What’s the difference between a
phenotype and a genotype?
Agenda for Friday Jan 30th
1. Go over HW (Dihybrid)
2. Incomplete and codominance
Complete Dominance
Purebred red snapdragons were crossed with
purebred white snapdragons all offspring were pink
Incomplete Dominance
• one allele for a trait is not completely dominant
over the other allele
• Combined (blended) phenotype
R = allele for red flowers
W = allele for white flowers
red x white ---> pink
RR x WW ---> 100% RW
Co-dominance
• In COdominance, both traits appear together in
the phenotype of hybrid (heterozygous)
WHITE
RED
ROAN
Practice
Set up genotypes for the phenotypes listed in each
set. Remember that the "medium" trait is always
heterozygous.
a) Birds can be blue, white, or white with blue-tipped
feathers.
b) Flowers can be white, pink, or red.
c) A Hoo can have curly hair, spiked hair, or a mix of
both curly and spiked.
1. Write the genotypes for the pictured phenotypes
2. Show the cross between a star-eyed and a circle
eyed.
What are the phenotypes of the offspring?
What are the genotypes?
3. Show the cross between a circle-star eyed, and a
circle eyed.
How many of the offspring are circle-eyed?
How many of the offspring are circle-star eyed?
Write the genotypic and phenotypic ratios.
4. Show the cross between two circle-star eyed.
How many of the offspring are circle-eyed?
How many of the offspring are circle-star eyed?
How many are star eyed?
In humans, straight hair and curly hair are
incompletely dominant traits that result in hybrids
that have wavy hair.
Cross a straight hair with a wavy hair.
What are the chances of having a curly haired child?
What are the chances of having a straight hair child?
Agenda for Monday Feb 2nd
1. Go over Incomplete/codominance wkst
2. Sex linked traits
Sex Determination
• Thomas Hunt Morgan – studied fruit flies in
the early 1900’s
Sex Determination
• Observed that one pair of chromosomes was
different between males and females
– Large one named “X” chromosome
– Smaller one named “Y” chromosome
– XX = female; XY = male
Sex Linkage
• Sex Linkage: the presence of a gene on a
sex chromosome (X or Y)
• X-linked genes: genes found on the X
chromosome
– X chromosome carries more genes
• Y-linked genes: genes found on the Y
chromosome
Fruit Fly Eye Color
• Fruit flies normally have red eyes
– Red is dominant; white is recessive
• A few males have white eyes
Morgan’s Fruit Fly Experiments
• Red-eyed female (XRXR) x White-eyed male (XrY)
XR
Xr
XRXr
XRXr
RESULTS:
F1 generation –
red-eyed
XRY
Y
XR
XRY
all
Morgan’s Fruit Fly Experiments
• Red-eyed female (XRXr) x Red-eyed male (XRY)
XR
XR
XRXR
XRY
Y
Xr
XRXr
XrY
RESULTS:
F2 generation –
3
red-eyed and 1 whiteeyed
** all white-eyed where
males…why?
Morgan’s Conclusions
• Gene for eye color is carried on the X
chromosome = eye color is an X-linked trait
• Y chromosome does not carry a gene for eye
color
• Red-eyed = XRXR, XRXr , XRY
• White-eyed = XrXr, XrY
In humans colorblindness (b) is an example of a
sex-linked recessive trait. A male without
colorblindness marries a female who isn’t
colorblind but carries the allele.
1. How many females will be colorblind?
2. What sex will any colorblind children be?
3. What percent will be male and colorblind?
In fruit flies red eye color (R) is dominant to white
eyes (r) and is a sex linked trait.
A heterozygous red eye female mates with a red
eye male.
1. How many will have red eyes?
2. What percent will have white eyes?
3. How many will be female and red eyed?
In fruit flies red eye color (R) is dominant to
white eyes (r) and is a sex linked trait.
A homozygous red eye female mates with a
white eye male.
How many males will have white eyes?
In humans colorblindness (b) is an example of a
sex-linked recessive trait. A male with
colorblindness marries a female who isn’t
colorblind and does not carry the allele.
What is the chance they will have a child that is
colorblind?
In humans colorblindness is an example of a sexlinked recessive trait. A male with colorblindness
marries a female who is a carrier.
What is the chance they will have a child that is
colorblind?
Agenda for Wednesday Feb 4th
1. Disorder project
What is meant by a sex-linked trait?
Agenda for Thursday Feb 5th
1. Go over HW
2. Multiple alleles
Quiz tomorrow
Projects due Monday
Multiple Alleles
• Multiple alleles – traits
determined by more than 2
alleles
– Increases possible number of
genotypes & phenotypes
– Blood groups
Blood Types
• A,B,O blood types
• A and B are dominant over O
– Co-dominant to each other
• O blood type is recessive
GENOTYPE
IAIA
IAi
IB IB
IBi
IAIB
ii
RESULTING PHENOTYPES
Type A
Type A
Type B
Type B
Type AB
Type O
• More alleles means more combinations
• 6 different genotypes & 4 different phenotypes
• 2 genotypes for both "A" & "B" blood --- either homozygous
(IAIA or IBIB) or heterozygous with one recessive allele for "O"
(IAi or IBi).
• The only genotype for "O" blood is homozygous recessive (ii).
• And lastly, what's the deal with "AB" blood? The "A" trait &
the "B" trait appear together in the phenotype. What is this
an example of?
Problems
• A woman with Type O blood and a man who is Type
AB have are expecting a child. What are the possible
blood types of the kid?
• What are the possible blood types of a child who's
parents are both heterozygous for "B" blood type?
One More
• What are the chances of a woman with Type
AB and a man with Type A having a child with
Type O?
Complete dominance - 2 traits
BB Bb bb
**Homozygous dominant same as heterozygous
Incomplete Dominance – 2 different letters
RR WW RW
** heterozygous is a blend of other 2 traits
Co-dominance - 2 different letters
GG TT GT
**heterozygous see both of the other 2 traits
Sex Linked Traits – use X’s and Y’s **no gene on Y
Multiple alleles – blood type, use I’s
What are the chances of a woman with Type AB
and a man with Type O having a child with
Type O?
Agenda for Friday Feb 6th
1. Quiz
2. Finish notes
3. Disorders
Epistasis
• Epistasis – one allele hides/suppresses another allele
Polygenic
• Phenotype depends on alleles in multiple
genes
– Skin color, height, eye color
– Continuous progression in expression of traits
Another example of a polygenic trait:
• Hair Color
– Hair color is controlled by alleles on chromosomes 3,
6, 10, and 18.
– The more dominant alleles that appear in the
genotype, the darker the hair!
Eye Color
• Has to do with the amount of melanin
– More melanin = darker eyes
• Multiple genes determine how much melanin
– More dominant genes = more melanin
• Heterochromia = different color eyes
– uneven distribution of melanin
Environmental
Influences
• Inherit tendency of
getting heart diseases
– Diet/exercise can also
influence
• Water/ temperature/
sunlight effect plants
In fruit flies red eye color (R) is dominant to
white eyes (r) and is a sex linked trait. Cross a
heterozygous female with a white eye male.
How many will be red eye and male?
Agenda for Tuesday April 29th
1. Sex Linked traits (go over HW)
2. Multiple alleles
3. Epistasis/polygenic
4. Talk about disorder project