Is Down Syndrome a Dominant or Recessive Trait?

You’re here because you are wondering if one of the Down syndrome or any other of the rare diseases like Huntington’s diseaseis dominant or recessive. However, firstyou need to stop and think if you really know what dominant traits and recessive traits are and how they differ before you think about what that means in terms of Down syndrome. What is Down syndrome? What is a dominant trait? What is a recessive trait? In fact, what is a trait?

Let’s address these question in the reverse order it was asked. Traits are essentially characteristics. In the world of genetics, traits are visible characteristics of an animal or of a human. Examples of traits include hair color, eye color, height, skin color, toe length, and so on. Traits are often discussed in the context of phenotypes in genetics whereby the phenotype is a collection of traits. The best way to think about this is in terms of eye color whereby eye color is a trait and blue eye color is a phenotype.

Now that you understand the concept of trait, let’s get into what dominant and recessive traits are. But, before we get into that, there are a few other things that should be addressed, such as genotype and alleles. The genotype is the collection of all of the genes an individual carries. The interaction between the genes, as well as the genes and environment,culminating in an individual’s phenotype. Genes typically come inseveral versions, called alleles. Think of alleles again in terms of traits—there is a single gene that determines a trait while alleles determine the phenotype (remember the eye color example). Alleles are typically denoted by letters, whereby capital letters indicate dominant alleles and lower case letters indicate recessive alleles coding for dominant and recessive traits, respectively. Let’s take eye color, for example, whereby B denotes brown eye color and b denotes blue color. Brown eye color is dominant and blue eye color is recessive. An individual who carries two copies of the brown color allele, or BB, is said to be homozygous for the brown eye color. Furthermore, an individual who carries two copies of the blue color allele is said to be homozygous for the blue eye color. However, there are also individuals who have the Bb genotype. They are called heterozygotes because they carry two different alleles. However, since the allele for brown eye color is dominant, an individual with the Bb heterozygous genotype will have brown eyes. Since the blue eye color allele is recessive, only individuals who are homozygous for it, or bb, will have blue eyes. Think of dominant and recessive alleles as stronger and weaker, whereby the dominant allele will always mask the recessive allele in phenotype and never the other way around. (This is only true for Mendelian type inheritance, which is what we will assume for the purposes of the post.)

So, now you know the difference between dominant and recessive. However, before we juxtapose it onto the Down syndrome scenario, let’s first get a bit into what Down syndrome is as well as the genetics behind it.

Down syndrome is a genetic disorder Down syndrome is a genetic disorder but cannot be classed as recessive disorder. Individuals who do not have the disease have two copies of each of the 23 chromosomes—one copy from the mother and one from the father. In other words, each cell contains a total of 46 chromosomes. Each healthy cell, that is. Those who have Down syndrome have an extra copy of chromosome 21. This “extra chromosome” is called trisomy 21. This means that they have an entire, part of, or half of chromosome 21 in addition to the rest of the chromosomes. Rather than being an illness, it is more of an umbrella term for all the manifestations that result from having additional DNA. This extra material affects several characteristics, namely physical features, cognition as well as the overall development of a person. Furthermore, in addition to the immediate observable side effects, it comes with an increased probability of some additional health issues.

Down syndrome also happens via another pathway, namely a translocation. A translocation is a rearrangement of genetic material between chromosome 21 and another chromosome. If the translocation is balanced, namely genetic material is neither gained nor lost, it comes without side effects. However, if a translocation carrier parent passes on this translocation to their offspring, things get messy, a misbalance of genetic material can occur, and it may lead to Down syndrome.

So what about the genetics of the syndrome? Is Down syndrome dominant or recessive? So far, we have only discussed inheritance in terms of two alleles, but now that we have more than two, things get a little tricky, but not too much.

Most of the time, Down syndrome is not inherited. Trisomy 21 is a random event that takes place during cell division. This typically happens in egg cells, but it is not uncommon for it takes place in egg cells, too. Trisomy 21 is neither dominant nor recessive simply by virtue of not being inheritable.

On the other hand, a translocation-induced trisomy 21 that leads to Down syndrome is indeed inherited, but it is neither dominant nor recessive because it is still a result of a faulty cell division machinery.

So, what is the overall verdict here?

There is no genetic marker associated with Down syndrome and, as such, there is nothing that can be passed down. (Translocation trisomy 21 is not a genetic mutation that is passed down, but simply a clean and balanced translocation, which is not a genetic marker.) Along those lines, as there is nothing that is passed down from one generation to another, there is nothing to be inherited, making Down syndrome a condition that is neither dominant nor recessive. If anything, one may call it spontaneous as it happens without prior indications. (There are theories that mothers over the age of 35 are at a higher risk of giving birth to babies with Down syndrome. However, that is beyond the scope of this post.)

Laura Day
 

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