You should have a working knowledge of the following terms:
Introductions and Goals
Mendel was able to carry out his work in an interpretable manner because he took careful notes, made accurate counts, and chose traits that exhibited a clear dominant or recessive expression pattern. In fact, alleles that show simple dominant or recessive expression patterns are sometimes called "Mendelian traits." Few alleles, however, behave in such a simple Mendelian manner. This tutorial will explore more complex patterns of expression. By the end of this tutorial you should have a basic understanding of:
- Genes with multiple alleles
- The distinction between the three types of dominance
- How genes can have multiple effects
- How genes can affect the expression of other genes
- Why some characters are controlled by more than one gene
In Mendel's experiments, offspring always looked like one of their two parents due to the complete dominance of one allele over the other. This is not always the case because some genes display incomplete dominance; that is, individuals with heterozygous alleles exhibit a phenotype intermediate between those with homozygous alleles. For example, this figure depicts the outcome of a cross between a snapdragon with red flowers and one with white flowers - the F1 hybrids have pink flowers. In this case neither of the alleles for flower color are completely dominant over the other. Therefore, individuals with heterozygous alleles have a phenotype unlike those with either set of homozygous alleles.
Figure. Incomplete dominance in snapdragon color. (Click image to enlarge).
As demonstrated in this figure, the Punnett square for this cross is like that for any other monohybrid cross. However, the ratio of phenotypes in the F2 generation is not 3:1 (dominant:recessive), as seen with completely dominant alleles, but rather a 1:2:1 ratio of red:pink:white flowers. In this example the alleles are symbolized differently than in the previous examples. Since neither allele dominates over the other, the use of an uppercase and lowercase version of the same letter is inappropriate. In this example the character (flower color) is indicated by a letter (C), and the alleles encoding the trait (white, blue or red) are listed as uppercase subscripts (recall, they are both uppercase because neither is dominant to the other). You may see other symbolic representations for incomplete dominance, but don't let this confuse you. The important thing to know is that some genes are expressed in an incomplete dominant manner.
At the following Web sites, find the correct answer to the multiple-choice monohybrid or dihybrid cross questions. Work out each problem for yourself. To view an explanation of the problem, select the "TUTORIAL" button. After viewing the correct answer, close the Monohybrid Cross Problem Set or Dihybrid Cross window to return to this page. (note: These sites are a part of the Monohybrid and Dihybrid Problem Sets provided by The Biology Project at the University of Arizona.)
Problem 9: Incomplete dominance - This problem is a part of the Monohybrid Cross Problem Set.
Problem 10: Disappearance of parental phenotypes in the F1 generation - This problem is also a part of the Monohybrid Cross Problem Set.
Problem 11: Incomplete dominance in a dihybrid cross - This problem is a part of the Dihybrid Cross Problem Set.
We have examined complete dominance, in which one allele is clearly dominant over the other, and incomplete dominance, in which neither allele dominates over the other. In a third type of dominance, codominance, both alleles are expressed in the phenotype of individuals that have heterozygous alleles. The human blood groups designated M, N, and MN are examples of codominance. These groups are distinguished by the presence of two specific proteins on the surface of red blood cells, such as those shown in this figure. Group M individuals have one of the two proteins, group N have the other protein, and group MN have both proteins. In the latter case, the heterozygous situation does not result in an intermediate phenotype; instead, the M and N phenotypes are both expressed.
Figure. MN blood groups are similar to the molecules shown here on a cell's surface. (Click image to enlarge)
The Three Types of Dominance
Distinguishing between the three types of dominance is sometimes difficult, however, a little review should help clarify things. Think of complete dominance, incomplete dominance, and codominance as a continuum. At one end is complete dominance, in which the phenotype for only one of the two alleles is expressed in individuals that are heterozygous for the alleles. At the other end is codominance, in which both alleles are equally expressed in individuals that are heterozygous for the alleles. In between there are various levels of incomplete dominance, in which individuals that are heterozygous for the alleles display an intermediate phenotype. The key to understanding the difference between the three types is to look at the phenotype of the individuals with heterozygous alleles, then classify the relationship accordingly.
So far we have only examined traits controlled by two alleles. This is easy to visualize because diploid organisms can only possess two alleles. Within a population, however, more than two alleles can exist (although any given individual only has two alleles).
