Transcript Punnet Squares

Genetic Crosses &
Punnett Squares
Important Terms
 Dominant
Trait: the trait that shows when
two different alleles for the trait are
inherited ex- tongue rolling
 Recessive
Trait: the trait that seems to
disappear when two different alleles for the
trait are inherited ex- non-rolling
Important Terms
 Phenotype: an organism’s physical
appearance
ex- tall, short, white flowers, green seeds
 Genotype: an organism’s gene code
ex- Tt, TT, tt, WW, Ww, yy
Important Terms
 Heterozygous: having dissimilar pairs of
alleles for any hereditary characteristic
ex- Tt, Ss, Ll, Mm (hybrid)
 Homozygous: having identical pairs of
alleles for any given pair of heredity
characteristic
ex- tt, SS, LL, mm (purebred)
Punnett Squares
Used to illustrates how
the parents’ alleles might
combine in offspring.
 Used to predict the
probability or % chance
of an offspring's genotype
and phenotype

Reginald Punnet developed the
Punnett Square in 1910
Punnett Squares

The top of the Punnett Square shows one of
the parent’s alleles, the side of the Punnett
square shows the other parent’s alleles for
the trait
t
T
T
t
Mendel’s Laws of Inheritance
Law of Dominance: In heterozygous genotypes,
dominant alleles hide recessive alleles when both
are inherited by an organism.
Mendel’s Laws of Inheritance
The Law of Segregation:
Each organism has 2
alleles for a gene and
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?

B = brown


Dominant trait =
brown hair
Recessive trait =
white hair

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
50 % brown
50 % white
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?

W = wrinkly

Dominant trait =
wrinkly skin

Recessive trait =
smooth skin

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