Transcript Using a Punnett Square PPT

#50 Using a Punnett
Square
Vocabulary
Homozygous:
Genotype with Identical alleles, same
letters (RR = dominant, rr = recessive)
Heterozygous
Genotype with different alleles, but
dominant one shows up (Bb always has
the dominant phenotype).
What is a PUNNETT SQUARE?
• A tool to predict the probability of
certain traits in offspring that shows the
different ways the alleles of each
genotype can combine
• A way to show possible phenotypes &
genotypes
What is a PUNNETT SQUARE?
• Letters stand for dominant and
recessive alleles
– An uppercase letter stands for a dominant
allele
– Lowercase letters stand for recessive
alleles
Using a PUNNETT SQUARE
To set up a Punnett square, draw a large
square, and then divide it into 4 equal
sections (also squares). It should look
something like this:
Using a PUNNETT SQUARE
Now you need two parents to mate, ones
with a known genotype
For example, a red flower (genotype Rr)
and a white flower (genotype rr). Rr x
rr
Using a PUNNETT SQUARE
Place one of the parents on top, and one
on the left. You should get a something
similar to this:
Using a PUNNETT SQUARE
Finally, take each letter in each column and
combine it with each letter from each
row in the corresponding square. You
should now have a picture close to this:
Using a PUNNETT SQUARE
• The two-letter combinations are the
possible genotypes of offspring
• They are: Rr, Rr, rr, and rr genotypes
• From this it is possible to determine the
probability (chance) that a flower will
have a red phenotype (2/4 or 50%) or a
white phenotype (2/4 or 50%)
1. Cross a homozygous dominant tall plant
with a homozygous recessive short plant

Recessive trait = short

Dominant trait = tall
What letters would you use to represent this
cross?

T = tall

t = short

What is the genotype of a homozygous
dominant tall plant?  TT

What is the genotype of a homozygous
recessive short plant?  tt

T
TT x tt
t
t
Tt
Tt

Genotype
0 % TT
100 % Tt
0 % tt

T
Tt
Tt
Phenotype
100 % tall
0 % short
#2. Cross a heterozygous brown haired
rabbit with a homozygous recessive white
haired rabbit.

Dominant trait =

brown hair
Recessive trait = white hair
o What letters would you use to represent this
cross?

B = brown

b = white

What is the genotype of a heterozygous
brown haired rabbit?  Bb

What is the genotype of a homozygous
recessive white haired rabbit?  bb

B
Bb x bb

b
b
Bb
Bb
Genotype
0 % BB
50 % Bb
50 % bb

b
bb
bb
Phenotype
50 % brown
50 % white
3. Cross a homozygous dominant long
haired guinea pig with a
heterozygous long haired guinea pig.

Dominant trait =
long hair

Recessive trait =
short hair
o What letters would you use to represent this
cross?

L = long

l = short

What is the genotype of a homozygous
dominant long haired guinea pig?  LL

What is the genotype of a heterozygous
long haired guinea pig?  Ll

L
LL x Ll
L
l
LL
Ll

Genotype
50 % LL
50 % Ll
0 % ll

L
LL
Ll
Phenotype
100 % long
0 % short
4. Cross a heterozygous wrinkly
elephant with another heterozygous
wrinkly elephant.

Dominant trait =
wrinkly skin

Recessive trait = smooth skin
 What letters would you use to represent this
cross?


W = wrinkly
w = smooth
What is the genotype of a heterozygous
wrinkly-skinned elephant?  Ww
 What is the genotype of a heterozygous
wrinkly-skinned elephant?  Ww


Ww x Ww
W
W
WW

w
Ww
Genotype
25 % WW
50 % Ww
25 % ww

w
Ww
ww
Phenotype
75 % wrinkly
25 % smooth
5.
A plant that is homozygous dominant for round
seeds is crossed with a heterozygous plant.
What is the probability that the offspring will have
wrinkled seeds?  R = round  r = wrinkled
RR x Rr
R
R
r
RR
Rr

Genotype
50 % RR
50 % Rr
0 % rr

R
RR
Rr
Phenotype
100 % round
0 % wrinkled