How is incomplete dominance expressed in a phenotype

Explanation : A phenotypic "blending" of two traits is referred to as incomplete dominance, indicating that neither trait is truly dominant over the other. Report an Error. Two pure breeding plants are crossed. One plant has red flowers and the other has white flowers. Possible Answers: Half of the offspring would have red flowers, and half would have pink. Half of the offspring would have red flowers, and half would have white.

All offspring would have both red and white flowers. Correct answer: All offspring would have pink flowers. Explanation : The genotypes of the offspring can be determined by crossing the red flowers, RR , with the white flowers, rr. RR x rr Offspring: all offspring are Rr. Possible Answers: Black or white, depending on each individual offspring. Correct answer: Black and white spotted. Explanation : Codominance means that more than one type of dominant allele for the same gene is present.

Possible Answers: The alleles for black and brown fur exhibit complete dominance. The alleles for black and brown fur exhibit incomplete dominance.

Correct answer: The alleles for black and brown fur exhibit incomplete dominance. Explanation : Incomplete dominance is when more than one type of dominant allele for the same gene is present. Possible Answers: Red and white. Correct answer: Red, white, and red-white spotted. Explanation : The possible genotypes for this trait are RR , Rr , and rr.

We know that RR is red and rr is white, since these genotypes are homozygous. This gives three total phenotypes: red, white, and red-white spotted. Possible Answers: Six. Correct answer: Six. Explanation : In the species the entire range of phenotypes will be expressed. This gives a total of six possible allele combinations and six different phenotypes. Possible Answers: A black dog and tan dog mate to produce a tan dog. A black dog and tan dog mate and produce a red dog.

A black dog and white dog mate to produce a black dog. A black dog and white dog mate to produce a gray dog. A black dog and tan dog mate to produce a dog with black and tan spots.

Correct answer: A black dog and tan dog mate to produce a dog with black and tan spots. Explanation : Codominance is evidenced when the phenotypes of both parents show up in the offspring.

Possible Answers: Incomplete dominance. Correct answer: Co-dominance. Explanation : In the example above, the flower has both red and white petals due to co-dominant inheritance pattern of the red and white petal alleles. Possible Answers: Complete dominance. Explanation : Incomplete dominance is described by a phenotype that is not completely dominant over another. Copyright Notice. View Tutors. Erin Certified Tutor. Kimberly Certified Tutor. Michael Certified Tutor.

Report an issue with this question If you've found an issue with this question, please let us know. Do not fill in this field. Louis, MO Or fill out the form below:. In the F 2 generation, approximately three-quarters of the plants had violet flowers, and one-quarter had white flowers. When true-breeding plants in which one parent had white flowers and one had violet flowers were cross-fertilized, all of the F1 hybrid offspring had violet flowers.

That is, the hybrid offspring were phenotypically identical to the true-breeding parent with violet flowers. However, we know that the allele donated by the parent with white flowers was not simply lost because it reappeared in some of the F2 offspring. Therefore, the F1 plants must have been genotypically different from the parent with violet flowers. In his publication, Mendel reported the results of his crosses involving seven different phenotypes, each with two contrasting traits.

A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position.

To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants, reporting results from 19, F 2 plants alone. His findings were consistent. First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.

A Punnett square applies the rules of probability to predict the possible outcomes of a monohybrid cross and their expected frequencies. When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, and the resulting offspring are monohybrids.

Mendel performed seven monohybrid crosses involving contrasting traits for each characteristic. On the basis of his results in F 1 and F 2 generations, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring and that every possible combination of unit factors was equally likely. To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea seeds.

The dominant seed color is yellow; therefore, the parental genotypes were YY homozygous dominant for the plants with yellow seeds and yy homozygous recessive for the plants with green seeds, respectively. A Punnett square, devised by the British geneticist Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies.

To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top for one parent and side for the other parent of a grid, representing their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square.

If the pattern of inheritance dominant or recessive is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds. Punnett square analysis of a monohytbrid cross : In the P generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype.

This cross produces F1 heterozygotes with a yellow phenotype. Punnett square analysis can be used to predict the genotypes of the F2 generation. Therefore, the offspring can potentially have one of four allele combinations: YY, Yy, yY, or yy. Notice that there are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm. Both of these possibilities must be counted. Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents.

They are grouped together. Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green. Indeed, working with large sample sizes, Mendel observed approximately this ratio in every F 2 generation resulting from crosses for individual traits.

Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote.

Called the test cross, this technique is still used by plant and animal breeders. In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic.

If the dominant-expressing organism is a homozygote, then all F 1 offspring will be heterozygotes expressing the dominant trait. Alternatively, if the dominant expressing organism is a heterozygote, the F 1 offspring will exhibit a ratio of heterozygotes and recessive homozygotes. Example of a test cross : A test cross can be performed to determine whether an organism expressing a dominant trait is a homozygote or a heterozygote.

With the inclusion of incomplete dominance, codominance, multiple alleles, and mutant alleles, the inheritance of traits is complex process. Discuss incomplete dominance, codominance, and multiple alleles as alternatives to dominance and recessiveness. However, the heterozygote phenotype occasionally does appear to be intermediate between the two parents. This pattern of inheritance is described as incomplete dominance, denoting the expression of two contrasting alleles such that the individual displays an intermediate phenotype.

