To the uninitiated, genetics may sound simple. You have a particular trait because your parents had it. Real-world genetics, however, is anything but simple.
To assert that a child has a trait (blond hair, asthma, a predilection for being overweight) because one of the parents had it is to presume that there is only one mode of inheritance. Unfortunately for the lay person (but fortunately for the employment of geneticists and genetic counsellors), traits can be inherited in many different ways.
Perhaps the simplest mode of inheritance is what is known as “autosomal dominant”. “Autosomes” constitute all chromosomes in a cell with the exception of the sex chromosomes, X and Y. A trait which is “autosomal” has its gene located on a non-sex chromosome. “Dominant” means exactly what the term conjures up: the presence of this trait will gag the expression of another. For example, Huntington’s disease is caused by a duplication of a DNA sequence in a particular gene. This duplication acts in an autosomal dominant fashion: if you inherit a copy from your dad which has the duplication and a copy from your mom which is healthy, the abnormal copy will “dominate” the healthy copy and you will develop the disease.
By contrast, a trait which is recessive requires both parental copies of a gene to be abnormal for the trait to be expressed. For instance, Tay-Sachs disease, in which fatty deposits in the brain lead to the death of the child at a young age, only occurs if both parents have an abnormal copy of the gene and the child receives both of these copies. The parents are said to be “carriers”; the child is “affected”.
Still with me so far? We’ve only scratched the surface.
If the gene responsible for a trait is on a non-sex chromosome, sons and daughters are equally likely to inherit it. However, if the gene is located on a sex chromosome, the odds abruptly change. You see, while women have two X chromosomes, men have one X and one Y chromosome. Men get their Y chromosome from their dad and their X from their mom. If the mom has a trait which is dominant but is located on one of her X chromosomes, then half of her children will inherit it, as if the gene was located on an autosome. However, if the father has the trait on his X chromosome, then only his daughters will be affected. His sons will remain unscathed because they must necessarily receive their dad’s Y chromosome (and not his X) in order to be boys.
An X-linked trait can also behave in a recessive manner. A good example of this is Duchenne muscular dystrophy, which is characterized by a rapidly progressing muscle weakness. Girls who have the mutation on one of their X chromosomes are protected by the other, healthy X. The boys, however, have no such protection, as they have only one copy of the X chromosome. If a gene on it is faulty, no second copy exists to compensate for it.
You can already see that the transmission of genes from parents to children and their expression to yield visible traits is not as simple as “parents have it therefore kids have it too”. The complexities, however, do not stop there.
Our blood groups (A, B, O), for instance, do not fit into any of these neat categories. They are “codominant”. If you inherit the “A” gene from your mom and the “B” gene from your dad, neither “gags” the other into submission; rather, both are expressed simultaneously, making you “AB”. This is because this gene encodes a protein which is expressed at the surface of blood cells. If both the “A” and the “B” versions of the gene are present, both proteins are made and expressed simultaneously. Neither one silences the other.
All of our discussion up until now has depended on the faithful expression of a trait whenever the gene is present. But what happens if a gene says a certain trait should be expressed but the trait is not? Believe it or not, this does happen and is referred to as “incomplete penetrance”.
Also, all this talk of “one gene, one trait” is great for the sake of simplicity but it provides a gravely deficient picture of reality. Some traits depend on more than one gene for their expression. If you are a fan of dogs, you may have noticed that labrador retrievers come in three colours: black, chocolate, and golden. Their coat colour is determined by the interplay between two different genes. Gene 1 determines if the dog is black or other; gene 2 determines if the dog is chocolate or golden. Even if gene 2 has the required sequence to yield a golden retriever, the presence of the “black” version of gene 1 will dominate over the golden colour and will result in a black labrador. This is known as “epistasis”.
Our most common diseases, like asthma, involve not one, not two, but dozens if not hundreds of genes in a complex web of interactions that is only now beginning to be mapped out. Moreover, some diseases are encoded not on our autosomes or sex chromosomes, but on another little genome that our cells carry called the “mitochondrial genome”. This little circle of DNA is passed on from the mother to all of her children. And let’s not forget that we are not our genes; the environment plays an important role in shaping us. And silencing marks known as “methyl groups” are more and more being recognized for the role they play in shaping the expression of the genetic material we inherit from our parents.
So, next time you contemplate the deep mystery of why you have asthma whereas neither of your parents suffer from it, you’ll have a better understanding of the complexities of genetic inheritance. If it were simple, it wouldn’t be a specialty of medicine.
(Feature picture by Imagicity)