All language subtitles for 37031x_MH_Chromosomes_Mitosis_Meiosis_03_Segregatio-en

MICHAEL HEMANN: Errors do occur.

And errors in meiotic chromosome segregation

are called non-disjunction events, non-disjunction meaning

that there is some error in the segregation

of these chromosomes.

So let's think about what happens if there's actually

a non-disjunction event, so an abnormal

or an aberrant segregation.

So what happens if you actually get

all of these chromosome ones that actually all

go in one direction?

After meiosis I, we have two progeny.

One of the progeny has the two homologs present,

and the other progeny has nothing.

So they've all segregated all of their chromosome

Is into one target cell.

So as this goes into meiosis II, we have the four gametes.

And in the four gametes, you have the two homologs present.

So you're going to end up with two gametes that

have two times as many-- or as much chromosome

I as they should normally have.

So they're going to enter fertilization

bringing two copies of a particular chromosome.

So this is characteristic of a meiosis I non-disjunction

event.

Now you can think of a meiosis II non-disjunction event.

So if you have a meiosis II non-disjunction event starting

from that cell there, the result is you have two gametes,

and one of these gametes will have two copies of chromosome I

and no copies of the other chromosome.

Or, the other gamete will have no chromosome I copies.

There's actually a distinction, a major distinction

between these, and it allows you to recognize

the difference between a meiosis I and a meiosis II

non-disjunction event.

So in a meiosis I non-disjunction event,

you actually have the two homologs

that are present in the final germ cells.

So these are two distinct chromosomes.

One is maternal, one is paternally derived.

In a meiosis II non-disjunction event,

you have two sister chromatids that are present.

And so when we learn about DNA markers,

we're going to be able to discriminate

between a meiosis I and a meiosis II

non-disjunction event based on the origin of the chromosomes

that are inherited.