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Predicting Inheritance using
Punnett Squares
Unit 5: Genetics
Chapter 11-2
Let’s Review our Vocab!
• Dominant traits cover up Recessive traits
– Dominant Allele = capital letter (B)
– Recessive Allele = lower case letter (b)
P = Parents
F1 = first generation offspring
F2 = second generation offspring
Dominant or Recessive?
• B=Black b=white
– Bb: Dom or Rec? Color?
• T=Tall t= short
– TT: Dom or Rec? Height?
• P=Purple p=white
– pp?: Dom or Rec? Color?
• Genotype: the actual alleles for a trait
• Represented by letters: AA, aa, Aa
– Homozygous Dominant: 2 dominant alleles (AA)
(purebred)
– Homozygous Recessive: 2 recessive alleles (aa)
(purebred)
– Heterozygous: 1 dominant allele & 1 recessive
allele (Aa) (hybrid)
Homozygous or Heterozygous?
• BB
• Kk
• ff
• Homozygous dominant
• Heterozygous
• Homozygous recessive
Purebred or Hybrid?
• rr
• Ww
• NN
• purebred
• hybrid
• purbred
• Phenotype: how the trait is seen
– Ex: Guinae pig fur color: black or white
P = Parents
F1 = first generation offspring
F2 = second generation offspring
Genotype or Phenotype?
•
•
•
•
•
•
•
•
Black
Bb
tt
Short
KK
Pp
Tall
White
•
•
•
•
•
•
•
•
Phenotype
Genotype
Genotype
Phenotype
Genotype
Genotype
Phenotype
Phenotype
Punnett Squares
• Used to predict the traits of each possible
offspring during a cross
• Five Step Plan
– 1) Legend
– 2) Parents
– 3) Punnett square-set it up
– 4) Punett square-fill it in
– 5) Answers in boxes
5-Step Plan for Punnett Squares
Flee Map
5-Step Plan
1) Legend
2) Parents
3) Punnett Square:
Set it up
4) Punnett Square:
Fill it in
5) Answers
Pg 10
Sample Problem
You do NOT have to write this slide!
• In guinea pigs, Black (B) is dominant to White (b)
• Black guinea pig (BB) x White guinea pig (bb)
• What are the chances that the offspring will be
black? White?
• How many offspring will be black? How many will be
white?
Five-Step Plan
• 1) Legend
– Always use the first letter of the dominant
trait to represent your alleles. (ex: in this
case, we use B for black)
– Set up a table:
Genotype
Phenotype
BB
Black
Bb
Black
bb
White
• 2) Parents - Write this as a “cross”
– BB x bb
• 3) Punnett Square - Set it up
– Draw a square and divide into four sections
– Write the genotype (letters) from one parent on
top (one letter per box)
– Write the genotype from the other parent along
the left side
B
B
b
b
• 4) Punnett Square - Fill it in
– Fill in the boxes with one letter from the top and
one letter from the left (2 letters per box)
B
B
b B b
B b
b
B b
B b
• 5) Answers
– Each box = a 25% chance of offspring inheriting
that genotype
– Always put a box around your final answer!!!
B
B
b
B b
B b
b
B b
B b
100% Black offspring
0% White offspring
Ratio: 4 : 0 OR
1 : 0 (Black : White)
• Now we have COMPLETELY answered the
question and we are done!
– Always double check!
Now you practice
on the
whiteboard!
Whiteboards
• Keep your notes & flow map in front of you.
• Please do not draw on whiteboard unless
instructed.
• For each problem you will do steps 1 & 2
(legend & parents) first.
• You will then do steps 3-5 (Punnett square and
answers).
Example:
Purple Flower (PP) x White Flower (pp)
P
p
p
P
Pp
Pp
Pp
Pp
What % of the offspring will be Purple?
White?
100 % Purple
0% White
What is the ratio of purple : white?
1: 0
(Hint: how many of each & reduce)
Example:
Tall (Tt) x short (tt)
t
T
t
Tt
tt
t
Tt
tt
What % of the offspring will be Tall?
Short?
50 % Tall
50% Short
What is the ratio of tall : short?
1: 1
(Hint: how many of each & reduce)
Example:
Brown eyes (Bb) x Brown eyes (Bb)
B
B
b
BB
Bb
b
Bb
bb
What % of the offspring will be Brown eyes? 75 % Brown
25% Blue
Blue eyes?
What is the ratio of brown eyes : blue eyes? 3: 1
(Hint: how many of each)
Example:
Dimples (DD) x No Dimples (dd)
D
d
d
Dd
Dd
D
Dd
Dd
What % of the offspring will have dimples? 100 % Dimples
0% No dimples
No dimples?
What is the ratio of dimples : no dimples? 1: 0
(Hint: how many of each & reduce)
Normal thumbs (T) are dominant to hitchiker’s
thumbs (t). A man with hitchiker’s thumb marries
a women that is heterozygous for a normal
thumb.
t
Tt
T
tt
t
t
Tt
tt
% of Normal thumbs?
% Hitchiker’s thumbs?
50 % Normal
50% Hitchiker’s
Ratio?
1: 1
Free earlobes (F) are dominant to attached
earlobes (f). A man with attached earlobes
marries a women that is homozygous for free
earlobes.
f
Ff
F
Ff
F
f
Ff
Ff
% Free Earlobes?
% Attached earlobes?
100 % Free
0% Attached
Ratio?
1: 0

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