Mendel's experimental work involved the crossing of what he called "typical" plants (homozygous dominant) with "atypical" plants (homozygous recessive). Mendel had discovered complete dominance, in which only one of the alleles is expressed, despite the presence of the other allele.
complete dominance: a situation where an allele will determine the phenotype, regardless of the presence of another allele
Not all traits are passed on from parent to offspring in the simple patterns that Mendel proposed. Variations in the patterns of heredity exist, and dominance is not always complete.
LEARNING TIP
Notation of Alleles
Notation of alleles for a specific gene can be represented using superscripts. For example, consider the alleles for colour in snapdragons shown in Figure 1. The gene is C for colour. The alleles are red (R) and white (W). When you combine the notations for genes and alleles, the result is CR for the red allele and CW for the white allele.
Incomplete Dominance and Codominance
Mendel's work provided an explanation of why the traits of parents did not blend in the offspring. Yet blended inheritance is common in nature. In snapdragons one of the genes that controls flower colour (C) has one allele for red (CR) and one allele for white (CW). A homozygous CRCR plant will produce red flowers, while a homozygous CWCW plant will produce white flowers. However, the heterozygous plants will produce pink flowers (CRCW). In this case, the actual flower colour (phenotype) is a result of varying amounts of red and white pigments. The homozygous (CRCR) plant produces red pigment, the homozygous (CWCW) plant produces white pigment, and the heterozygous (CRCW) plant produces both red pigment and white pigment. Neither of the alleles is dominant, because the red pigment cannot mask the white pigment and the white pigment cannot mask the red pigment. This type of interaction, in which a heterozygous phenotype is a blend of the two homozygous phenotypes, is known as incomplete dominance. Interestingly, in this case, incomplete dominance still results in the same Mendelian genotype ratio of 1:2:1 (Figure 1).
incomplete dominance: a situation where neither allele dominates the other and both have an influence on the individual; results in partial expression of both traits
Figure 1 (a) Colour in snapdragons is an example of incomplete dominance. (b) When crossed, red-flowering and white-flowering snapdragons produce pink-flowering offspring. A cross between these pink F1 individuals produces an F2 generation with a ratio of 1 red to 2 pink to 1 white (1:2:1).
Image: (a) Several red, pink, white, and yellow snapdragons. (b) A Punnett square; F1: CR and CW F2: 1/4 red = CRCR 2/4 pink = CRCW 1/4 white = CRCW.
In the F1 generation there is only a single phenotype with equal numbers of CR and CW alleles. Notice that in the F2 generation there are now three different phenotypes but the total numbers of CR and CW alleles remain equal.
Print Page 195
Another type of interaction between alleles occurs when both allele products appear in the offspring at the same time. In this case, a mixed phenotype is generated. This type of interaction is called codominance. A classic example of codominance appears in shorthorn cattle. A red bull crossed with a white cow will produce a roan calf (Figure 2). Roan calves have intermingled pure white and pure red hair.
codominance: a situation where both alleles are expressed fully to produce offspring with a third phenotype
Figure 2 In codominance, one allele does not mask the other allele. Both alleles influence the final phenotype. In shorthorn cattle, roan calves have intermingled red and white hair.
Image: A Punnett square. P generation: At the top of the Punnett square is a red bull. Over top of the top left square and top right square is Hr. On the left side of the Punnett square is a white cow. Hw is next to the side of the top left square and the side of the bottom left square. There is a roan cow on the right side of the Punnett square. All four inner squares are labeled HrHw.
Image: F1 generation: roan cow × roan bull; four arrows point to F2 generation: red bull HrHr; roan cow HrHw; roan bull HrHw; white cow HwHw.
Codominance and Dominance: ABO Blood Types
Human blood type is both a codominant and dominant genetic trait. There are four major blood types: A, B, AB, and 0. The blood type gene has three possible alleles. They are IA, IB, and i. Each allele codes for a different enzyme that places different types of sugars on the surface of a red blood cell. If you are IAIA (type A), an enzyme places one type of sugar on the surface of the cell. If you are IBIB (type B), another enzyme places a different sugar on the cell surface. If you are IAIB (type AB), both sugars are placed on the cell surface. Type AB blood is an example of codominance. The allele i codes for an enzyme that makes a simpler surface molecule that lacks the extra sugars of the A, B, or AB blood types. If an individual is ii, he has type O blood. If i is paired with IA or IB, then the individual expresses the dominant allele (IA or IB) and is either type A or type B. Type IAi blood and type IBi blood are examples of dominant inheritance. Table 1 shows the distribution and expression of the blood type alleles. One of the gametes is provided by the father and the other is provided by the mother.
