Punnett Squares
Punnett Squares
A Punnett square illustrates how the
parents’ alleles might combine in offspring.
 The top of the Punnett Square are one of the
parent’s alleles, the side of the Punnett
square shows the other parent’s alleles for
the trait

 Heterozygous: having dissimilar pairs of
alleles for any hereditary characteristic
 Homozygous: having identical pairs of
alleles for any given pair of heredity
characteristics
Mendel’s Laws of Inheritance
1. Law of Dominance: In heterozygous genotypes,
dominant alleles hide recessive alleles when both
are inherited by an organism. (Hh means the
dominant trait will show up in the phenotype)
Mendel’s Laws of Inheritance
1. The Law of
Segregation
(separation):
Each organism
has 2 alleles for
a gene. Only
one allele from
each gene pair
will be passed on
to the offspring.
Homozygous/Heterozygous
Hmzg / Htzg

Cross a homozygous
dominant tall plant
with a homozygous
recessive short plant.
What letters would
you use to represent
this cross?

Dominant trait = tall

Recessive trait = short

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.
o What letters
would you use
to represent
this cross?



Dominant trait =
brown hair
Recessive trait =
white hair
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

Bb x bb
b
b
B
Bb
Bb
b
bb
bb

Genotype
0 % BB
50 % Bb
50 % bb Phenotype
% brown
50
% white
50
3.. Cross a homozygous dominant
long haired guinea pig with a
heterozygous long haired guinea pig.
o What letters
would you use
to represent
this cross?

L = long


Dominant trait =
long hair
Recessive trait =
short
hair

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.
 What letters
would you use
to represent
this cross?

Dominant trait =
wrinkly skin
Recessive trait =
smooth
skin
 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

RR x Rr
R
R
r
RR
Rr

r = wrinkled
Genotype
50 % RR
50 % Rr
0 % rr

R
RR
Rr
Phenotype
100 % round
0 % wrinkled