Phenotype

Phenotype in Genetics

The term phenotype is used most often when studying genetics. In genetics, we are interested in an organism's genes (genotype), which genes get expressed, and how that expression looks (phenotype).

While an organism's phenotype certainly has a genetic component, it's important to remember that there can be a huge environmental component affecting phenotype as well (Fig. 1).

Genetic and Environmental Factors Can Both Decide Phenotype

Some traits are decided 100% genetically. Often, genetic diseases like sickle cell anemia, maple-syrup urine disease, and cystic fibrosis, get their diseased phenotypes due to a mutated gene. If someone has the mutated gene, no amount of lifestyle changes can make the disease more or less likely to appear. Here, genotype decides phenotype.

Phenotypic Characteristics and Identical Twins

Another classical example of the environment's influence on phenotype is in identical twins. Monozygotic (identical) twins have the same DNA sequences, hence the same genotype. They are not, however, phenotypically identical. They have phenotypic differences, in looks, behavior, voice, and more, that are observable.

Scientists have often studied identical twins to observe the environment's impact on genes. Their identical genomes make them excellent candidates to help us decipher what else is involved in determining phenotype.

Two typical twin studies compare the following groups:

• Monozygotic vs dizygotic twins

• Monozygotic twins raised together vs. monozygotic twins raised apart.

When comparing monozygotic twins to dizygotic twins, scientists are attempting to discover phenotypic factors that are more heavily influenced by genetics. If all sets of twins were raised together, then any trait shared more heavily by monozygotic twins is a trait that has higher genetic control over phenotype.

The same can be said when comparing monozygotic twins raised apart to those raised together. Suppose monozygotic twins raised apart share a trait at the same rate as monozygotic twins raised together. In that case, the similarity of genetics appears to play a stronger part than the vast variation in their environments.

Types of Phenotypes

What types of phenotypes do twin studies help us elucidate? Virtually any trait can be examined this way, although twin studies are often used to examine psychological or behavioral phenotypes. Two identical twins will have the same eye color or ear size. But do they respond identically, or even similarly, to certain behavioral stimuli? Did they make similar choices growing up, even if they grew up many miles apart, with different adoptive parents, having never met one another? How much of these phenotypic variations are due to their upbringing and environment, and how much is due to their genetic similarity?

Ultimately, the modern practice of twin studies has led to the development of three broad categories of phenotypes: those with a high amount of genetic control, those with a moderate amount, and those with more complex and nuanced inheritance patterns.

Difference Between Genotype and Phenotype

What are some instances in which genotype and phenotype may differ? "The Father of Genetics," Austrian monk Gregor Mendel, discovered the Law of Dominance (Fig. 2), which helped explain why the genotype and phenotype of an organism are not always intuitive.

If you were to see a green pea, for example, then its phenotype for color is green. Its phenotype is its observable characteristic. But would we necessarily know its genotype? Does the fact that it is green mean that both alleles that decide the color code for the "green" trait? Let's answer those questions one at a time.

1. Would we necessarily know the genotype of a green pea from seeing its color?

No. Let's say that, as Mendel discovered, peas can have two possible colors. Green and yellow. And let's say that we know that green color is the dominant trait (G) and yellow color is the recessive trait (g). So yes, a green pea can be homozygous for the green trait (GG), but according to The Law of Dominance, a pea with a heterozygote genotype (Gg) will also appear green.

Ultimately, we can't determine just from looking at a green pea if it's (Gg) or (GG), so we cannot know its genotype.

2. Does the fact that it is green mean both alleles which decide the color code for the green trait?

Again, no. Because green is the dominant trait, the plant only needs one green allele to appear green. It might have two, but it only needs one. If the plant were yellow, as yellow is the recessive allele, yes, the plant would need two yellow alleles to appear yellow, and then we would know its genotype - (gg).