What you’ll learn to do: Discuss the role environment plays on phenotypes

In recent years, scientists have begun to research how our environment can impact our phenotypes: most common diseases are a result of both your genes and your environment. Your environment can include personal choices, such as what foods you eat and how much you exercise, and external factors, such as stress, clean water, and air quality. Most diseases, especially common diseases, are a combination of your genetic risk and your environment. Only a small number of diseases are a result of just a single mutation in a gene. Examples of these single-gene disorders are Huntington disease and Tay Sachs.

It is becoming difficult to group diseases into either purely “genetic” or “environmental” because most diseases are a little bit of both. For example, emphysema can be the result of both smoking and a disorder called alpha-1-AT deficiency.

In this outcome, we’ll learn about a few different ways our environment can impact us.

Polygenic Inheritance and Environmental Effects

How is Height Inherited?

Many heritable human characteristics don’t seem to follow Mendelian rules in their inheritance patterns. For example, consider human height. Unlike a simple Mendelian characteristic, human height displays:

• Continuous variation. Unlike Mendel’s pea plants, humans don’t come in two clear-cut “tall” and “short” varieties. In fact, they don’t even come in four heights, or eight, or sixteen. Instead, it’s possible to get humans of many different heights, and height can vary in increments of inches or fractions of inches. As an example, consider the bell curve-shaped graph in Figure 1, which shows the heights of a group of male high school seniors.
• A complex inheritance pattern. If you’ve paid attention to the heights of your friends and family, you may have noticed that many different patterns of inheritance are possible. Tall parents can have a short child, short parents can have a tall child, and two parents of different heights may or may not have a child of intermediate height. In addition, siblings with the same two parents may have a range of heights, ones that don’t fall into clear, distinct categories. Simple models involving one or two genes can’t accurately predict all of these inheritance patterns.

Figure 1. Heights of male high school seniors. Image modified from “Continuous variation: Quantitative traits,” by J. W. Kimball (CC BY 3.0).

Some human characteristics, such as height, eye color, and hair color, don’t come in just a few distinct forms. Instead, they vary in small gradations, forming a spectrum or continuum of possible phenotypes.

How, then, is height inherited? Height and other similar features are controlled not just by one gene, but rather, by multiple (often many) genes that each make a small contribution to the overall outcome. This inheritance pattern is called polygenic inheritance (poly– = many). For instance, a recent study found over 400 genes linked to variation in height^[1]. When there are large numbers of genes involved, it becomes hard to distinguish the effect of each individual gene, and even harder to see that gene variants (alleles) are inherited according to Mendelian rules. In a further complication, height doesn’t just depend on genetics: it also depends a lot on environmental factors, such as a child’s overall health and the type of nutrition he or she receives while growing up.

Pleiotropy and Human Disorders

Pleiotropy

When we discussed Mendel’s experiments with purple-flowered and white-flowered plants, we didn’t mention any other phenotypes associated with the two flower colors. However, Mendel noticed that the flower colors were always correlated with two other features: the color of the seed coat (covering of the seed) and the color of the axils (junctions where the leaves met the main stem)^[2].

Figure 2. Based on similar diagram by Ingrid Lobo

Genes like this, which affect multiple, seemingly unrelated aspects of an organism’s phenotype, are said to be pleiotropic (pleio– = many, –tropic = effects)^[3]. The seemingly unrelated phenotypes can all be traced back to a defect in a single gene with several jobs.

Importantly, alleles of pleiotropic genes are transmitted in the same way as alleles of genes that affect single traits. Although the phenotype has multiple elements, these elements are specified as a package, and the dominant and recessive versions of the package would appear in the progeny of a monohybrid cross in a ratio of 3:1.

Pleiotropy in Human Genetic Disorders

Genes affected in human genetic disorders are often pleiotropic. For example, people with the hereditary disorder Marfan syndrome may have a constellation of seemingly unrelated symptoms^[4]:

• Unusually tall height
• Thin fingers and toes
• Dislocation of the lens of the eye
• Heart problems (in which the aorta, the large blood vessel carrying blood away from the heart, bulges or ruptures).

These symptoms don’t appear directly related to one another, but as it turns out, they can all be traced back to the mutation of a single gene.

Effect of the Environment

Characteristics that are influenced by environmental as well as genetic factors are called multifactorial. The idea of “nature versus nurture” — in other words, the relative influence of genetics versus environmental factors — has been and still is debated. Just looking at the genes of a given organism will not determine how that organism will develop and act. Even identical twins will show different characteristics, depending on the environment in which they live. Everyone is a product of their environment as well as their genetics.

Figure 3. Taking a newborn blood sample for PKU testing. By Staff Sgt Eric T. Sheler, U.S. Air Force (Phenylketonuria testing) Public Domain

Even when influenced by the environment, phenotypes have a normal range of expression. For instance, human height varies based on nutrition and genetics, but not many people are shorter than 4½ feet or taller than 7 feet. The range of phenotypic possibilities is called the norm of reaction. Hydrangeas, for example, may be blue, pink, or purple, but they are never naturally orange. Hydrangeas are blue in acidic soil with available aluminum, and they are pink in alkaline soil without available aluminum.

You may have heard about PKU, a pleiotropic disorder caused by defects in a single gene coding for an enzyme that converts the amino acid phenylalanine to tyrosine. Newborns are tested for this defect very early in life (Figure 3), so that if the results are positive, they can be given a diet limiting phenylalanine ingestion. That way, the toxic buildup is prevented and the children can develop normally. PKU is an example in which environmental factors can modify gene expression.