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Non-binary traits

  1. May 4, 2017 #1
    In biology, you learn that every trait is on a binary. Then, you learn that there are exceptions to that binary, but most of them don't allow for a continuous range-- for codominance and incomplete dominance there are still only 3 possible phenotypes. But most traits we learn like that clearly don't fit into those categories.

    For example, we were told that hitchhiker's thumb is binary, you either have it (dominant) or you don't (recessive). But everyone who had it was a little different; some of the the thumbs were just slightly bent, some were very bent, and others were in the middle. But, this is not allowed from the binary that we are taught. That really confuses me.

    Or, for hair color. I know that both pheomelanin and eumelanin contribute to hair color. Everyone has 8 alleles for eumelanin that is either incomplete dominant or in incomplete recessive. This allows for 9 shades of hair color. Then, there is pheomelanin which is just regular dominant and recessive. This allows for 18 shades of hair color. I know for a fact that hair color has more than 18 shades. But, genetically, from what I have learned, there should only be 18. Am I missing something?

    Finally, just like the hitchhiker's thumb, many traits that are binary appear, by looks, to not be binary. It just feels like everyday obvious observations come into conflict with biology. I am definitely not trying to question biology, I just want to know if I am missing something, or if traits like hitchhiker's thumb are really binary.

    Thanks in advance.
  2. jcsd
  3. May 4, 2017 #2


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  4. May 5, 2017 #3


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    Most traits are not binary, but genetics classes may give that impression because they focus on binary traits as the genetics for these traits are very well understood and simple to understand (Mendel's Laws). Furthermore, many genetic diseases are examples of binary traits, so knowing about binary traits is very medically relevant. However, many of the traits we encounter in our everyday lives—height, weight, intelligence, skin color, etc.—are not binary traits and don't show Mendelian modes of inheritance. There are two main reasons why this is the case:

    1. Most traits are influenced by more than one gene. Traits that are binary and show Mendelian modes of inheritance are very often controlled by a single gene (e.g. whether you have cystic fibrosis depends on whether or not you have a functional copy of the CTFR gene). Most traits, however, are what we would refer to as complex traits which are influenced by very many genes. For example, I've written about how traits like height are influenced by hundreds to thousands of genes. Even if each gene exerts a binary influence on height, the combined effect of these hundred of genes will create a continuous distribution of heights (for example, imagine that an individual's height is determined by a series of 100 coin flips. Each individual coin flip produces a binary outcome, but the total number of heads will essentially be a continuous distribution with a bell-shaped curve).

    2. Most traits are influenced by both genetics and environment (i.e. nature and nurture). Studies have shown that most complex traits have some genetic component and some environmental component. For the case of height, about 80% of the variation in height is due to genetics and 20% is due to environment (for example, diet). Thus, even if genetics produces a binary outcome, environment can alter the expression of that trait in different individuals, creating a distribution of outcomes. The classic extreme case of this is the genetic disease phenylketonuria, which can cause intellectual disability due to the inability to metabolize certain amino acids. However, if individuals affected by the disease adopt a specific diet that largely avoids the problematic amino acid, they will show no symptoms of the disease.
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