1. KCI 2014
BEYOND MENDELIAN GENETICS Conditions that Mendel didn’t explore: Incomplete Dominance Co-dominance Multiple Alleles Polygenic Traits Lethal gene

2. 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

3. 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
Another example of an intermediate expression may be the pitch of human male voices.  The lowest and highest pitches apparently are found in men who are homozygous for this trait (AA and aa), while the intermediate range baritones are heterozygous (Aa).  The child-killer disease known as Tay-Sachs  is also characterized by incomplete dominance.  Heterozygous individuals are genetically programmed to produce only 40-60% of the normal amount of an enzyme that prevents the disease.
Fortunately for Mendel, the pea plant traits that he studied were controlled by genes that do not exhibit an intermediate expression in the phenotype.  Otherwise, he probably would not have discovered the basic rules of genetic inheritance.
3 phenotypes are observed in F2 in a 1:2(blended traits):1 ratio

4. Incomplete dominance
However, the alleles of flower color do segregate according to Mendel’s law of independent assortment
In human, traits showing incomplete dominance:
Some genes are incompletely penetrant.   That is to say, their effect does not normally occur unless certain environmental factors are present.  For example, you may inherit the genes that are responsible for type 2 diabetes but never get the disease unless you become greatly overweight, persistently stressed psychologically, or do not get enough sleep on a regular basis.   Similarly, the genes that cause the chronic autoimmune disease, multiple sclerosis , may be triggered by the Epstein-Barr virus  and possibly other specific environmental stresses.  New research suggests that abundant exposure to the sun in childhood can provide some protection from developing MS.  Subsequently, people who grow up in tropical and subtropical regions of the world have significantly lower rates of MS as adults.

5. 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)

6. Sickle cell disease in humans. Trait concerned: Shape of blood cells
Incomplete dom.
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
Sickle cell anemia: cannot say its autosomal recessive or dominant but a mutation on the HBB gene which provides instruction to make beta globin part of the 4-subunit hemoglobin
This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.
Heterozygous HbAHbS advantage: prevent parasites from further infecting the cell
Living With and Managing Sickle Cell Disease (Nicholas)

7. Probability of offspring in F1
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
Genotype
25% HbA HbA
50% HbA HbS
25% HbS HbS
Phenotype
25% normal blood shape
50% with sickle cell trait
25% crescent blood shape (sickle cell anemia)
HbA
HbS
HbA HbA
HbA HbS
HbS HbS
 In regard to the presence or absence of anemia, the HbA allele is dominant. In regard to blood-cell shape, however, there is incomplete dominance. Finally, as we shall now see, in regard to hemoglobin itself, there is codominance (?). 
Answer: 25% chance their offspring will have sickle cell anemia (HbSHbS)

9. 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.)
Roan cow is the product of mating of shorthorn cattle: a red bull and a white cow.

10. Codominance
Why small r?
Correct the letter HR, not Hr as expressed in your textbook

11. 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 

12. 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
Multiple Alleles: when more than two different alleles exist for the same trait. (Remember: each individual will only have two alleles for a trait but there are several alleles to choose from.)
Some traits are controlled by far more alleles.  For instance, the human HLA system, which is responsible for identifying and rejecting foreign tissue in our bodies, can have at least 30,000,000 different genotypes.  It is the HLA system which causes the rejection of organ transplants. The more we learn about human genetics the more it becomes clear that multiple-allele series are very common.  In fact, it now appears that they are more common than simple two allele ones.

14. Reminder

15. Human blood types: example of both codominance and dominance
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
Alleles of the I gene
What do they enable RBC’s membrane to have? IA
Polysaccharide A IB
Polysaccharide B i
No polysaccharide
Possible alleles from a female
Genotype
Blood type
IAIA
IAi
Type A
IBIB
IBi
Type B
IAIB
Type AB ii
Type O IA IB i
IAIA
IAIB
IAi
IBIB
IBi ii
Possible alleles from a male
RECAP:

16. Multiple Alleles (Cont.) – Rabbit Coat
In rabbits, coat color is controlled by multiple alleles. Full color (C), white (c), light-gray or chinchilla (cch) and white with black points or a Himalayan (ch). Full color is dominant to all the other alleles. Chinchilla is dominant to Himalayan and white.

17. cch c C Ccch Cc ch cchch chc P: cchc x Cch F1:
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:
Genotypic Ratios:
1 Ccch: 1Cc : 1 cchch: 1 chc
Phenotypic Ratios:
2 dark grey: 1 light grey: 1 Himalayan
cch c C
Ccch Cc ch
cchch
chc
One trait (i.e. rabbit coat color)
is influenced by multiple alleles

18. Complete attached practice problems on incomplete-, co-dominance and multiple alleles Taking up Practice problem

19. 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? _____ %
SpongeBob - Incomplete Dominance
ANSWER KEY:
1. Red - RR, Blue - BB, Purple - RB
2A. RB - purple
2B. 0%
2C. 100%
2D. 0%
3A. RR - red, BB- blue, RB - purple
3B. 25%
3C. 50%
3D. 25%
_F R____ Blue - _ F B _____ Purple - __ F B FR ___
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? _____ %

20. 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? ____%

21. 3. In a certain fish, blue scales and red scales are codominant
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 =

22. 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.
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.
Human hair color is controlled by one gene
with four alleles (with some incomplete dominance):
HBr = brown HBd = blonde hR = red hbk = black
The possible genotypes and phenotypes:
HBdHBd or HBdhbk = blonde HBdHBr = mousy brown
HBdhR = strawberry blonde HBrHBr or HBr hbk = brown
HBrhR = auburn hRhR or hRhbk = red hbkhbk = black
What do you think your parent’s phenotypes and genotypes for hair color are?
What are your phenotype and genotype for hair color?
If someone with auburn hair has children with someone with red hair (but whose mother had black
hair), what are the genotype and phenotype probabilities for their children?

23. 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.

24. Incomplete dom.
Show:
a. Pink x Red
b. Pink x Pink
c. White x White