Table 1 The Distribution and Expression of the Blood Type Alleles
Genotype: IAIA Blood type: A Able to receive blood from: A, O
Genotype: IAi Blood type: A Able to receive blood from: A, O
Genotype: IBIB Blood type: B Able to receive blood from: B, O
Genotype: IBi Blood type: B Able to receive blood from: B, O
Genotype: IAIB Blood type: AB Able to receive blood from: A, B, AB, O
Genotype: ii Blood type: O Able to receive blood from: O
Because different blood types exist, it is important that individuals who receive blood transfusions receive blood that is compatible with their own. For example, an individual with type A blood produces an immune response against type B and type AB blood. An individual with type B blood produces an immune response against type A blood and type AB blood. If an incompatible blood type is transfused, the patient's life may be put at risk. In an emergency situation when there is no time to test the patient's blood type, or if a certain blood type is in short supply, type 0 blood may be used (Figure 3). Type 0 blood is known as the "universal donor." Type AB blood is the "universal recipient? [GO TO NELSON SCIENCE]
Figure 3 Blood banks are always in need of blood donations. Type 0 blood is valuable because it is compatible with all blood types.
Image: A woman sitting in a chair having blood drawn from her arm.
CAREER LINK
Phlebotomist
Phlebotomists are technicians who are specially trained and are certified to draw blood. To learn more about a career in phlebotomy, GO TO NELSON SCIENCE
Print Page 196
The frequency of the blood type alleles varies throughout the world. Genetically isolated populations sometimes have very high frequencies for particular alleles. For example, about 80% of the Native Americans of the Blackfeet Nation Pikuni Indians in Montana have type A blood because the frequency of the IA allele is very high in this population.
Codominance can provide an even greater variation in the population: there are genes that have many more alleles than just three or four. For example, the gene that helps determine the acceptance or rejection of a transplant has more than 200 different types of alleles. [GO TO NELSON SCIENCE]
WEB LINK
To learn more about blood types, GO TO NELSON SCIENCE
5.2 Summary
- Alleles that determine the phenotype regardless of the presence of other alleles follow a pattern of inheritance called complete dominance.
- A heterozygous individual with an intermediate phenotype between the phenotypes of the two homozygous individuals follows a pattern of inheritance called incomplete dominance.
- Codominance occurs when both alleles are fully expressed. Type AB blood is an example of codominance.
- Blood type is an example of a gene with multiple alleles. The three blood type alleles are IA, IB, and i. Different combinations of the three alleles produce type A, type B, type AB, and type 0 blood.
Investigation 5.2.1
Gummy Bear Genetics (page 215)
You can now complete Investigation 5.2.1.
In this observational study you will use the type and number of "offspring" you have to predict the possible genotypes of the parents. Before starting, review the different Mendelian monohybrid crosses.
5.2 Questions
Key
K/U: Knowledge and Understanding T/I: Thinking and Investigation C: Communication A: Application
*1. Explain in your own words the meaning of dominance, codominance, and incomplete dominance. K/U
*2. In some chickens, the gene for feather colour is controlled by codominance. The allele for black is FB and the allele for white is FW. The heterozygous phenotype is known as erminette. T/I A (a) What is the genotype for black chickens? (b) What is the genotype for white chickens? (c) What is the genotype for erminette chickens? (d) If two erminette chickens are crossed, what is the probability that they would have a black chick? A white chick?
3. A geneticist notes that crossing a round radish with a long radish produces oval radishes. When oval radishes were crossed with oval radishes, the F2 generation had these phenotypes: 100 long, 200 oval, and 100 round radishes. Use symbols to explain the results obtained for the F1 and F2 generations. T/I C
4. How would Mendel's conclusions have differed if he had worked with plants whose alleles were incomplete dominant? K/U T/I
5. Thalassemia is an inherited anemic disorder in humans. Individuals can exhibit major anemia, minor anemia, or neither. Assume only one gene is involved with two alleles in the inheritance of this condition. What type of inheritance is thalassemia governed by? What are the corresponding genotypes to the three scenarios? K/U T/I
6. List the possible genotypes for an individual with type A blood. K/U T/I A
*7. Suppose a father of blood type A and a mother of blood type B have a child of type 0. What are the possible blood types of the mother and father? K/U T/I A
8. Suppose a father of blood type B and a mother of blood type 0 have a child of type 0. What are the chances that their next child will be blood type 0? Type B? Type A? Type AB? K/U T/I A
*9. Why is blood type inheritance an example of both codominance and complete dominance? K/U
*10. Another characteristic of human blood is the presence or absence of a blood protein called the Rh factor. People with the protein are Rh+ and those without it are Rh-. Use the Internet and other resources to answer the following questions: [GO TO NELSON SCIENCE] K/U T/I A (a) What are the genotypes of individuals who are Rh—and Rh+? Is this an example of complete dominance, incomplete dominance, or codominance? (b) How can the Rh blood type of two parents be of concern during a pregnancy? How can possible harmful complications be avoided?