The human ABO blood groups are an example of multiple alleles, and the relationship between phenotype and genotype is depicted in the figure above. There are four possible phenotypic blood types for this particular gene: A, B, AB, and O. The letters refer to two specific carbohydrate molecules on the surface of red blood cells. (Note, these are not the same molecules as the MN blood groups.) Individuals can have the A carbohydrate (blood type A), the B carbohydrate (blood type B), both the A and B carbohydrates (blood type AB), or neither carbohydrate (blood type O).
The ABO blood groups are formed by various combinations of three different alleles; IA (codes for carbohydrate A), IB (codes for carbohydrate B), and i (codes for the lack of any carbohydrate). As shown in this figure, individuals with one or two IA alleles will have blood type A, those with one or two IB alleles will have blood type B, those with both the IA and IB alleles will have blood type AB, and those with the genotype ii will have blood type O.
Figure. AB antigens. (Click image to enlarge)
Figure. Multiple alleles for the ABO blood groups. (Click image to enlarge)
The Punnett square can be used to predict the genotype frequencies resulting from multiple allele crosses. However, one cannot be certain of an individual's genotype if they are blood type A or B because there are two possible genotypes for each of these blood types. Therefore, many cross problems that examine blood types are similar to test crosses; that is, the parental genotype is uncertain. A few examples will aid in your understanding.
At the following Web sites, find the correct answer to the multiple-choice monohybrid cross questions. Work out each problem. To view an explanation of the problem, select the "TUTORIAL" button. After viewing the correct answer, close the Monohybrid Cross Problem Set window to return to this page. (These sites are a part of the Monohybrid Problem Sets provided by The Biology Project at the University of Arizona.)
Problem 11: Co-dominant alleles: The Human ABO markers - This problem is a part of the Monohybrid Cross Problem Set.
Problem 13: Predicting human blood types - This problem is also a part of the Monohybrid Cross Problem Set.
So far we have only considered genes that affect a single phenotypic character. This actually is a rare situation because it is more common that one gene can have multiple effects (pleiotropy). For example, albino individuals lack pigment in their skin and hair, and also have crossed eyes at a higher frequency than pigmented individuals (see photograph). This occurs because the gene that causes albinism can also cause defects in the nerve connections between the eyes and the brain. These two traits are not always linked, again showing the complexity of genetic interactions in determining phenotypes.
Figure. Pleiotropy in individuals with albinism. (Click image to enlarge)
Mendel also recognized this effect. He observed that pea plants with red flowers had red coloration where the leaf joined the stem, but that their seed coats were gray in color. Plants with white flowers had no coloration at the leaf-stem juncture and displayed white seed coats. These combinations were always found together, leading Mendel to conclude that they were likely controlled by the same hereditary unit (i.e., gene).
Sometimes a gene at one location on a chromosome can affect the expression of a gene at a second location (epistasis). A good example of epistasis is the genetic interactions that produce coat color in horses and other mammals. In horses, brown coat color (B) is dominant over tan (b). Gene expression is dependent on a second gene that controls the deposition of pigment in hair. The dominant gene (C) codes for the presence of pigment in hair, whereas the recessive gene (c) codes for the absence of pigment. If a horse is homozygous recessive for the second gene (cc), it will have a white coat regardless of the genetically programmed coat color (B gene) because pigment is not deposited in the hair. The figure above demonstrates this scenario. Several of the white horses have genotypes for brown or tan coat color in the first gene, but are completely white because they are homozygous recessive for the gene controlling pigment deposition.
Figure. An example of epistasis. (Click image to enlarge)
At the following Web sites, find the correct answer to the multiple-choice dihybrid cross questions. Work out each problem yourself using paper and pencil. To view an explanation of the problem, select the "TUTORIAL" button. After viewing the correct answer, close the Dihybrid Cross Problem Set window to return to this page. (These sites are a part of the Dihybrid Problem Sets provided by The Biology Project at the University of Arizona.)
Problem 12: What is the genotype of the agouti parent? - This problem is a part of the Dihybrid Cross Problem Set.
Problem 13: AaBb dihybrid cross involving epistasis - This problem is also a part of the Dihybrid Cross Problem Set.
