Definition: the expressed version of a trait
(what you actually see)
A. Determined by the two genes present
that were inherited from the parents.
 EX: Blue eyes, freckles, lactose intolerance,
flower color, fur color
Definition: The genes that are present for
any trait.
A. These are the genes that were passed on
from your parents in their egg and sperm.
B. The genotype may not always be evident
(clear) from the traits expressed.
C. Some traits are stronger than others and
can mask (hide) the expression of the
“weaker” trait.
Allele
Def: an alternate version of
expression for a gene
Ex: Gene – Hair color
Alleles – brown, black, red, blonde
Ex: Gene- Freckles
Alleles- freckles present or absent
A. Alleles can be dominant or
recessive
B. You can get different ones from
each parent
Copy of chromosome from dad
Allele for dimples
Present
Absent
Copy of chromosome from mom
 Recessive:
The “weaker” version  you must have both
recessive genes to have that phenotype
expressed.
 Dominant:
A. The “stronger” version
B. Dominance does not mean the genes are
“aggressive”, it just expresses itself more often.
C. 3 types of Dominance:
Complete Dominance
Incomplete Dominance
Co-dominance
Definition: the actual combination of the
genes you inherited (the alleles)
A. Alleles in a genotype are represented by
letters:
capital letter = Dominant
lowercase letter = Recessive
B. There are 2 possible combinations:
heterozygous
homozygous
1 copy of
2 copies of the
each allele
same allele
Aa
AA or aa
(both dom. or both rec.)
 Complete Dominance:
- A. You will have the trait (phenotype) it
codes for if you received the dominant
gene from one or both parents.
- Ex: Freckles
Genotype:
AA
Aa
aa
Phenotype:
freckles
freckles
no freckles
 Incomplete Dominance:
A. You must be homozygous to have the
phenotype (show the trait)
B. If you are heterozygous then the
phenotype is an “intermediate” (blending
of the traits).
- Ex: hair texture
Genotype:
Phenotype:
AA
curly
Aa
wavy
aa
straight
 Co-dominance:
A. when two alleles are equal to each other,
but are dominant to another
B. Being heterozygous for these two alleles
means both phenotypes will be expressed
- Ex: Blood Type
Genotype:
Phenotype:
AA or AO
A
BB or BO
B
AB
AB
OO
O
How can two people
with brown eyes have a
blue eyed baby?
Figuring out possible
outcomes…
Punnet
Squares
How Do Monohybrid crosses work?
Ff x ff
2 Parents
One has Freckles Ff
F
One does not ff
What is the chance that
they will have a child
without freckles
At the end of meiosis, the
chromosomes carrying the
genes for those traits will
separate and go into
opposite gametes.
f
f
Ff
Ff
f
ff
ff
How Do Dihybrid Crosses Work?
Just the same as a
monohybrid test
cross,
the traits from the
parents will separate
from each other and
end up in separate
gametes.
Simply put, the “A” genes
will separate from
each other, as will the
“B’s”.
Father: AaBb
Possible Gametes:
AB Ab aB ab
Mother: AaBb
Possible Gametes:
AB Ab aB ab
AaBb x AaBb
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb