Beyond Mendel 11.3Into the Genetic Frontier
Genes can be inherited based on various patterns
• Mendel’s Dominance vs. Recessive
• Incomplete dominance
• Co-dominance
• Multiple alleles
• Polygenic genes
Incomplete Dominance• In this pattern neither allele is considered recessive
• Heterozygous conditions produce a blend of the two traits creating a third phenotype.
Co-dominance• In this pattern much like
incomplete dominance, neither allele is considered recessive
• In a heterozygous condition where each type of allele is present the phenotype displays both traits from each allele
• There are 3 possible phenotypes
Multiple alleles
• In this pattern there is more than just two alleles (3 or more)
• Combinations of other patterns
• Ex. Some alleles behave in a dominant/recessive fashion while other alleles display a co-dominant or incomplete dominant relationship
• This can lead to multiple phenotypes
There are 4 Human Blood Types
A, B, AB & O = 4 phenotypesBlood type is controlled by one
gene, with 3 different alleles – A, B and O
The 3 alleles can be combined to produce 6 different genotypes
AA BBAO BOAB OO
Inheritance of Blood TypeIs a mix of complete dominance and
codominance - AA & AO produce type A blood - BB & BO produce type B blood - AB produces type AB blood - OO produces type O blood
From this we can infer that both A & B alleles are dominant to the O allele
The A allele is codominant with the B allele
Determining Possible Outcomes
How would a person who is homozygous for B blood be notated?
- IB IB
How would a person who is
heterozygous for B blood be
notated?
- IB i
Notating Blood Type IA = A allele IB = B allele i = O allele
Example a person who heterozygous for A blood = IA iAnd a person with O blood = i i
What exactly is blood type?Each blood type is the result of a specific
combination of proteins found on the blood cells called antigens and proteins found in the blood plasma called antibodies
- type A blood = A antigens & B antibodies - type B blood = B antigens & A antibodies - type AB blood = both A & B antigens & no
antibodies - type O blood = no antigens & both A & B
antibodies
Why is Blood Typing Important?
If a person is given blood with antigens that their blood plasma contains antibodies for, the antibodies will attach to the blood causing it to clump
- this clumping is called agglutination
BloodGroup(Phenotype) O
GenotypesAntibodiesPresent inBlood
Red Blood CellsReactions When Blood from Groups Below IsMixed with Antibodies from Groups at Left
A B AB
O
A
B
AB
ii
IAIB
IBIB
orIBi
IAIA
or IAi
Carbohydrate A
Carbohydrate B
Anti-A
Anti-B
Anti-AAnti-B
—
Figure 9.20
Polygenic traits
Some traits which have many possible
phenotypes are controlled by more than one gene.
- the interactions of these genes allows for the large numbers of phenotypes.
- human examples: Hair, eye and skin color
The Inheritance of Eye Color Only partially understood. at least 3 different genes with 2
alleles each are responsible for eye color.
– bey 1 bey 2 and geyThese genes code for the
formation of the pigment melaninThe function of only 2 of the 3
genes is currently understood
The Inheritance of Eye Color as it is Currently Understood
Known as the 2 gene modelOne gene known as bey 2 has 2 alleles B
for brown eyes and b for blue eyesThe other gene known as gey has 2
alleles G codes for green eyes and g for blue eyes
The 2 Gene Model
- B is dominant to all other alleles- BBGG,BBGg,BBgg,BbGG BbGg,Bbgg = Brown eyes
- G is dominant to b bbGG , bbGg = Green eyes
- bbgg only = blue eyes
Genes and the Environment
◦ The characteristics of any organism are not determined solely by the genes that organism inherits.
◦ Genes provide a plan for development, but how that plan unfolds also depends on the environment.
◦ The phenotype of an organism is only partly determined by its genotype.
Environment Can Influence Gene Expression
example: Siamese cats
- Siamese coloring is a partial albinism. Most of the cat is white
-Black fur is only expressed in areas where the temperature is lower than the rest of the body
- if black hair is shaved and the area kept warm then the hair that grows back will be white
- if white hair is shaved and the area is cooled, then the hair will grow back black
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.
In snapdragons, flower color is controlled by incomplete dominance. The two alleles are red (R) and white (W). The heterozygous genotype is expressed as pink. What happens when you breed 2 pink dragons?
