Transcript Document

Sperm Cell
Meiotic Cell Division
and
Punnett Squares
Egg Cell
You don’t need to take any notes, unless
you want to, but watch and listen!!!
Meiotic Cell Division
• What is Meiotic Cell Division? A process of
cell division which produces sex cells.
• What is the end result of Meiotic Cell Division?
4 sex cells (sperm or egg) with ½ the number
of chromosomes.
• For purposes of discussion, we’ll talk about
Guinea Pigs for this powerpoint.
• The father guinea pig donates one set of
chromosomes (alleles) and the mother guinea pig
donates one set of chromosomes (alleles) also.
BB allele combination = Black Guinea Pig
•Lets just pretend that the father guinea pig has 2
alleles for fur color, B and B.
•The letter B represents fur color. Big B is DOMINANT
for Black fur and small b is recessive for white fur.
•The father guinea pig has the BB allele combination,
so if the number of chromosomes is reduced by half,
then each sperm cell will have only 1 chromosome.
Through the process of Meiosis, one
sperm cell will get a big ‘B’ and the
other sperm cell will get the other big
‘B’ . Remember, the number of
chromosomes is reduced by half!!!!!
bb allele combination = White Guinea Pig
•The mother guinea pig also has 2 alleles for fur color,
b and b.
Again, the letter B represents Fur color. Capital B is
DOMINANT for black fur and small b is recessive for
white fur.
The mother has the bb allele combination, so if the
number of chromosomes is reduced by half, then
each egg cell will have only 1 chromosome.
Remember, through the process of
Meiosis, one egg cell will get a small ‘b’
and the other egg cell will get the other
small ‘b’ . Remember, the number of
chromosomes is reduced by half!!!!!
Punnett Squares
• A Punnett Square is a tool that geneticists use
to determine the POSSIBLE allele
combinations for the offspring, which were
inherited from the parents.
• The allele combination for the father goes on
the TOP of the Punnett Square!!!
• The allele combination for the mother goes on
the LEFT SIDE of the Punnett Square!!!
Lets use our 2 guinea pigs as an example
of how to set up a Punnett Square!
First, draw a square like the one below!!
The allele combination for the mother
Guinea pig is bb. (homozygous recessive)
The allele combination for the father Guinea
pig is BB. (homozygous DOMINANT)
So the cross would be
(mother guinea pig) bb X (father guinea pig) BB
(mother guinea pig) bb X (father guinea pig) BB
bb X BB
Remember, the process of meiotic cell division
reduces the number of chromosomes by ½.
bb X BB
One sperm cell gets a big B and the other
sperm cell gets the other big B!!
bb X BB
Remember, the father’s alleles go on the
top and the mother’s alleles on the left side.
bb X BB
One sperm cell gets a big B and the other
sperm cell gets the other big B!!
bb X BB
One egg cell gets a small b and the
other egg cell gets the other small b!!
bb X BB
If this sperm cell fertilized this egg cell, then
the new allele combination would be Bb!!
bb X BB
Bb
If this sperm cell fertilized this egg cell, then
the new allele combination would be Bb!!
bb X BB
Bb
If this sperm cell fertilized this egg cell, then
the new allele combination would be Bb!!
bb X BB
Bb
If this sperm cell fertilized this egg cell, then
the new allele combination would be Bb!!
bb X BB
Bb
So the completed Punnett Square should
look like this!!
bb X BB
Bb
Bb
Bb
Bb
OR
If you don’t want to draw the sperm cell and
egg cell, then it should look like this!!
bb X BB
B
B
b
Bb
Bb
b
Bb
Bb
What are the POSSIBLE genotypes,
or allele combinations?
bb X BB
B
B
b
Bb
Bb
b
Bb
Bb
What are the POSSIBLE genotypes?
There is only 1 POSSIBLE genotype… Bb!
bb X BB
B
B
b
Bb
Bb
b
Bb
Bb
There is only 1 POSSIBLE genotype… Bb!
But there are 4 chances to get that genotype!!!
bb X BB
B
B
b
Bb
1
Bb
2
b
Bb
3
Bb
4
What are the POSSIBLE phenotypes,
or physical characteristics?
bb X BB
B
B
b
Bb
Bb
b
Bb
Bb
What are the POSSIBLE phenotypes?
There is only 1 POSSIBLE phenotype… Black fur!
bb X BB
B
B
b
Bb
Bb
b
Bb
Bb
There is only 1 POSSIBLE phenotype… Black fur!
But there are 4 chances to get that phenotype!!!
bb X BB
B
B
b
Bb
1
Bb
2
b
Bb
3
Bb
4
What is the probability the two guinea pigs will have an
offspring with black fur? 100%!!!!!
bb X BB
B
B
b
Bb
25%
Bb
25%
b
Bb
25%
Bb
25%
Now try one on your own
without talking to your
neighbors. See if you can
figure it out in your head!

The genotypes of the parents have changed. In
your head, visualize filling in the Punnett Square!
Bb X Bb
B
B
b
b
If you know the answer to this, raise your hand. What is
the probability that the offspring will have black fur?
Bb X Bb
B
B
b
b
If you know the answer to this, raise your hand. What is
the probability that the offspring will have black fur?
75%!!!!!
Bb X Bb
B
B
b
BB
Black
25%
Bb
Black
25%
b
Bb
Black
25%
bb
White
25%
If you know the answer to this, raise your hand. What is
the ratio of black fur to white fur?
Bb X Bb
B
B
b
b
If you know the answer to this, raise your hand. What is
the ratio of black fur to white fur?
3 to 1%!!!!
Bb X Bb
B
b
B
BB
Black
#2
Bb
Black
#3
b
Bb
Black
#1
Bb
White
Now try the Punnett Square
worksheet on your own.
When you finish, you can
start on the homework
assignment.