Genetics (Recap)
Let’s look at this again
Gene
 The part of a chromosome that codes for a
specific trait
 Has two Alleles
◦ One from mom and other from dad
◦ Can either be dominant or recessive
 If Dominant (T)  then the trait will be expressed
 If Recessive (t)  then the trait can only be
expressed when there’s another recessive allele
What is a Gene?
A gene is homozygous (pure) when both
alleles are the same
◦ Ex. TT (homozygous dominant) or tt (homozygous
recessive)
A gene is heterozygous (hybrid) when both
alleles are different
◦ Ex. Tt
Genotype
◦ Name of the gene:
Phenotype
TT, Tt or tt
◦ What the gene looks like or does…
◦ Ex. albinism genotype is aa but the phenotype is
lack of skin or hair color
How Are They Written?
A cross that involves only one gene:
 Example:
A homozygous dominant tall plant is
crossed with a heterozygous tall plant. Give
the genotypic and phenotypic ratios for the
offspring:
T
T

TT x Tt
Genotype Ratio:
2TT and 2Tt
Phenotype Ratio:
4 Tall Plants
T
t
T T
T T
T t
T t
What is a Monohybrid Cross?
In silkworms a single gene determines the
color of the cocoon. The yellow cocoon allele is
dominant. It is indicated by “C”. The white
allele is recessive, and indicated by “c”.
Complete the following Punnett Square
assuming a homozygous dominant male
fertilizes a homozygous recessive female.
Genotype Ratio:
4Cc
C
CC x cc
c
C c
C
C c
Phenotype Ratio:
4 yellow cocoons
c
C c
C c
What Are Some Examples?
The length of fur in guinea pigs is controlled by
a single gene. The dominant allele “F”
produces short fur. The recessive allele “f”
produces long fur. Complete the following
Punnett Square assuming a heterozygous male
fertilizes a homozygous recessive female
Genotype Ratio:
2 Ff and 2 ff
F
Ff x ff
f
F f
f
f f
Phenotype Ratio:
2 short haired
2 long haired
f
F f
f f
What Are Some Examples?

A Cross that involve TWO traits
In race horses, black hair (F) and a trotting
gait (G) are dominant traits. Recessive
traits are chestnut hair (f) and a pacing
gait (g). The genes for hair color and gait
are on two different chromosomes.
Determine the possible offspring from a
cross between two heterozygous black
Trotters.
FfGg x FfGg
Gamete Combinations:
FG Fg fG
FG Fg fG
fg
fg
What is a Dihybrid Cross?
FG
FG
Fg
fG
fg
FFGG
FFGg
FfGG
FfGg
FFGg
FFgg
FfGg
Ffgg
fG
FfGG
FfGg
ffGG
ffGg
fg
FfGg
Ffgg
ffGg
ffgg
Fg
Genotype Ratio:
FFGG l, FFGg ll, FfGG ,ll FfGg llll,
FFgg l , Ffgg ll,
ffgg l,
ffGG l, ffGg ll,
Remember:
FF or Ff:
ff:
GG or Gg:
gg:
So:
Black Hair
Chestnut Hair
Trotting Gait
Pacing Gait
Black Hair Trotters:
Chestnut Trotters:
Black Hair Pacers:
Chestnut Pacers:
9
3
3
1
When traits mix like paint
 Example:
◦ When Red snapdragons are crossed with white
snapdragons the resulting offspring are pink.
RR x WW
R
Genotype Ratio:
4 RW
W
R W
R
R W
Phenotype Ratio:
4 Pink
W
R W
R W
What is Incomplete Dominance?
When traits mix like paint
 Example:
◦ Cross a pink snapdragon with another pink
snapdragon.
RW x RW
R
Genotype Ratio:
1 RR, 2 RW, 1 WW
R
R R
W
R W
Phenotype Ratio:
1 Red, 2 Pink, 1 White
W
R W
WW
What is Incomplete Dominance?


When both forms of a trait are displayed at
the same time
AIA X
I
ii
Example
◦ ABO blood Types
◦ Type A:
IAIA, or IAi
◦ Type B:
IBIB, or IBi
◦ Type AB:IAIB
◦ Type O:
ii
Cross a homozygous type A
with a type O
 Genotype Ratio:
4 IAi
 Phenotype Ratio:
4 Type A
i
i
IA
IA
IAi
IAi
IAi
IAi
What is Codominance?
Cross a type AB with a type O
 Homozygous type A:
IAIB
 Type O:
ii
 Genotype Ratio:
2 IAi; 2 IBi


Phenotype Ratio:
2 Type A
2 Type B
Probabilities:
Type A: 2 out of 4
Type B: 2 out of 4
i
i
IAIB X
ii
IA
IB
IAi
I Bi
IAi
I Bi