DIHYBRID CROSSES
Sometimes more than 1 allele can
determine a trait.
For example you can follow 2 traits of peas.
R = Round
r = wrinkled
Round is dominant
Y = Yellow
y = green
Yellow is dominant
When you follow two traits that means you will
use 2 letters instead of 1 in a punnet square.
However, you no longer use the 4 square
punnet square.
Now you will use a 16 square…
For each parent, you have 2 alleles you follow
1. Round or wrinkled
R or r
2. Yellow or Green
Y or y
Each Parent will
have a total of 4
letters… RrYy
So if each parent has 4 alleles and you cross
4 alleles x 4 alleles = 16 (why the 16 square
box is needed)
A few basic rules when figuring out the
alleles…
1. Put the letters alphabetically
2. Dominant (big letters go first always)
Let’s take a one parent pea plant that is
heterozygous for both traits.
R = Round
r = wrinkled
Y = Yellow
y = green
RrYy
Each gamete (sperm or egg) from this pea
must give:
1 – “R or r” from this parent
1 – “Y or y” from this parent
How do you figure out the possible combinations?
How to find the right combo of alleles. To produce
sperm or egg.
1
2
R r Yy
3
RY Ry
1
2
4
rY
3
ry
4
Remember, the alleles we just figured out are
for just one parent.
Every offspring needs two letter “R or r” and
two letter “Y or y”
So let’s cross it with another heterozygous
plant…
RrYy
x
RrYy
So let’s cross two heterozygous RrYy x RrYy
It is set up in a 16 square block.
Now take your alleles from the first parent
and set them on the box this way..
1
2
4
3
R r Yy
RY Ry
1
2
rY
3
ry
4
Remember, every square will have 4 letters plus the rules are
in effect!
RY
RY
Ry
rY
ry
Ry
rY
ry
Next you just fill in the boxes….
Ry
rY
ry
RY RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
rY RrYY
RrYy
rrYY
rrYy
ry RrYy
Rryy
rrYy
rryy
RY
Ry
R = Round
r = wrinkled
RY
RY RRYY
Ry
RRYy
rY RrYY
ry RrYy
Phenotype
Y = Yellow
y = Green
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
Round Yellow
9
Round Green
3
Wrinkled Yellow
3
Wrinkled Green
1
Anytime you are working a Dihybrid Punnet
square…
And you are crossing two hybrids exactly like
our example.
You will always get a 9:3:3:1 Phenotypic ratio.
Now the hard Part.
What is the Genotypic ratio?
Yes, I want you to do this now…
RRYY = (1) = 6.25%
RRYy = (2) = 12.5%
RRyy =
(1) = 6.25%
RrYY =
RrYy =
Rryy =
rrYY =
rrYy =
rryy =
(2) = 12.5%
(4)
(2)
(1)
(2)
(1)
= 25%
= 12.5%
= 6.25%
= 12.5%
= 6.25%
16
100%
What if you wanted to follow 3 traits at one
time? How big would your punnett square be?
Each parent has 6 alleles - RrYyXx
So: 6 from mom x 6 from dad = 64 squares
This is a sample question from the Old TAKS
tests.
G
g
B
b
This is an
example of the
questions you will
get for the Test.
Now I will show
you how to solve
it without having
to do the large
punnet square.
Both dominant traits
G = Gold B = Banded
4
1
5
3
Since all they are asking for is both dominant
Which
traits, would give him the least amount of
Golden banded hampsters?
Count how many Capital letters are in cross.
VOCABULARY !
Test Cross
Dihybrid cross
9:3:3:1 ratio