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Bio NOTES: Traits and Probability
The Punnett square is a grid system for predicting all possible
genotypes resulting from a cross.
- The axes represent
the possible gametes
of each parent.
- The boxes show the
possible genotypes
of the offspring.
The Punnett square gives the
ratio of possible genotypes
and phenotypes.
Monohybrid crosses examine
the inheritance of only
one specific trait
homozygous dominant
x
homozygous recessive
=
all heterozygous
heterozygous
x
heterozygous
=
1 homozygous dominant
2 heterozygous
1 homozygous recessive
genotype ratio – 1:2:1
phenotype ratio – 3:1
homozygous recessive
x
heterozygous
=
2 heterozygous
2 homozygous recessive
genotype ratio – 1:1
phenotype ratio – 1:1
Example: Two pea plants
cross, one homozygous
dominant yellow seeds and
the other homozygous
recessive green seeds.
Key: Y=_______
Yellow and y= _______
Green
yy
YY X ____
P, Parents: ____
y
Y
Yy
Y
Yy
y
Yy
Yy
Yy X ____
Yy
Ex. Continued: What
F1, Parents: ____
offspring would result
from an F1 cross? Results:
Y
y
What are the genotypes and
phenotypes of the F1
generation?
Results:
F1 Genotypes:
100% Yy
F1 Phenotypes:
100% Yellow
F2 Genotypes:
25% YY, 50% Yy, 25%yy
F2 Phenotypes:
75% Yellow, 25% green
Y YY
Yy
y Yy
yy
An organisms phenotype is based on its genotype & its environment!!
•A plant may have the GENES to grow tall, but not the sunlight.
Monohybrid Crosses can be used in REVERSE to find the
genotypes of an “unknown” parent.
Ex): A homozygous
Hairy and h= _______
Bald
Key: H=_______
hh
Mom Cat Geno: ____
recessive bald cat had 4
kittens, two were bald like Dad Geno: HH or Hh? _________________
Father is Hh!!
their mother and the
h
h
h
h
F1: 
other two were hairy.
H Hh
Hh
Hh
2 Bald H Hh
What was the genotype of
H Hh
Hh
hh
the father cat?
2 Hairy h hh
Test Cross: A process that uses an organisms phenotype to determine its
genotype. The unknown is crossed with an organism that is homozygous
recessive (because it cannot hide any traits).
T T
T
t
Ex): In pea plants, tall plants are
dominant to short. Tall plants
t Tall Tall
t Tall Short
could be TT or Tt.
t Tall Tall
t Tall Short
Determine the genotype of an tall
TT X t t
Tt t t
_____
____X
pea plant.
?? X t t
Dihybrid Cross
A dihybrid cross involves two traits.
Mendel’s dihybrid crosses with heterozygous plants yielded a
9:3:3:1 phenotypic ratio.
Mendel’s dihybrid crosses led to his
second law,
the law of independent
assortment.
The law of independent
assortment states that allele
pairs separate independently of
each other during meiosis.
Heredity patterns can be calculated with probability.
• Probability is the likelihood that something will happen.
• Probability predicts an average number of occurrences,
not an exact number of occurrences.
number of ways a specific event can occur
• Probability =
number of total possible outcomes
• Probability applies to
random events such as
meiosis and fertilization.
Questions and Practice Problems:
Phenotype
An organisms physical characteristics are called its: _________
Genotype
An organisms genetic make up is called its: ____________
Alternative forms of a gene are called: ___________
Alleles
Examples of allele alternatives are: Tall and _______
Short pea plants
Curly Hair
Straight Hair & __________
5. Indicate: Homozygous Dominant (HD), Homozygous Recessive (HR) or
Heterozygous (HE)
HE bb=____
HR E1E2=___
HE Xx=____
HE
AA=____
HD Rr=____
6. If green hair is dominant to white hair indicate the phenotypes below.
Green Hair Gg=____________
Green Hair gg=____________
White Hair
GG = _____________
1.
2.
3.
4.
7. Cross a homozygous dominant three-eyed alien with a homozygous
recessive one-eyed alien. Determine the F1 genotypes & phenotypes.
Key:
Results:
T
T
__=__________
T Three eyes
F1 Genotypes:
t __________
One eye
__=
TT X ____
tt
P, Parents: ____
t
Tt
Tt
t
Tt
Tt
100% Tt
F1 Phenotypes:
100% Three eyes