Mendel's experimental work involved the crossing of what he called "typical" plants (homozygous dominant) with "atypical" plants (homozygous recessive). Mendel had discovered complete dominance, in which only one of the alleles is expressed, despite the presence of the other allele.
complete dominance: a situation where an allele will determine the phenotype, regardless of the presence of another allele
Not all traits are passed on from parent to offspring in the simple patterns that Mendel proposed. Variations in the patterns of heredity exist, and dominance is not always complete.
LEARNING TIP
Notation of Alleles
Notation of alleles for a specific gene can be represented using superscripts. For example, consider the alleles for colour in snapdragons shown in Figure 1. The gene is C for colour. The alleles are red (R) and white (W). When you combine the notations for genes and alleles, the result is CR for the red allele and CW for the white allele.
Incomplete Dominance and Codominance
Mendel's work provided an explanation of why the traits of parents did not blend in the offspring. Yet blended inheritance is common in nature. In snapdragons one of the genes that controls flower colour (C) has one allele for red (CR) and one allele for white (CW). A homozygous CRCR plant will produce red flowers, while a homozygous CWCW plant will produce white flowers. However, the heterozygous plants will produce pink flowers (CRCW). In this case, the actual flower colour (phenotype) is a result of varying amounts of red and white pigments. The homozygous (CRCR) plant produces red pigment, the homozygous (CWCW) plant produces white pigment, and the heterozygous (CRCW) plant produces both red pigment and white pigment. Neither of the alleles is dominant, because the red pigment cannot mask the white pigment and the white pigment cannot mask the red pigment. This type of interaction, in which a heterozygous phenotype is a blend of the two homozygous phenotypes, is known as incomplete dominance. Interestingly, in this case, incomplete dominance still results in the same Mendelian genotype ratio of 1:2:1 (Figure 1).
incomplete dominance: a situation where neither allele dominates the other and both have an influence on the individual; results in partial expression of both traits
Figure 1 (a) Colour in snapdragons is an example of incomplete dominance. (b) When crossed, red-flowering and white-flowering snapdragons produce pink-flowering offspring. A cross between these pink F1 individuals produces an F2 generation with a ratio of 1 red to 2 pink to 1 white (1:2:1).
Image: (a) Several red, pink, white, and yellow snapdragons. (b) A Punnett square;
F1: CR and CW
F2: 1/4 red = CRCR
2/4 pink = CRCW
1/4 white = CRCW.
In the F1 generation there is only a single phenotype with equal numbers of CR and CW alleles. Notice that in the F2 generation there are now three different phenotypes but the total numbers of CR and CW alleles remain equal.
Print Page 195
Another type of interaction between alleles occurs when both allele products appear in the offspring at the same time. In this case, a mixed phenotype is generated. This type of interaction is called codominance. A classic example of codominance appears in shorthorn cattle. A red bull crossed with a white cow will produce a roan calf (Figure 2). Roan calves have intermingled pure white and pure red hair.
codominance: a situation where both alleles are expressed fully to produce offspring with a third phenotype
Figure 2 In codominance, one allele does not mask the other allele. Both alleles influence the final phenotype. In shorthorn cattle, roan calves have intermingled red and white hair.
Image: A Punnett square. P generation: At the top of the Punnett square is a red bull. Over top of the top left square and top right square is Hr. On the left side of the Punnett square is a white cow. Hw is next to the side of the top left square and the side of the bottom left square. There is a roan cow on the right side of the Punnett square. All four inner squares are labeled HrHw.
Image: F1 generation: roan cow × roan bull; four arrows point to F2 generation: red bull HrHr; roan cow HrHw; roan bull HrHw; white cow HwHw.
Codominance and Dominance: ABO Blood Types
Human blood type is both a codominant and dominant genetic trait. There are four major blood types: A, B, AB, and 0. The blood type gene has three possible alleles. They are IA, IB, and i. Each allele codes for a different enzyme that places different types of sugars on the surface of a red blood cell. If you are IAIA (type A), an enzyme places one type of sugar on the surface of the cell. If you are IBIB (type B), another enzyme places a different sugar on the cell surface. If you are IAIB (type AB), both sugars are placed on the cell surface. Type AB blood is an example of codominance. The allele i codes for an enzyme that makes a simpler surface molecule that lacks the extra sugars of the A, B, or AB blood types. If an individual is ii, he has type O blood. If i is paired with IA or IB, then the individual expresses the dominant allele (IA or IB) and is either type A or type B. Type IAi blood and type IBi blood are examples of dominant inheritance. Table 1 shows the distribution and expression of the blood type alleles. One of the gametes is provided by the father and the other is provided by the mother.
