Goal 3 - Incomplete and Co-dominance Notes
INCOMPLETE DOMINANCE AND CO-DOMINANCE (pages 315-316; 317-318)
In many ways Gregor Mendel was quite lucky in discovering his genetic laws. He happened to use pea plants, which happened to have a number of easily observable traits that were determined by just two alleles. And for the traits he studied in his peas, one allele happened to be dominant for the trait & the other was a recessive form. Things aren't always so clear-cut & "simple" in the world of genetics, but luckily for Mendel (& the science world) he happened to work with an organism whose genetic make-up was fairly clear-cut & simple.
In many ways Gregor Mendel was quite lucky in discovering his genetic laws. He happened to use pea plants, which happened to have a number of easily observable traits that were determined by just two alleles. And for the traits he studied in his peas, one allele happened to be dominant for the trait & the other was a recessive form. Things aren't always so clear-cut & "simple" in the world of genetics, but luckily for Mendel (& the science world) he happened to work with an organism whose genetic make-up was fairly clear-cut & simple.
Incomplete Dominance
INCOMPLETE DOMINANCE
If Mendel were given a mommy black mouse & a daddy white mouse & asked what their offspring would look like, he would've said that a certain percent would be black & the others would be white. He would never have even considered that a white mouse & a black mouse could produce a GREY mouse! For Mendel, the phenotype of the offspring from parents with different phenotypes always resembled the phenotype of at least one of the parents. In other words, Mendel was unaware of the phenomenon of INCOMPLETE DOMINANCE.
I remember Incomplete Dominance in the form of an example like so:
RED Flower x WHITE Flower ---> PINK Flower
With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.
It's like mixing paints, red + white will make pink. Red doesn't totally block (dominate) the pink, instead there is incomplete dominance, and we end up with something in-between.
We can still use the Punnett Square to solve problems involving incomplete dominance. The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive trait, the letters we use are both going to be capital (because neither trait dominates the other). So the cross I used up above would look like this:
R = allele for red flowers
R’ = allele for white flowers
Homozygous red flower crossed with a homozygous white flower
red (RR) x white (R’R’) ---> pink RR’ ---> 100%
R R
R’ RR’ RR’
R’ RR’ RR’
The trick is to recognize when you are dealing with a question involving incomplete dominance. There are two steps to this:
1) Notice that the offspring is showing a 3rd phenotype. The parents each have one, and the offspring are different from the parents.
2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.
Sample Questions
1. A cross between a blue blahblah bird (B) & a white blahblah (B’) bird produces offspring that are silver. The color of blahblah birds is determined by just two alleles.
B’ B’
B BB’ BB’
B BB’ BB’
Mom:BB - blue Dad: B’B’ - white Offspring:BB’ - silver
a) What are the genotypes of the parent blahblah birds in the original cross? BB and B’B’
b) What is/are the genotype(s) of the silver offspring? BB’
c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?
B B’
B BB blue BB’ silver
B’ BB’ silver B’B’ white
Mom: BB’-silver Dad: BB’ - silver Offspring:50% silver, 25% blue,25% white
2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants?
R R’
R RR red RR’ orange
R’ RR’ orange R’R’ yellow
Mom: RR’ Dad: RR’ Offspring:
3. Two pink flowers are crossed. What are the genotypes and phenotypes of the offspring?
R R’
R RR red RR’ pink
R’ RR’ pink R’R’ white
Mom: Dad: Offspring:
4. A white flower is crossed with a pink flower. What are the genotypes and phenotypes of the offspring?
R R’
R' RR’ pink R’R’ white
R’ RR’ pink R’R’ white
Mom: Dad: Offspring:
5. A red flower is crossed with a pink flower. What are the genotypes and phenotypes of the offspring?
R R’
R RR red RR’ pink
R RR red RR’ pink
Mom: Dad: Offspring:
6. A blue alien is crossed with a yellow alien. What are the genotypes and phenotypes of the offspring?
B’ B’
B BB’ green BB’ –green
B BB’ green BB’ -green
Mom: Dad: Offspring:
7. A green alien is crossed with a green alien. What are the genotypes and phenotype of the offspring?
B B’
B BB blue BB’ green
B’ BB’ green B’B’ yellow
Mom: Dad: Offspring:
8. A black dog is crossed with a white dog. The result of the cross is a grey dog. What type of inheritance is this? Show the Punnett square. INCOMPLETE
B’ B’
B BB’ – grey BB’ –grey
B BB’ - grey BB’-grey
Mom: Dad: Offspring:
9. A black eyed alien and a white eyed alien meet get married and have a grey eyed child. What type of inheritance is this? Show the Punnett square. INCOMPLETE
B’ B’
B BB’ – grey BB’ – grey
B BB’ – grey BB’ –grey
Mom: Dad: Offspring:
If Mendel were given a mommy black mouse & a daddy white mouse & asked what their offspring would look like, he would've said that a certain percent would be black & the others would be white. He would never have even considered that a white mouse & a black mouse could produce a GREY mouse! For Mendel, the phenotype of the offspring from parents with different phenotypes always resembled the phenotype of at least one of the parents. In other words, Mendel was unaware of the phenomenon of INCOMPLETE DOMINANCE.
