you have normal color vision but if you
cannot
it means that you may be suffering from one form
of red-green colorblindness our ability to perceive
red and green is determined by
another allele which resides
on this part of the X chromosome for which there is no corresponding
part on the Y
again it means that males who possess
the recessive allele will express
red-green color blindness in their phenotypes
a female who carries the recesses allele
for red-green colorblindness on one of her X chromosomes
a carrier female or a heterozygote
has children with a man with normal vision
one who carries the dominant allele on his single
X chromosome and of course there is no
allele to be carried on the tiny Y chromosome
the gametes produced are shown here with the male gametes at the top
and the female gametes down (the side) possible genotypes of their children
would reveal that there is no chance
of a daughter having color blindness
but a 50 percent chance of a son
being color blind
there is a 50 percent chance however of
a daughter being similar to the mother
and being a carrier of red green color blindness