Recessive Traits In Humans

As we mentioned earlier, many of the early ideas about how inheritance in humans works turned out to do a poor job of explaining what actually happens. But when we take Mendel's ideas about inheritance in peas and apply them to humans, a lot of confusing things start to make sense. By proposing that information is different from what gets made using that information, by proposing the existence of the particles that we now call genes and by proposing that some traits are dominant over other traits, Mendel provided ideas that help us understand many things we see in human patterns of inheritance.

Let's look at the human trait albinism, specifically a form known as oculo-cutaneous albinism, which is manifested in people who make little or no melanin pigment (Figure 4.1). The common perception of albinism, people with stark white skin and hair and red eyes is over-simplified and incorrect. In fact, there is some variation in how pale people with albinism are, and the stereotype of red eyes is wrong. Some may even have yellowish hair or other signs of coloration, such as freckles. Most commonly, they have blue or gray eyes. Sometimes, their eyes may take on a purplish or reddish tint if the light is just right, resulting from the red tints from the retina showing through the pale coloring of the iris. This does not normally happen in most individuals with blue or gray eyes because the pigment in a pigment epithelium layer behind the iris normally blocks the red tints in the retina from being seen.

In most ways, people who lack melanin are just like the rest of us. But even though they are highly diverse in terms of a variety of traits, they do have some features in common, such as their unusual coloring and vision problems. The lack of melanin during development of the eyes causes abnormal routing of the optic nerves into the brain and results in inadequate development of the retina. They often use glasses, but their vision often cannot be corrected to 20/20 acuity with either glasses or surgery. They are unusually sensitive to bright light. Some are legally blind, but others see well enough to drive a car when using special lenses. Some are not blind but have vision problems that can't be helped by corrective lenses. In some cases, skin cancer can

FIGURE 4.1 Rick Guidotti's elegant photo spread in Life magazine featured individuals with albinism as unique and beautiful, emphasizing the need to avoid stuffing them into some prejudicial box with a label on it just because they happen to share a genetic trait. In contrast to the attitudes during the nineteenth century when people with albinism were featured in circus sideshows, this twentieth-century article succeeded in communicating a great sense of the unique and positive value in each of those photographed, including the beautiful woman featured here.* (Courtesy of Rick Guidotti for Positive Exposure.)

FIGURE 4.1 Rick Guidotti's elegant photo spread in Life magazine featured individuals with albinism as unique and beautiful, emphasizing the need to avoid stuffing them into some prejudicial box with a label on it just because they happen to share a genetic trait. In contrast to the attitudes during the nineteenth century when people with albinism were featured in circus sideshows, this twentieth-century article succeeded in communicating a great sense of the unique and positive value in each of those photographed, including the beautiful woman featured here.* (Courtesy of Rick Guidotti for Positive Exposure.)

be a problem, especially in equatorial regions, if they don't use sunscreen and take other steps to protect themselves well enough from sunlight.

Although most individuals with simple albinism are as healthy as the rest of us, two very rare forms of syndromic albinism are associated with serious medical problems. Hermansky Pudlak syndrome includes albinism and multiple other characteristics, including problems with bleeding. Chediak-Higashi syndrome characteristics include susceptibility to infection and development of malignant lymphoma.

According to the National Organization on Albinism and Hypopigmen-tation, one person in every 17,000 has some form of albinism. Based on those numbers, that would mean that more than one in every 100 individuals may be a carrier, some one with one normal copy of an albinism gene and one defective copy of that same gene.

Albinism is hereditary, which may not seem obvious if you look at the pig-mented families into which most people with albinism are born. When we

* At www.rickguidotti.com, there is more information about Rick Guidotti's Positive Exposure campaign which is aimed at challenging stigmatization of genetically unusual individuals and celebrating the differences that are the result of genetic diversity.

look at how this trait is passed along in Figure 4.2, we see that Mendel's ideas are not unique to pea plants. The same rules apply here.

Thus there is a human trait, absence of pigmentation, that is recessive to the dominant trait, presence of pigmentation. If someone has a pigmented version of the albinism gene (the pigmented allele) along with the albinism allele, the albinism trait is not manifested and they have color in their skin and hair. If the individual is homozygous for albinism alleles, skin and hair color will be white. As with the pea plants, the heterozygous individual's coloring is determined by their dominant pigmented allele and is not a blended average of the two traits. As happens with the true-breeding yellow pod strain

FIGURE 4.2 A family tree for a family with the albinism trait shows what happens when someone with the albinism trait marries someone pigmented. In the first generation, we see two different couples in which an individual with albinism marries someone who does not have the albinism trait. Notice that in the second generation, all of their children have pigment in hair and skin, but the marriage of two individuals who each have a parent with albinism can lead to about one quarter of their children having the albinism trait. The pigmented individuals in the second generation are considered carriers, individuals who lack the trait but carry the information, which can then be passed along to the next generation. In many families into which an child with albinism is born, there may be no known ancestors with albinism. In rare cases, the trait might be reported for a distant ancestor, but most often there will be no evident family history of albinism. Thus recessive information can pass through many generations before a carrier chances to marry another carrier to a produce a child with the trait. In this case, for the information to result in the trait, the child must receive an albinism allele from each parent so that they end up with two albinsim alleles. So what is wrong with this picture? The individuals with albinism in this picture are depicted as having skin as white as their hair, but the very pale skin of someone with albinism is not white like a sheet of paper.

of peas, if someone with albinism marries someone else with the same form of albinism, their children will also fail to make pigment.

There are individuals in the population with normal levels of skin and hair color who do not know that they carry an albinism allele. In fact, they won't ever know it unless they pick someone else with an albinism allele as their mate. Having one albinism allele does not dilute out the coloration brought about by the other skin color genes.

The term carrier is used for people who carry a recessive allele without showing any phenotypic differences to indicate that they have the recessive allele. Another way to think about it is that carriers are individuals who carry the information (genotype) without manifesting the trait (phenotype), just like the heterozygous green pea pods.

We talk about albinism as if this were some uniform condition, but in fact there are different forms of albinism, some of which do not look at all like our classic concept of someone with albinism. For instance, some individuals have what is called "yellow albinism," which may involve some coloration in both skin and hair. There is also a form of ocular albinism that only affects pigmentation in the eyes. Also, although syndromic forms of albinism can involve features other than coloration, simple albinism does not cause uniformity of anything outside of coloration. Thus individuals with simple albinism are as diverse as the rest of the human race in terms of intelligence, talents, temperament, agility, strength, and health status.

We talk about albinism as a recessive trait rather than talking about pigmentation as a dominant trait because, when we are talking about an unusual or rare characteristic, we are usually trying to identify what is going on with the mode of inheritance of the unusual phenotype within a family or population. We could actually say that pigmentation is dominant over albinism. The inheritance of skin pigmentation may look simple: if you have two copies of the albinism allele, you have the coloration of the albinism trait. However, if you have at least one copy of the normal dominant copy of the albinism gene, there are actually a lot of other genes that contribute to coloration and make inheritance of skin and hair color rather complex.

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