modes of inheritance exceptions to mendel’s principles
TRANSCRIPT
Modes of Inheritance
Exceptions to Mendel’s Principles
Exceptions to Mendel’s principles:
1.Some alleles are neither dominant nor recessive.
2.Many traits are controlled by more than one gene (polygenic traits)
Incomplete Dominance The phenotype of the heterozygote is in
between those of the two homozygotes. In other words, in heterozygotes neither
allele is dominant.
How to Spot these: Look for a third phenotype that is in
between in the heterozygote
The R allele codes for a redred pigmentpigment
The W allele codes for a defective enzyme that produces no pigmentno pigment
The heterozygous cross will one R allele and thus produce only half of the red pigment. And Pink is And Pink is Made.Made.
Incomplete Dominance
Incomplete Dominance
A third (new) phenotype appears in the heterozygous condition.
Flower Color in 4 O’clocks (a type of flower0
RR = red WW= white RW = pink
Ex. RR = red flowerRR = red flower, WW = white flowerWW = white flower.A cross between the red and white flowers (RR x WW) yields all pink flowerspink flowers (RW)
RR RR
WRW
(pink)RW
(pink)
WRW
(pink)RW
(pink)
Incomplete Dominance
Cross a pink flower and a white flower.
What is the genotype of the pink flower? What is the genotype of the white flower? Draw the punnett square. What is the genotypic ratio (fraction or %)? What is the phenotypic ratio (fraction or
%)?
Practice!
Genotypes:Pink Flower: RWWhite Flower: WW
2RW : 2WW
2pink : 2white
R W
W
W
WWRW
WWRWGenotypic Ratio:
Answer!
Phenotypic Ratio:
Codominance The phenotype of the heterozygote has
two alleles that are expressed at the same time.
In other words, in heterozygotes neither allele is dominant.
How to Spot these: Look for a third phenotype that shows
both of the homozygote traits in the heterozygote
The heterozygous condition, both alleles are expressed equally
Sickle Cell Anemia in Humans
NN = normal cells
SS = sickle cells NS = some of each
Codominance
Black is dominant in roosters (B)
So is white (W)
The cross of a BB and WW results in a BW or black/white mix… however, it’s not grey
Codominance
Another Similar Example…Color in Animals:Color in Animals: “Red” and “white”
are both dominant and when crossed—both colors are partiallypartially expressed as in “roan”.
Practice!Cross an individual with sickle-cell anemia and
another who is a carrier but not sick.
What is the genotype of the individual with sickle cell anemia?
What is the genotype of the carrier? Draw the punnett square. What is the genotypic ratio? What is the phenotypic ratio?
Genotypes:Carrier: NSSickle Cell: SS
2NS2SS
2carrier 2sick
Genotypic Ratio:
Answer!
Phenotypic Ratio:
N S
S
S
NS
NS
SS
SS
Multiple Alleles
There are more than two alleles for a trait
Blood type in humans Blood Types?
Type A, Type B, Type AB, Type O Blood Alleles?
A, B, O (in book – IA, IB, I)
Rules for Blood Type A and B are codominant
AA = Type A BB = Type B AB = Type AB
A and B are dominant over O AO = type A BO = type B OO = type O
Practice!Cross a mother who is O with a father who is AB
What is the genotype of mother? What is the genotype of the father? Draw the Punnett square. What is the genotypic ratio? What is the phenotypic ratio?
Genotypes:Mother: OOFather: AB
2AO2BO
2 type A2 type B
Genotypic Ratio:
Answer!
Phenotypic Ratio:
O O
A
B
AO
BO
AO
BO
More Practice!Cross a mother who is heterozgous B with a
father who is heterozygous A
What is the genotype of mother? What is the genotype of the father? Draw the Punnett square. What is the genotypic ratio? What is the phenotypic ratio?
Genotypes:Mother: BOFather: AO
1AB, 1BO,1AO, 1OO
1 type A: 1 type B: 1 type AB : 1 Type OO
Genotypic Ratio:
Answer!
Phenotypic Ratio:
A O
B
O
AB
OO
BO
AO
Polygenic traits Traits controlled by two or more genes. Examples:
Human height Eye color Skin color