I remember Incomplete Dominance in the form of an example like so:
RED Flower x WHITE Flower ---> PINK Flower
With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.
It's like mixing paints, red + white will make pink. Red doesn't totally block (dominate) the pink, instead there is incomplete dominance, and we end up with something in-between.
We can still use the Punnett Square to solve problems involving incomplete dominance. The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive trait, the letters we use are both going to be capital (because neither trait dominates the other). So the cross I used up above would look like this:
R = allele for red flowers
R’ = allele for white flowers
Homozygous red flower crossed with a homozygous white flower
red (RR) x white (R’R’) ---> pink RR’ ---> 100%
R R
R’ RR’ RR’
R’ RR’ RR’
The trick is to recognize when you are dealing with a question involving incomplete dominance. There are two steps to this:
1) Notice that the offspring is showing a 3rd phenotype. The parents each have one, and the offspring are different from the parents.
2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.
Sample Questions
1. A cross between a blue blahblah bird (B) & a white blahblah (B’) bird produces offspring that are silver. The color of blahblah birds is determined by just two alleles.
B’ B’
B BB’ BB’
B BB’ BB’
Mom:BB - blue Dad: B’B’ - white Offspring:BB’ - silver
a) What are the genotypes of the parent blahblah birds in the original cross? BB and B’B’
b) What is/are the genotype(s) of the silver offspring? BB’
c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?
B B’
B BB blue BB’ silver
B’ BB’ silver B’B’ white
Mom: BB’-silver Dad: BB’ - silver Offspring:50% silver, 25% blue,25% white
2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants?
R R’
R RR red RR’ orange
R’ RR’ orange R’R’ yellow
Mom: RR’ Dad: RR’ Offspring:
3. Two pink flowers are crossed. What are the genotypes and phenotypes of the offspring?
R R’
R RR red RR’ pink
R’ RR’ pink R’R’ white
Mom: Dad: Offspring:
4. A white flower is crossed with a pink flower. What are the genotypes and phenotypes of the offspring?
R R’
R' RR’ pink R’R’ white
R’ RR’ pink R’R’ white
Mom: Dad: Offspring:
5. A red flower is crossed with a pink flower. What are the genotypes and phenotypes of the offspring?
R R’
R RR red RR’ pink
R RR red RR’ pink
Mom: Dad: Offspring:
6. A blue alien is crossed with a yellow alien. What are the genotypes and phenotypes of the offspring?
B’ B’
B BB’ green BB’ –green
B BB’ green BB’ -green
Mom: Dad: Offspring:
7. A green alien is crossed with a green alien. What are the genotypes and phenotype of the offspring?
B B’
B BB blue BB’ green
B’ BB’ green B’B’ yellow
Mom: Dad: Offspring:
8. A black dog is crossed with a white dog. The result of the cross is a grey dog. What type of inheritance is this? Show the Punnett square. INCOMPLETE
B’ B’
B BB’ – grey BB’ –grey
B BB’ - grey BB’-grey
Mom: Dad: Offspring:
9. A black eyed alien and a white eyed alien meet get married and have a grey eyed child. What type of inheritance is this? Show the Punnett square. INCOMPLETE
B’ B’
B BB’ – grey BB’ – grey
B BB’ – grey BB’ –grey
Mom: Dad: Offspring:
Co-Dominance
CO-DOMINANCE
The prefix "co-" means "together".
Cooperate = work together. Coexist = exist together. Cohabitate = habitat together.
The genetic gist to codominance is pretty much the same as incomplete dominance. A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype. With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).
In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.
I remember codominance in the form of an example like so:
red x white ---> red & white spotted
With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.
When it comes to punnett squares & symbols, it's the same as incomplete dominance. Use capital letters for the allele symbols. My example cross from above would look like so:
R = allele for red flowers
W = allele for white flowers
red x white ---> red & white spotted RR x WW ---> 100% RW
W W
R RW RW
R RW RW
The symbols you choose to use don't matter, in the end you end up with hybrid organisms, and rather than one trait (allele) dominating the other, both traits appear together in the phenotype. Wa-la, codominance.
A very very very very very common phenotype used in questions about codominance is roan fur in cattle. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of codominance.
W W
R RW RW
R RW RW
Another common phenotype in questions is about codominance is the black and white feathered checkered chicken. Chickens may be black (BB = all black feathers), white (WW = all white feathers), or checkered (BW = black and white feathers together).
