KCI 2014
BEYOND MENDELIAN GENETICS
Conditions that Mendel didn’t explore:
Incomplete Dominance
Co-dominance
Multiple Alleles
Polygenic Traits
Lethal gene
Limitations of Mendelian genetics
• Some genes are NOT independently
assorted simply because they are too
close to each other on a chromosome
• some alleles are incompletely dominant
• some genes are sex-linked
• A single trait can be controlled by many
genes which can have multiple alleles
Incomplete dominance
Flower color of snapdragons
• A condition in which
neither allele for a
gene completely
conceals the
presence of the
other; it results in
intermediate
expression of trait
3 phenotypes are
observed in F2 in a
1:2(blended traits):1
ratio
Incomplete dominance
• However, the alleles of flower color do
segregate according to Mendel’s law of
independent assortment
• In human, traits showing incomplete
dominance:
How to represent incomplete dominance?
• Use superscripts rather than upper-case and
lower-case letters since neither allele is
dominant over the other
• Eg. Snapdragon flower colour: both alleles
affect the colour of flower C
• 2 alleles are therefore CR (red) and CW (white)
Incomplete dom.
https://www.youtube.com/watch?v=R4-c3hUhhyc
Sickle cell disease in humans.
Trait concerned: Shape of blood cells
Hb: letter chosen to represent hemoglobin
HbAHbA: normal biconcave disk
HbSHbS: sickle cell S shape
HbAHbS sickle cell trait (some
normal and some sickle blood
cells under prolonged low O2
condition)  resistant to
malaria giving the individuals
a heterozygous advantage
If both parents have sickle cell trait (heterozygous),
what is the chance of their offspring having sickle
cell anemia?
P: HbAHbS x HbAHbS
F1:
Probability of offspring in F1
HbA
HbS
Genotype
25% HbA
HbA
50% HbA
HbS
25% HbS
HbS
HbA
HbA HbA
HbA HbS
Phenotype
25% normal
blood
shape
HbS
HbA HbS
HbS HbS
50% with
sickle
cell
trait
25%
crescent
blood
shape
(sickle
cell
anemia)
Answer: 25% chance their offspring will have sickle cell
anemia (HbSHbS)
Codominance
•
Is the condition in which both alleles for a trait
are equally expressed in a heterozygous
offspring; both alleles are dominant
E.g
a. Shorthorn roan cow
Cattle can be red (HRHR = all red
hairs), white (HWHW = all white hairs),
or roan (HRHW = red & white hairs
together)
b. Human AB blood type (two types of
protein ("A" & "B") appear together on
the surface of blood cells.)
Codominance
Correct the letter HR, not Hr as expressed in your textbook
What’s the difference between incomplete
dominance and codominance?
• E.g.
a. Incomplete dominance:
- Both alleles are partially
expressed
- Offspring shows a third phenotype
that is something between the
parents’ phenotypes (i.e. a blend)
b. Co-dominance:
- Both alleles are fully expressed
- Offspring shows a third phenotype
that is completely different from
both the parents’ phenotype
Correct the letter r to R in this
co-dominance cross 
Multiple alleles
• So far we’ve studied traits controlled by one gene with
two alleles
• An individual has only 2 alleles for a gene but many
different alleles for a gene can exist within a
population
• A gene with more than 2 alleles is said to have
multiple alleles
Reminder
Human blood types: example of both
codominance and dominance
Possible alleles from a male
• One single gene determine a
person’s blood type: A, B or O
• This gene is designated as I and
have 3 alleles that code for antigen
proteins attached to the RBC’s
membrane
Possible alleles from a female
Alleles
of the I
gene
What do they enable
RBC’s membrane to
have?
IA
Polysaccharide A
IB
Polysaccharide B
i
No polysaccharide
Genotype
Blood type
Type A
IA
IB
i
IAIA
IAi
IA
IAIA
IAIB
IAi
Type B
IB
IAIB
IBIB
IBi
IBIB
IBi
IAIB
Type AB
ii
Type O
i
IAi
IBi
ii
RECAP: http://wps.aw.com/bc_goodenough_boh_3/104/26722/6840887.cw/content/index.html
Multiple Alleles (Cont.) – Rabbit Coat
Predict the genotypic ratio and phenotypic ratio of a cross between a
heterozygous Chinchilla (cchc) with a heterozygous dark grey (Cch).
P: cchc x Cch
F1:
cch
c
C
Ccch
Cc
ch
cchch
chc
Genotypic Ratios:
1 Ccch: 1Cc : 1 cchch: 1
c hc
Phenotypic Ratios:
2 dark grey: 1 light grey:
1 Himalayan
One trait (i.e.
rabbit coat color)
is influenced by
multiple alleles
Complete attached practice
problems on incomplete-, codominance and multiple alleles
Taking up Practice problem
Incomplete dominance practice problem
SpongeBob loves growing flowers for his pal Sandy! Her favorite
flowers, Poofkins, are found in red, blue, and purple.
1. Write the correct genotype for each color if R represents a red gene and
B represents a blue gene.
Red Blue Purple -
2. What would happen if SpongeBob crossed a Poofkin with red flowers
with a Poofkin with blue flowers. (a) Give the genotypes and phenotypes for
the offspring.
(b) How many of the plants would have red flowers? _____%
(c) How many of the plants would have purple flowers? _____ %
(d) How many of the plants would have blue flowers? _____ %
3. What would happen if SpongeBob crossed two Poofkins with purple flowers?
(a) Give the genotypes and phenotypes for the offspring.
(b) How many of the plants would have red flowers? _____%
(c) How many of the plants would have purple flowers? _____ %
(d) How many of the plants would have blue flowers? _____ %
Codominance practice problem
1. In some chickens, the gene for feather color is controlled by
codominance. The allele for black is B and the allele for white
is W. The heterozygous phenotype is known as erminette.
• a. What is the genotype for black chickens? ____
• b. What is the genotype for white chickens? ____
• c. What is the genotype for erminette chickens? ____
2. If two erminette chickens were crossed,
what is the probability that:
a. They would have a black chick? ____%
b. They would have a white chick? ____%
3. In a certain fish, blue scales and red scales are codominant. When a fish has
the genotype B R, it has a patchwork of blue and red scales. What happens if
you breed this fish with a fish that only has Blue Scales.
Step 1: Determine the genotypes of the
parents
CB CR =
CB CB =
CR CR =
Multiple alleles practice problem
1. In the 1950’s, a young woman sued film star/director Charlie
Chaplin for parental support of her illegitimate child. Charlie
Chaplin’s blood type was already on record as type AB. The
mother of the child had type A and her son had type O blood.
a. Complete a Punnett square for the possible cross of Charlie
and the mother.
b. The judge ruled in favor of the mother and ordered Charlie
Chaplin to pay child support costs of the child. Was the judge
correct in his decision based on blood typing evidence?
Explain why or why not.
*Use Punnett squares to support your answer.
Alien Genotyping:
2. In these aliens, horns are controlled by three allelles
• One center horn (A) is codominant with two horns (B). If an
alien inherits both alleles (AB), then the alien has 3 horns. A
recessive allele (O) results in an alien have no horns. Match the
genotype to each of the pictures below.
Incomplete dom.
Show:
a. Pink x Red
b. Pink x Pink
c. White x White