Codominant vs Incomplete Dominant- What’s the
difference?
Beyond Dominant and Recessive
• Incomplete Dominance
One allele is not completely dominant over
the other – something in the middle is
expressed
Ex. Red and White Snapdragons
Result can be heterozygous (Rr) or two
separate dominant alleles (RW) each
resulting in a mixture of both alleles
Another way that incomplete
dominance can be expressed
• Red= RR
• White= WW
• RW= pink- each allele
is equally expressed
to result in a blended
product
One way to express incomplete
dominance
• RR (Red) X rr
(White)= (Rr)Pink
• Rr- results in a
blended result of
PINK
Incomplete Dominance Practice
• In certain cats, tail length is determined by
a gene that demonstrates incomplete
dominance. The allele that causes a long
tail (T) is not completely dominant over the
allele that causes no tail (t). If a cat is
heterozygous forthis trait (Tt), then the cat
will have a short tail. What is the
probability that the offspring will be manx
cats (no tail) if a short – tailed cat is bred
with a manx cat (no tail)?
Incomplete Dominance Practice
• Incomplete dominance is seen in
snapdragons. The allele that causes red
flowers (F) is not completely dominant
over the allele that causes white flowers
(f). When a plant is heterozygous for the
trait of flower color (Ff), pink flowers result.
Cross two pink snapdragons, and provide
the genotype and phenotype of all
offspring.
Beyond Dominant and Recessive
• Codominance
Both alleles are expressed in the phenotype
Ex. Cow Hair Color
RR – Red
WW – White
RW – Roan (Red & White)
Practice
Codominance/Incomplete Dominance #1-4
Beyond Dominant and Recessive
• Multiple Alleles
Genes have more then two alleles
Ex. Blood Type
Type A blood- AA or AO alleles
A is dominant to O
Type B blood- BB or BO alleles
B is dominant to O
Type AB- codominant- A and B alleles
A nor B is dominant so both are expressed on organisms RBC
Type O- recessive- OO alleles
Both alleles must be recessive in order to have type O.
More on blood types…..
• The blood type determines what antibodies are
located within the blood. Type A blood has type B
antibodies. If type B blood is put into their bodies,
their immune system reacts as if it were a foreign
invader, the antibodies clump the blood - can cause
death.
• Type AB blood has no antibodies, any blood can be
donated to them - they are called the "universal
acceptors"
• Type O blood has no surface markers on it,
antibodies in the blood do not react to type O blood,
they are called the "universal donors"
Co-dominance Practice
• In humans, blood types A and B are
equally dominant (codominant). Both types
are dominant to type O. A man with type
AB blood marries a woman with type O
blood. Give the genotypes and
phenotypes of all possible offspring.
Co-dominance Practice
• If a man with blood type A, one of whose
parents had blood type O, marries a
woman with blood type O, what
percentage of their offspring would have
blood type OO?
Polygenic Traits
• Traits that are controlled by the interaction
of several genes.
• Example:
– Reddish brown eyes in varying degrees found
in fruit flies is controlled by 3 genes
– Human skin color is controlled by 4 different
genes which result in a variety of skin color.
Sex-linked Genetics
Ex. Colorblindness
Sex Chromosomes- last pair
(23rd) in a karyotype
MALE KARYOTYPE
FEMALE KARYOTYPE
Sex Chromosomes- last pair
(23rd) in a karyotype
• Male – XY and Females – XX
• The 23rd pair of chromosomes will
determine the gender of an individual
• Very few genes are located on the Y
chromosome……Most are located on the
X
• Sex linked alleles will ALWAYS be
tracked on the X chromosome ONLY
when we conduct practice genetic
problems
Sex-Linked Genes
• Ex. Colorblindness is carried on the sexchromosomes
• It is a recessive trait – Xc
How many genes do females need to
express the trait (colorblindness)?
2 Xc Xc
How many genes do males need to
express the trait (colorblindness)?
1 XcY
Sex-Linked Punnett Square
• Let C = Normal Vision and c = Colorblind
• Cross: Normal Male ( ) x Carrier Female (
)
Sex-Linked Punnett Square
C
• Let C = Normal Vision and c = Colorblind
• X C Y x X C X c = Normal Male x Carrier Female
1st put male genotype
C
X
Y
on the top of the table
& female genotype on
the left side
C
X
X
c
Sex-Linked Punnett Square
C
• C – Normal Vision and c - Colorblind
• X C Y x X C X c - Normal Male x Carrier Female
2nd, cross them
XC
Y
X
C
X
c
XC XC
C
X X
c
XCY
c
X Y
Sex-Linked Punnett Square
C
list the sex
• C – Normal Vision and c - Colorblind 3rd,
and appearance
C
C
c
X Y x X X -Normal Male x Carrier Femaleof each possible
offspring
XC
Y
Offsprings:
X
1 Normal Female
C
XC XC
XCY
1 Normal (Carrier)
Female
1 Normal Male
X
c
C
X X
c
c
X Y
1 Colorblind Male
Sex-linked Practice
• Hemophilia is a disease caused by a gene
found on the X chromosome. Therefore, it
is referred to as a sex – linked disease.
The recessive allele causes the disease. A
normal man marries a woman that is
heterozygous for the trait. Give the
genotypes and phenotypes of all possible
offspring. Will any of their children have
the disease?
PRACTICE and HW
• Complete problems 1-3 on the sex linked
genetic practice problems sheet NOW!
• Complete the remaining 3 Co-dominant
and Incomplete dominant practice
problems and Sex Linked practice
problems # 4-8 from today’s class for HW