The characters that Mendel studied are sometimes referred to as discrete characters because they can only be classified on an "either-or" basis (e.g., purple or white flowers, green or yellow seeds). Many characters cannot be classified in this manner because they vary in a population across a continuum (gradient). For example, the figure above illustrates that skin color in humans is a quantitative character. Quantitative characters usually indicate that the character is controlled by more than one gene (polygenic inheritance). In the example shown, a simplification of the genetics of skin color in humans shows that three genes interact to determine the level of pigment in an individual's skin. The dominant alleles (A, B, and C) each contribute one "unit" of pigment to the individual, and their effects are cumulative such that individuals with more of these alleles will be darker than those with fewer alleles. The recessive alleles (a, b, and c) do not contribute any units of pigment. Therefore, skin color is related to the number of dominant alleles present in each individual's genotype. A cross of two completely heterozygous parents produces seven phenotypes in their offspring, ranging from very light to very dark skin. The distribution of skin color in the offspring would resemble a bell-shaped curve because there would be more individuals with intermediate skin colors than either extreme. As the number of genes involved increases, the differences between the various genotypes become more subtle and the distribution fits the curve more closely. Other examples of polygenic inheritance in humans include height, hair color, and eye color. This helps to explain the slight variations in these characters that we see in different individuals.
Figure. A simplified model for polygenic inheritance of skin color. (Click image to enlarge)
This tutorial explored the more complex expression patterns of alleles. These patterns of expression do not contradict the ideas and conclusions of Mendel, rather they reveal that genes and their products can interact and/or be expressed in complex ways. In all cases, these genes are still transmitted from generation to generation on chromosomes that segregate independently during meiosis. The differences lie in how the gene product behaves within the cell, and the number of such products that contribute to a given character trait.
Some alleles can show incomplete dominance. In the snapdragon flower color and snake body color examples, we saw that three phenotypes could be traced to two alleles. In other words, two separate homozygous phenotypes resulted (as is usually seen with characters transmitted in a Mendelian manner) and a third phenotype associated with a heterozygous genotype (clearly different than the case observed with Mendelian traits, where the heterozygous phenotype is the same as the homozygous dominant genotype). How can two alleles yield three phenotypes? Consider the snapdragon. When a plant receives two alleles (a double dose) of the red allele, the flower is very red. These alleles encode for enzymes involved in the production of red pigment. The alternative alleles produce an enzyme that is nonfunctional and cannot participate in pigment synthesis. When a plant receives two copies of this nonfunctional alternative allele, pigment is not produced and the flower is white. When the plant receives one copy (a single dose) of the functional red allele, it can produce some pigment, but not as much as with two fully functional alleles, and so, the color is less red (pink).
In the example above, note that the reason for the phenotypic pattern was that one allele was nonfunctional and the other functional allele resulted in a phenotype that was dependent on there being one or two copies of the functional alleles. There is another situation, however, termed codominance, in which both alleles are functional and expressed. The M and N blood groups are examples of codominance. These alleles encode for proteins that are located on the surface of red blood cells. They are similar but nonidentical; that is, they differ in four amino acids. If the individual is homozygous, then the phenotype is either M or N. If heterozygous, however, then both are expressed on the surface of red blood cells.
So far we have considered fairly simple cases, where the number of alleles is limited to two. In fact, many genes have multiple alleles. This may seem impossible because in the diploid state there is only room for two alleles (one on the maternal chromosome and one on the paternal chromosome) of a gene. At the population level, however, many more allele forms are possible (although in any given diploid individual, only two occur at any given time). The ABO blood type is one example.
In some cases, genes and their alleles may be expressed in complex ways. That is, no single trait can be attributed to a given allele. Pleiotropy describes this situation, and includes the examples of pigmentation and crossed eyes in the case of albinism.
In the case where one gene product is used by (or dependent on) another product, epistasis can occur. This is fairly common because gene products do not function in isolation. In the example of fur color in mice, one can see that pigment synthesis and pigment deposition are two processes that must occur in order for a specific phenotype (i.e., color) to be observed.
Lastly, we considered the case of polygenic inheritance, whereby many genes and their alleles are involved in the expression of a given phenotype. If you think about it, you will likely recognize that polygenic inheritance and epistasis are related. In fact, many traits are determined by multiple genes, which makes the analysis of expression patterns complex.