Cross a homozygous dominant
green plant with a heterozygous
green plant. What are the genotypic
and phenotypic ratios.
Agenda for Monday April 11th
1. Complex patterns of inheritance
– Incomplete and Co-dominance
Incomplete Dominance
• Complete dominance: a heterozygous individual’s
phenotype will show the dominant trait
• When red snapdragons (RR) were crossed with
white snapdragons (rr) offspring were pink (Rr)
Incomplete Dominance
• one allele for a trait is not completely dominant
over the other allele
• Combined phenotype (blended)
R = allele for red flowers
W = allele for white flowers
red x white ---> pink
RR x WW ---> 100% RW
Co-dominance
• In COdominance, the "recessive" & "dominant"
traits appear together in the phenotype of
hybrid (heterozygous) organisms
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.
D) A Sneech can be tall, medium, or short.
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?
Welcome
In humans, straight hair (ss) and curly hair (cc) are codominant traits that result in hybrids that have wavy
hair (sc). Cross a Straight hair female with a wavy
haired male.
• What are the chances of having a curly haired child?
• What are the chances of having a straight hair child?
• What are the chances of having a wavy hair child?
Agenda
• Quiz
• 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
XY
X
XX
Y
X
Y
X
X
X
X
XX
XX
Female
Female
XY
XY
Male
Male
50% Female; 50% Male
Sex Linkage
• Sex Linkage: the presence of a gene on a sex
chromosome (X or Y)
Sex Linkage
• 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
• A few males have white eyes
• Red is dominant;
white is recessive
Welcome to Class
What is sex linkage?
Agenda
• Finish sex linked traits notes
– worksheet
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 with
colorblindness marries a female who is not
colorblind but carries the (b) allele.
Using a Punnett square, determine the
genotypic and phenotypic probabilities for
their potential offspring.
In fruit flies red eye color (R) is dominant to
white eyes (r). In a cross between two flies,
50% of the male and 50% of the female
offspring had red eyes. The other half of the
males and females had white eyes.
What are the phenotype, and all possible
genotypes, of the offspring?
Welcome to Class
In fruit flies red eye color (R) is dominant to
white eyes (r). Cross a heterozygous female with
a white eye male. What are the phenotypic and
genotypic ratios?
Agenda
• Review Sex linked traits
• Multiple Alleles
Epistasis
• Epistasis – when one allele hides another
• Labrador Coat Color
– 2 alleles
– Dominant allele E determines pigment
• ee will have no pigment
– Dominant allele B determines how dark the pigment
• EEbb or Eebb = chocolate brown
• eebb, eeBb, or eeBB will be yellow because e masks effects
of dominant B allele
Multiple Alleles
• Multiple alleles – traits that are determined by
more than 2 alleles
– We have only 2 alleles (1 from mom, 1 from dad)
– Blood groups (ABO)
– O is recessive (i allele)
ALLELE
IA
IB
i
CODES FOR
Type "A" Blood
Type "B" Blood
Type "O" Blood
GENOTYPES
IAIA
IAi
RESULTING PHENOTYPES
Type A
Type A
IBIB
IBi
Type B
Type B
IAIB
Type AB
ii
Type O
• More alleles means more combinations
• there are 6 different genotypes & 4 different phenotypes
for blood type
• Note: 2 genotypes for both "A" & "B" blood --- either
homozygous (IAIA or IBIB) or heterozygous with one recessive
allele for "O" (IAi or IBi).
• Note too that the only genotype for "O" blood is
homozygous recessive (ii).
• And lastly, what's the deal with "AB" blood? What is this an
example of? The "A" trait & the "B" trait appear together in
the phenotype.
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?