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Meiosis and Punnett Squares

meiosis is a type of cell division in

which a para cell divides to produce

four

sex cells in males this is how sperm is

made and in females this is how eggs are

made in humans this is also why sperm

and egg cells only have half the amount

of chromosomes that a regular cell has

so for instance all of your skin cells

have 46 chromosomes but if you're a

female you're excels only have 23

chromosomes and if you're male your

sperm cells only have 23 chromosomes so

23 obviously it's half of 46

so what meiosis does is it takes the

original parent cell as seen in this

diagram and it splits the chromosome

number in half and by doing so it also

produces four daughter cells this might

look similar to mitosis which is a type

of cell division that we learned earlier

this year but mitosis produces two

identical cells again meiosis however

produces four cells and they're all

different from each other and if you

compare it back to the original cell

here we've been learning about punnett

squares and this kind of ties together

fertilization with punnett squares and

why we use punnett squares for genetics

and what it has to do with sperm and

eggs in fertilization so here's a

Punnett Square and on the top you guys

are used to seeing the letters for a

dominant or recessive allele and then on

the side you would also see the letters

for a dominant recessive allele but what

this diagram is showing you is that

meiosis is what creates sperm meiosis is

what creates eggs and female and then

the punnett square shows you the result

of fertilization so when the egg and the

sperm

our fertilized this offspring it's in

this box right here will have a certain

allele combination according to what it

inherits from its father's sperm and

from its mother's egg so here's another

way of looking at it here we have this

parent cell which we're going to call

that the the primary spermatocyte and

then we have this parent cell which is

the primary oocyte in humans this is a

male right here this cell that starts

out with 46 chromosomes that makes sperm

it's called the primary spermatocyte

this cell and females that starts out

with 46 chromosomes and makes the eggs

is called the primary I was like so I

want you guys to write that down on your

paper during meiosis the primary oocyte

in a female makes eggs now the primary

oocyte has 46 chromosomes but when those

chromosomes split into 23 and 23 amongst

all the eggs the alleles and the genes

that were originally in this cell also

split the same thing happens in a male

during meiosis the primary spermatocyte

has a certain amount of elite alleles

for all 46 chromosomes but these split

and so this is why we end up with one

letter that goes into this cell one

letter that goes into this cell so let's

just write this down so here we have the

primary oocyte let's really simplify it

let's say that it started out with full

set of chromosomes and for one of the

genes it had a capital T lowercase T

well when meiosis occurs and this cell

splits into the eggs

these alleles also split so the capital

T you might go with this AG and the

lowercase T might go at this AG now

depending on which one of these eggs

produces the child the child is either

going to end up with the dominant allele

or the recessive allele the same things

going to happen up here with the primary

spermatocyte in a male so let's write

that down so here we can see that the

primary spermatocyte which also similar

to the primary oocytes started out with

a capital T lowercase T but during

meiosis this self split and produce

sperm cells and so the chromosome number

also splits so this capital T might go

to this sperm cell this lowercase T

might go to this sperm cell and

depending on the combination of egg and

sperm you know if it's this sperm that

fertilizes this egg then the offspring

is going to end up with capital T

capital T and you can go ahead and fill

that in on your note sheet but if it's

this firm that fertilizes this egg then

the offspring is going to have capital T

and lowercase T just like if it's this

firm that fertilizes this egg the result

would be lowercase T lowercase T and the

child would have the recessive trait so

on your sheet there's some information

go ahead and fill up the punnett square

if you haven't done so already and I'd

like to just read this out loud to you

it says we've already learned about

mitosis in which one cell divides into

two daughter cells that are exact

replicas of each other another type of

cell division that only occurs in the

ovaries and females and in testes and

males is called meiosis in meiosis one

parent cell divides two produced for sex

cells sperm or egg the give rise to new

individuals keep in mind that this

simplified version of opponent square

only shows you

the primary spermatocyte Ando site

dividing to make two eggs or two sperm

but it is in fact four as we'll see in

just a minute

since this only occurs in the

reproductive organs the cells that go

through meiosis are very specialized

sperm stem cells are called primary

spermatocytes these go through meiosis

to produce four sperm cells and eggs

stem cells are called primary oocytes

and these go through meiosis to produce

four egg cells

so if you look to the other side of your

notes this is a more complicated version

of a punnett square

and basically what this shows you is

that here we have the female in this

case it's talking about a pea plant and

it shows you how this original cell

splits and you can see that the

chromosome number gets split in half

amongst the egg cells this is the

original cell after the chromosomes are

replicated but then once it splits you

can see that you know originally had two

chromosomes

well once these but the eggs are either

going to have a capital R a lowercase R

a capital R or a lowercase R which is

one allele only as compared to the two

appear so it is half the number of the

original again relating it back to

humans eggs have 23 chromosomes whereas

all the rest of your cells have 46 and

same thing with the sperm over here so

as you can see you get to capital ours

here but they're not both listed in the

punnett square because that would be

repetitive and then you have to

lowercase ours and we only list one here

and then the same thing with the sperm

over here we have the capital R and we

have the lowercase R and once you

combine everything all together you can

see what the offspring are so a punnett

square basically shows you the possible

allele combinations after fertilization

which is when the egg and the sperm come

together to make the offspring to make

the embryo to make the baby to make the

child during meiosis the chromosomes

pairs separate they end up in different

sex cells and this punnett square shows

you the separation of alleles that occur

during meiosis this is why we have such

variation amongst organisms that go

through sexual reproduction because you

never know what chromosome is going to

end up

the AG that makes the organism and might

be the capital R and might be the

lowercase R at the same point you don't

know which chromosome is gonna be in the

sperm it might be the capital R might be

the lowercase R this is why it's kind of

luck of the draw

when you're trying to figure out if a

child is going to be a boy or a girl a

female only has two X chromosomes so

when these split amongst our eggs a

females eggs will only contain X

chromosomes whereas within male

he has XY so when his sperm cells divide

through meiosis some sperm might end up

with X's some sperm might end up with

Y's and depending on whether or not it's

the sperm cell with an X that fertilizes

the egg or if it's the sperm cell with a

Y that fertilizes the egg you're going

to end up with either a male offspring

or a female offspring so again xx will

give you a little girl XY will give you

a little boy so the last thing I want

you guys to think about is how are

meiosis and punnett squares related to

one another and basically it's that

punnett squares show you all the

possible genotypes in the offspring when

the egg and the sperm combine