The allele for red flowers is incompletely dominant over the allele for white flowers. However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses. Example of incomplete dominance : These pink flowers of a heterozygote snapdragon result from incomplete dominance. A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote.

An example of codominance is the MN blood groups of humans. The M and N alleles are expressed in the form of an M or N antigen present on the surface of red blood cells.

In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct. However, the genotypic ratio characteristic of a Mendelian monohybrid cross still applies.

Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification. Although individual humans and all diploid organisms can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed. All other phenotypes or genotypes are considered variants of this standard, meaning that they deviate from the wild type.

The variant may be recessive or dominant to the wild-type allele. An example of multiple alleles is coat color in rabbits. Here, four alleles exist for the c gene.

The chinchilla phenotype, c ch c ch , is expressed as black-tipped white fur. The Himalayan phenotype, c h c h , has black fur on the extremities and white fur elsewhere.

Note, however, that partial dominance is not the same as blending inheritance ; after all, when two F 1 pink flowers are crossed, both red and white flowers are found among the progeny. In other words, nothing is different about the way these alleles are inherited; the only difference is in the way the alleles determine phenotype when they are combined.

As opposed to partial dominance, codominance occurs when the phenotypes of both parents are simultaneously expressed in the same offspring organism. Indeed, "codominance" is the specific term for a system in which an allele from each homozygote parent combines in the offspring, and the offspring simultaneously demonstrates both phenotypes. An example of codominance occurs in the human ABO blood group system.

Many blood proteins contribute to blood type Stratton, , and the ABO protein system in particular defines which types of blood you can receive in a transfusion. In a hospital setting, attention to the blood proteins present in an individual's blood cells can make the difference between improving health and causing severe illness. There are three common alleles in the ABO system. These alleles segregate and assort into six genotypes, as shown in Table 1.

As Table 1 indicates, only four phenotypes result from the six possible ABO genotypes. How does this happen? To understand why this occurs, first note that the A and B alleles code for proteins that exist on the surface of red blood cells; in contrast, the third allele, O, codes for no protein. Thus, if one parent is homozygous for type A blood and the other is homozygous for type B, the offspring will have a new phenotype, type AB.

In people with type AB blood, both A and B proteins are expressed on the surface of red blood cells equally. Therefore, this AB phenotype is not an intermediate of the two parental phenotypes, but rather is an entirely new phenotype that results from codominance of the A and B alleles. All of these heterozygote genotypes demonstrate the coexistence of two phenotypes within the same individual. In some instances, offspring can demonstrate a phenotype that is outside the range defined by both parents.

In particular, the phenomenon known as overdominance occurs when a heterozygote has a more extreme phenotype than that of either of its parents. A well-known example of overdominance occurs in the alleles that code for sickle-cell anemia. Sickle-cell anemia is a debilitating disease of the red blood cells, wherein a single amino acid deletion causes a change in the conformation of a person's hemoglobin such that the person's red blood cells are elongated and somewhat curved, taking on a sickle shape.

This change in shape makes the sickle red blood cells less efficient at transporting oxygen through the bloodstream. The altered form of hemoglobin that causes sickle-cell anemia is inherited as a codominant trait. Specifically, heterozygous Ss individuals express both normal and sickle hemoglobin, so they have a mixture of normal and sickle red blood cells. In most situations, individuals who are heterozygous for sickle-cell anemia are phenotypically normal.

Under these circumstances, sickle-cell disease is a recessive trait. Individuals who are homozygous for the sickle-cell allele ss , however, may have sickling crises that require hospitalization.

In severe cases, this condition can be lethal. Producing altered hemoglobin can be beneficial for inhabitants of countries afflicted with falciparum malaria, an extremely deadly parasitic disease. Sickle blood cells "collapse" around the parasites and filter them out of the blood. Thus, people who carry the sickle-cell allele are more likely to recover from malarial infection.

In terms of combating malaria, the Ss genotype has an advantage over both the SS genotype, because it results in malarial resistance, and the ss genotype, because it does not cause sickling crises. Allelic dominance always depends on the relative influence of each allele for a specific phenotype under certain environmental conditions. For example, in the pea plant Pisum sativum , the timing of flowering follows a monohybrid single-gene inheritance pattern in certain genetic backgrounds.

While there is some variation in the exact time of flowering within plants that have the same genotype, specific alleles at this locus Lf can exert temporal control of flowering in different backgrounds Murfet, Investigators have found evidence for four different alleles at this locus: Lf d , Lf , lf , and lf a.

Plants homozygous for the lf a allele flower the earliest, while Lf d plants flower the latest. A single allele causes the delayed flowering.

Thus, the multiple alleles at the Lf locus represent an allelic series, with each allele being dominant over the next allele in the series. Mendel's early work with pea plants provided the foundational knowledge for genetics, but Mendel's simple example of two alleles, one dominant and one recessive, for a given gene is a rarity.

In fact, dominance and recessiveness are not actually allelic properties. Rather, they are effects that can only be measured in relation to the effects of other alleles at the same locus. Furthermore, dominance may change according to the level of organization of the phenotype. Variations of dominance highlight the complexity of understanding genetic influences on phenotypes. Murfet, I. Flowering in Pisum : Multiple alleles at the Lf locus.

Heredity 35 , 85—98 Parsons, P. The evolution of overdominance: Natural selection and heterozygote advantage.