W W
B BW BW
B BW BW
Another example of codominance is human blood type AB, in which two types of protein ("A" & "B") appear together on the surface of blood cells.
B B
A AB AB
A AB AB
Sample Questions
1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.
R W
W RW roan WW white
W RW roan WW white
Mom: Dad: Offspring:
2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?
R R
R RR red RR red
R RR red RR red
Mom: Dad: Offspring:
3. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritence does this illustrate? b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat? 50%
B B
B BB black BB black
T BT tabby BT tabby
Mom: Dad: Offspring:
4. A cross between a black chicken and a checkered chicken. What are the genotypes and phenotypes of the offspring?
B W
B BB black BW checkered
B BB black BW checkered
Mom: Dad: Offspring:
5. A cross between a checkered chicken and a white chicken. What are the genotypes and phenotypes of the offspring?
W W
B BW – checkered BW checkered
W WW – white WW white
Mom: Dad: Offspring:
6. A cross between a red cow and a white bull. What are the genotypes and the phenotypes of the offspring?
W W
R RW roan RW roan
R RW roan RW roan
Mom: Dad: Offspring:
7. A cross between a roan bull and a red cow. What are the genotypes and the phenotypes of the offspring?
R R
R RR red RR red
W RW roan RW roan
Mom: Dad: Offspring:
8. A cross between a red flower and a purple flower yields a red and purple striped flower? What type of inheritance is this? Show the Punnett square. CODOMIANCE
P P
R RP –striped RP striped
R RP – striped RP striped
Mom: Dad: Offspring:
9. A cross between a chestnut horse and a white horse yield a palomino (A brown and white spotted horse? What type of inheritance is this? Show the Punnett square. CODOMINANCE
W W
C CW palomino CW palomino
C CW palomino CW palomino
Mom: Dad: Offspring:
The prefix "co-" means "together".
Cooperate = work together. Coexist = exist together. Cohabitate = habitat together.
The genetic gist to codominance is pretty much the same as incomplete dominance. A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype. With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).
In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.
I remember codominance in the form of an example like so:
red x white ---> red & white spotted
With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.
When it comes to punnett squares & symbols, it's the same as incomplete dominance. Use capital letters for the allele symbols. My example cross from above would look like so:
R = allele for red flowers
W = allele for white flowers
red x white ---> red & white spotted RR x WW ---> 100% RW
W W
R RW RW
R RW RW
The symbols you choose to use don't matter, in the end you end up with hybrid organisms, and rather than one trait (allele) dominating the other, both traits appear together in the phenotype. Wa-la, codominance.
A very very very very very common phenotype used in questions about codominance is roan fur in cattle. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of codominance.
W W
R RW RW
R RW RW
Another common phenotype in questions is about codominance is the black and white feathered checkered chicken. Chickens may be black (BB = all black feathers), white (WW = all white feathers), or checkered (BW = black and white feathers together).
W W
B BW BW
B BW BW
Another example of codominance is human blood type AB, in which two types of protein ("A" & "B") appear together on the surface of blood cells.
B B
A AB AB
A AB AB
Sample Questions
1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.
R W
W RW roan WW white
W RW roan WW white
Mom: Dad: Offspring:
2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?
R R
R RR red RR red
R RR red RR red
Mom: Dad: Offspring:
3. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritence does this illustrate? b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat? 50%
B B
B BB black BB black
T BT tabby BT tabby
Mom: Dad: Offspring:
4. A cross between a black chicken and a checkered chicken. What are the genotypes and phenotypes of the offspring?
B W
B BB black BW checkered
B BB black BW checkered
Mom: Dad: Offspring:
5. A cross between a checkered chicken and a white chicken. What are the genotypes and phenotypes of the offspring?
W W
B BW – checkered BW checkered
W WW – white WW white
Mom: Dad: Offspring:
6. A cross between a red cow and a white bull. What are the genotypes and the phenotypes of the offspring?
W W
R RW roan RW roan
R RW roan RW roan
Mom: Dad: Offspring:
7. A cross between a roan bull and a red cow. What are the genotypes and the phenotypes of the offspring?
R R
R RR red RR red
W RW roan RW roan
Mom: Dad: Offspring:
8. A cross between a red flower and a purple flower yields a red and purple striped flower? What type of inheritance is this? Show the Punnett square. CODOMIANCE
P P
R RP –striped RP striped
R RP – striped RP striped
Mom: Dad: Offspring:
9. A cross between a chestnut horse and a white horse yield a palomino (A brown and white spotted horse? What type of inheritance is this? Show the Punnett square. CODOMINANCE
W W
C CW palomino CW palomino
C CW palomino CW palomino
Mom: Dad: Offspring: