CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

10. Patterns of Inheritance

10.6. Modified Mendelian Patterns

Mendel’s principles are most clearly observed under very select conditions in which alleles have consistent dominant/ recessive interactions. So far, we have considered only a few straightforward cases. Most, however, may not fit these fundamental patterns. This section discusses several common inheritance patterns that do not fit the patterns that are generally associated with Mendelian genetics.

Codominance

In some inheritance situations, alleles lack total dominant and recessive relationships and are both observed phenotypically to some degree. This behavior is not consistent with Mendel’s law of dominance. This inheritance pattern is called codominance. In codominance, the phenotype of both alleles is expressed in the heterozygous condition. Consequently, a person with the heterozygous genotype can have a phenotype very different from either of his or her homozygous parents. In problems involving codominant alleles, all capital symbols are used, and superscripts are added to represent the different alleles. The capital letters call attention to the fact that each allele can be detected phenotypically to some degree, even when in the presence of an alternative allele. For example, the coat colors (C) of shorthorn cattle are phenotypically red (C^RC^R), roan (C^RC^W), and white (C^WC^W). The roan coat is composed of individual hairs, which are either red or white. Together, they create the intermediate effect of roan. Roan coat color can be seen in several other species, including horses (figure 10.5).

FIGURE 10.5. Codominance

The color of this breed of horse, an Arab, also displays the color called roan. Notice that there are places on the body where both white and red hairs are displayed.

Another example of codominance occurs in certain horses. A pair of codominant alleles (D^R and D^W) is known to be involved in the inheritance of these coat colors. Genotypes homozygous (D^RD^R) for the D^R allele are chestnut-colored (reddish); heterozygous genotypes (D^rD^w) are palomino-colored (golden color with lighter mane and tail). Genotypes homozygous (D^WD^W) for the D^W allele are almost white and called cremello.

Incomplete Dominance

In incomplete dominance, the phenotype of a heterozygote is intermediate between the two homozygotes on a phenotypic gradient; that is, the phenotypes appear to be “blended” in heterozygotes. A classic example of incomplete dominance in plants is the color of the petals of snapdragons. There are 2 alleles for the color of these flowers. Because neither allele is recessive, we cannot use the traditional capital and lowercase letters as symbols for these alleles. Instead, the allele for white petals is the symbol F^W, and the one for red petals is F^R (figure 10.6).

FIGURE 10.6. I ncomplete Dominance

The colors of these snapdragons are determined by two alleles for petal color, F^W and F^R. There are three phenotypes because of the way in which the alleles interact with one another: (a) red, (b) white, and (c) pink. In the heterozygous condition, neither of the alleles dominates the other.

There are three possible combinations of these 2 alleles:

Genotype Phenotype

F^WF^W White flower

F^rF^r Red flower

F^rf^w Pink flower

Notice that there are only 2 different alleles, red and white, but there are three phenotypes—red, white, and pink. Both the red-flower allele and the white-flower allele partially express themselves when both are present, and this results in pink. The gene products of the 2 alleles interact to produce a blended result.

Problem Type: Incomplete Dominance

Cross 4: If a pink snapdragon is crossed with a white snapdragon, what phenotypes can result, and what is the probability of each phenotype? Notice that the same principles used in earlier genetics problems still apply. Only the interpretation process between genotypes and phenotypes in the gene key is altered. (Table 10.5)

TABLE 10.5. Solution Pathway

Gene Key

Gene: flower color

This cross results in two different phenotypes—pink and white. No red flowers can result, because this would require that both parents be able to contribute at least 1 red allele. The white flowers are homozygous for white, and the pink flowers are heterozygous.

Multiple Alleles

So far, we have discussed only traits that are determined by only 2 alleles: for example, A, a. However, there can be more than 2 different alleles for a single trait. The term multiple alleles refers to situations in which there are more than 2 possible alleles that control a particular trait. However, an organism still can have only a maximum of 2 of the alleles for the characteristic because diploid organisms have only 2 copies of each gene. A good example of a characteristic that is determined by multiple alleles is the ABO blood type. There are 3 alleles for blood type:

Alleles*

I^A = blood has type A antigens on red blood cell surface

I^B = blood has type B antigens on red blood cell surface

i = blood type O has neither type A nor type B antigens on red blood cell surface

In the ABO system, A and B show codominance when they are together in an individual, but both alleles are dominant over the O allele. These 3 alleles can be combined as pairs in six ways, resulting in four phenotypes. Review the gene key and the following problem to further explore the genetics of blood type.

Problem Type: Multiple Alleles

Cross 5: One aspect of blood type is determined by 3 alleles—A, B, and O. Allele A and allele B are codominant. Allele A and allele B are both dominant to allele O. A male heterozygous with blood type A and a female heterozygous with blood type B have a child. What are the possible phenotypes of their offspring?

Gene Key

Gene: blood type

The solution for this problem is shown in Table 10.6.

TABLE 10.6. Solution Pathway

Polygenic Inheritance

Thus far, we have considered phenotypic characteristics that are determined by single genes. However, some characteristics are determined by the interaction of several genes. This is called polygenic inheritance. In polygenic inheritance, a number of different pairs of alleles combine their efforts to determine a characteristic. Skin color in humans is a good example of this inheritance pattern. According to some experts, genes for skin color are located at a minimum of three chromosomal locations or loci. At each of these loci, the allele for dark skin is dominant over the allele for light skin. Therefore, a wide variety of skin colors is possible, depending on how many dark-skin alleles are present (figure 10.7). The number of total dark-skin alleles (capital D in figure 10.7) from all three genes determines skin color.

