Chromosome 21 – How accidental inheritance can lead to Downs syndrome
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Chromosome 21 – How accidental inheritance can lead to Downs syndrome


Our genetic information, our
DNA, is conveniently packaged into little bundles called
chromosomes. Humans have 23 pairs of
them, 46 altogether. They contain the instructions
that the body uses to build itself, the recipe for
a human, if you will. Accidentally duplicating copies
of the DNA instructions can cause problems later on. That’s what happens in Down’s
syndrome, a genetic condition that causes developmental and
physical disabilities. It’s a result of accidentally
inheriting additional copies of some of the DNA
instructions. Down’s patients typically have
an extra chromosome 21. So instead of 46 chromosomes,
they have 47. Where does this extra chromosome
come from? It happens during the formation
of sperm and eggs, a process called meiosis. During meiosis, your 46
chromosomes are shared out so that only half of them, 23, go
into each sperm or egg cell. I’ll show you using just one
of the sets of chromosomes, although actually it’s
happening to all 23 at the same time. Now originally, one of
the pair will have come from your father. Let’s say it’s this blue one. And the other one will
have originally come from your mother. Let’s say it’s this red one. First of all, each chromosome
makes a copy of itself. It duplicates, making four
altogether, two stuck together from the father and two stuck
together from the mother. These then move to the middle of
the cell, where they match up and almost wrap around
each other. It’s then that something called
crossing over occurs, where genetic material is
exchanged between chromosomes. This mixing process
can generate new combinations of genes. It’s one of the most vital
features of sexual reproduction. Now the chromosomes are pulled
in opposite directions by fibres that come from the
sides of the cells. They’re pulled apart once
and then once again. And the four separate
chromosomes go into four separate sex cells. Remember, this is actually
happening with 23 pairs, not just 1. And so each sex cell ends
up with 23 chromosomes. In males, those all go on
to become sperm cells. In females, it’s slightly
different. Only one of them goes on
to become an egg cell. During fertilisation, you get 23
from the male sperm and 23 from the female egg, making 46
in total again, 23 pairs. Down’s syndrome occurs when
during this pulling apart process of meiosis something
goes wrong with chromosome number 21. Instead of the four chromosomes
separating and being pulled apart, at some
point two of them go together. And both go into one sex cell. During fertilisation that sex
cell fuses with another one to form an embryo. It means that there are three
copies of chromosome 21, rather than the usual two. In total, the embryo
has 47 chromosomes. Chromosome 21 being the shortest
human chromosome, the foetus can normally survive. If this sort of thing happens to
other chromosomes with more genetic information, the outcome
is usually fatal. The intricate mechanics of how
our sex cells are created can make the difference between
life or death.

23 thoughts on “Chromosome 21 – How accidental inheritance can lead to Downs syndrome

  1. Good video.  Thankfully more rigorous than the balding hair one, but why is there no vocabulary in here.  The process described is called NONDISJUNCTION.  I get that you are trying to pull in the masses, but ONE WORD is not that bad.  

  2. The chromosomes of Male and Female mix and devide and than create sex cells (egg and sperm cells). Either I had a window place in biology or that is just completly wrong. "They mix.. in males they all go and make up sperm cells, in females its slightly diffrent only one of them makes up an egg cell"

  3. Downs Syndrome really confuses me in respect to WHY it actually creates a "defect".

    All the right chromosomes are there (Albeit twice in the case of C21).
    All the infomation is there and available to "Code" a human without the defect.
    …but it doesn't, even though there is 'above and beyond' all the data needed.
    Its like everything (and more) is there but it doesn't bother using it.

    Do scientists yet know why this is the case or is it something yet to be discovered?

  4. Muy bien explicado. Conocía la razón de la trisomía del par 21, pero no sabía porqué esto no se daba en otros pares de cromosomas y porqué no había, por tanto, seres humanos reflejando sus propias características comunes a esas otras posibles trisomías.

  5. Please explain how a 54 year old Down syndrome male 4th in a family of 5 children inherited his mother's RhD negative blood along with the second sister who lovingly takes care of him and has to constantly explain to doctor's about our low blood pressure, cool skin, paleness and slower breathing.

  6. Forgot to add that Downs are very sensitive to epilepsy medication less is better, my brothers seizures are triggered by sewer and drain smells especially ammonia.

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