Table 1 The Distribution and Expression of the Blood Type Alleles
Genotype: IAIA
Blood type: A
Able to receive blood from: A, O
Genotype: IAi
Blood type: A
Able to receive blood from: A, O
Genotype: IBIB
Blood type: B
Able to receive blood from: B, O
Genotype: IBi
Blood type: B
Able to receive blood from: B, O
Genotype: IAIB
Blood type: AB
Able to receive blood from: A, B, AB, O
Genotype: ii
Blood type: O
Able to receive blood from: O
Because different blood types exist, it is important that individuals who receive blood transfusions receive blood that is compatible with their own. For example, an individual with type A blood produces an immune response against type B and type AB blood. An individual with type B blood produces an immune response against type A blood and type AB blood. If an incompatible blood type is transfused, the patient's life may be put at risk. In an emergency situation when there is no time to test the patient's blood type, or if a certain blood type is in short supply, type 0 blood may be used (Figure 3). Type 0 blood is known as the "universal donor." Type AB blood is the "universal recipient? [GO TO NELSON SCIENCE]
Figure 3 Blood banks are always in need of blood donations. Type 0 blood is valuable because it is compatible with all blood types.
Image: A woman sitting in a chair having blood drawn from her arm.
CAREER LINK
Phlebotomist
Phlebotomists are technicians who are specially trained and are certified to draw blood. To learn more about a career in phlebotomy, GO TO NELSON SCIENCE
Print Page 196
The frequency of the blood type alleles varies throughout the world. Genetically isolated populations sometimes have very high frequencies for particular alleles. For example, about 80% of the Native Americans of the Blackfeet Nation Pikuni Indians in Montana have type A blood because the frequency of the IA allele is very high in this population.
Codominance can provide an even greater variation in the population: there are genes that have many more alleles than just three or four. For example, the gene that helps determine the acceptance or rejection of a transplant has more than 200 different types of alleles. [GO TO NELSON SCIENCE]
WEB LINK
To learn more about blood types, GO TO NELSON SCIENCE
5.2 Summary
- Alleles that determine the phenotype regardless of the presence of other alleles follow a pattern of inheritance called complete dominance.
- A heterozygous individual with an intermediate phenotype between the phenotypes of the two homozygous individuals follows a pattern of inheritance called incomplete dominance.
- Codominance occurs when both alleles are fully expressed. Type AB blood is an example of codominance.
- Blood type is an example of a gene with multiple alleles. The three blood type alleles are IA, IB, and i. Different combinations of the three alleles produce type A, type B, type AB, and type 0 blood.
Investigation 5.2.1
Gummy Bear Genetics (page 215)
You can now complete Investigation 5.2.1.
In this observational study you will use the type and number of "offspring" you have to predict the possible genotypes of the parents. Before starting, review the different Mendelian monohybrid crosses.
5.2 Questions
Key
K/U: Knowledge and Understanding
T/I: Thinking and Investigation
C: Communication
A: Application
*1. Explain in your own words the meaning of dominance, codominance, and incomplete dominance. K/U
*2. In some chickens, the gene for feather colour is controlled by codominance. The allele for black is FB and the allele for white is FW. The heterozygous phenotype is known as erminette. T/I A
(a) What is the genotype for black chickens?
(b) What is the genotype for white chickens?
(c) What is the genotype for erminette chickens?
(d) If two erminette chickens are crossed, what is the probability that they would have a black chick? A white chick?
3. A geneticist notes that crossing a round radish with a long radish produces oval radishes. When oval radishes were crossed with oval radishes, the F2 generation had these phenotypes: 100 long, 200 oval, and 100 round radishes. Use symbols to explain the results obtained for the F1 and F2 generations. T/I C
4. How would Mendel's conclusions have differed if he had worked with plants whose alleles were incomplete dominant? K/U T/I
5. Thalassemia is an inherited anemic disorder in humans. Individuals can exhibit major anemia, minor anemia, or neither. Assume only one gene is involved with two alleles in the inheritance of this condition. What type of inheritance is thalassemia governed by? What are the corresponding genotypes to the three scenarios? K/U T/I
6. List the possible genotypes for an individual with type A blood. K/U T/I A
*7. Suppose a father of blood type A and a mother of blood type B have a child of type 0. What are the possible blood types of the mother and father? K/U T/I A
8. Suppose a father of blood type B and a mother of blood type 0 have a child of type 0. What are the chances that their next child will be blood type 0? Type B? Type A? Type AB? K/U T/I A
*9. Why is blood type inheritance an example of both codominance and complete dominance? K/U
*10. Another characteristic of human blood is the presence or absence of a blood protein called the Rh factor. People with the protein are Rh+ and those without it are Rh-. Use the Internet and other resources to answer the following questions: [GO TO NELSON SCIENCE] K/U T/I A
(a) What are the genotypes of individuals who are Rh—and Rh+? Is this an example of complete dominance, incomplete dominance, or codominance?
(b) How can the Rh blood type of two parents be of concern during a pregnancy? How can possible harmful complications be avoided?