FIGURE 10.7. Polygenic Inheritance

Skin color in humans is an example of polygenic inheritance. There are several different genes for skin color located on different chromosomes, each with dark and light alleles. The total number of dark D alleles present have an additive effect on skin color. The top portion of the figure shows examples of genotypes that can produce the different skin colors. The number of dark D alleles is more important than how the D alleles are distributed in the different genes.

Polygenic inheritance is common with characteristics that show great variety within the population. Some obvious polygenic traits in humans are height, skin color, eye color, and intelligence. The many levels of height, skin color, eye color, and intelligence makes it difficult to separate individuals into meaningful categories. There is an entire range of expression for polygenic characteristics. For example, height in humans ranges from tall to short, with many intermediate heights. Eye color varies in some populations from deep brown to the lightest blue. Although it is still unclear how many genes are involved in determining these characteristics, at least two or three different genes have been identified. (Outlooks 10.1). Polygenic traits are different from a characteristic such as blood type because blood type is determined by one gene locus; thus, there are a limited number of well-defined phenotypes (A, B, O, AB).

OUTLOOKS 10.1

The Inheritance of Eye Color

It is commonly thought that eye color is inherited in a simple dominant/recessive manner, in which brown eyes are considered dominant over blue eyes. However, the real pattern of inheritance is more complicated than this. Eye color is determined by the amount of a brown pigment, melanin, present in the iris of the eye. If there is a large quantity of melanin on the anterior surface of the iris, the eyes are dark. Black eyes have a greater quantity of melanin than do brown eyes.

If melanin is absent from the front surface of the iris, the eyes appear blue, not because of a blue pigment but because blue wavelengths of light are reflected from the iris. The iris appears blue for the same reason that deep bodies of water tend to appear blue. There is no blue pigment in the water, but blue wavelengths of light are returned to the eye from the water. Just as black and brown eyes are determined by the amount of pigment present, colors such as green, gray, and hazel are produced by the various amounts of melanin in the iris. If a very small amount of brown melanin is present in the iris, the eye tends to appear green, whereas relatively large amounts of melanin produce hazel eyes. If you examine the irises of people with green or hazel eyes, you will notice that specific parts of the iris have the brown pigment.

Several genes are probably involved in determining the quantity and placement of melanin. These genes interact in such a way that a wide range of eye color is possible. Eye color is probably determined by polygenic inheritance, just as skin color and height are. Some newborn babies have blue eyes that later become brown. This is because their irises have not yet begun to produce melanin.

Blue eyes are due to a lack of pigment, not the presence of blue pigment. In blue eyes, blue light is reflected while other colors are absorbed. Green eyes absorb some blue light.

Pleiotropy

Even though a single gene may produce only one type of protein, it often has a variety of effects on the phenotype of a person. The term pleiotropy (pleio = changeable) describes the multiple effects a single gene has on a phenotype. A good example of pleiotropy—PKU—has already been discussed. In addition to the mental retardation phenotype, several other phenotypes are associated with PKU. Whereas mental retardation is caused by the buildup of phenylpyruvic acid, other phenotypes are caused by a lack of tyrosine, the next product in the pathway. Tyrosine is used by the human body to create two other important molecules—growth hormone and melanin. Growth hormone is needed for normal growth, and melanin is a skin pigment. Individuals with PKU have low levels of tyrosine because of the faulty enzyme; this results in abnormal growth and unusually pale skin, in addition to the presence of phenylpyruvic acid that can cause mental retardation.

Another example of pleiotropy is Marfan syndrome. This syndrome is a disorder of the body’s connective tissue, but it can also have effects in many other organs. (Consider the phenotypic characteristics of the individual shown in figure 10.8.

FIGURE 10.8. Marfan Syndrome

It is estimated that about 40,000 (1 out of 10,000) people in the United States have this autosomal dominant abnormality. Notice the common lanky appearance to the body and face of (a) this person with Marfan syndrome and (b) former U.S. president Abraham Lincoln. Photos (c) and (d) illustrate their unusually long fingers.

Some feel that the former U.S. president Abraham Lincoln also had Marfan syndrome. Do you see similarities?) The symptoms of Marfan syndrome generally include the following:

Skeletal

• Long arms and legs, disproportionate in length to the body

• Abnormally long fingers

• Skinniness

• Curvature of the spine

• Abnormally shaped chest, chest caves in or protrudes outward

Eye Problems

• Nearsightedness

Heart and Aortic Problems

• Weak or defective heart valves

• Weak blood vessels that rupture

• Inflammation of the heart

Lung and Breathing Problems

• Collapsed lungs

• Long pauses in breathing during sleep (sleep apnea)

Both PKU and Marfan syndrome are examples of alleles that have many different effects in an organism. Cystic fibrosis also shows pleiotropy. Review How Science Works 10.1—what information there supports this statement?

10.6. CONCEPT REVIEW

19. What is the difference between the terms dominant and codominant?

20. What is the probability of a child having type AB blood if one of the parents is heterozygous for type A blood and the other is heterozygous for type B? What other genotypes are possible in this child?

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*The symbols, I and i stand for the technical term referring to the antigenic carbohydrates attached to red blood cells, the immunogens. These alleles are located on human chromosome 9. The ABO blood system is not the only system used to type blood. Others include the Rh, MNS, and Xg systems.