The Complexity of Human Genetic Inheritance

Scenario 1

A newly married couple just moved into a serviced apartment with its own security guard. The husband was working in another state, although within the same country. Hence, he only comes home once a month, usually at the end of every month. The wife conceived and delivered a baby boy. But to the surprise of everyone, the baby looked so much like their security guard. The husband suspected foul play and decided to go for a paternity test. The result confirmed that he is actually the father of the baby. Still not contented, samples were sent to an international lab and the result came out the same. How come?

Scenario 2

A black Nigerian couple - Ben and Angela in the United Kingdom gave birth to a white baby. The first instinct of everyone back at home was that the wife must have cheated on the husband. Further inquiries actually showed that there was no case of infidelity on the part of the wife. Several hypotheses were formulated to support the possibility of such occurrence but none could be tested or confirmed.

Scenario 3

When my first son was given birth, she looked so much like his mum. A few months later, he looked like me in every way. He is two years now and one cannot really pick who exactly he looks like anymore. In fact, none of us the parent.

....and the complexity goes on

The early days in my genetics class were very interesting and the only aspects that resonate quite well on my mind - and the minds of virtually everyone. Then, inheriting characters was as simple as just taking one allele from your mother and another from your father. When the two alleles come together, a genotype for a particular trait is formed. What would be expressed morphologically now depends on the dominance or recessiveness of each allele.

Taking human height as an example, if your father is short and your mother is tall with the tallness allele in your mother being dominant (meaning shortness is recessive), a combination of shortness allele and tallness allele from the respective parents will result in you being tall. In other words, the tallness of your mother will manifest in you.

If bother of your parents are homozygous for their respective traits, none of your siblings would be short as far as height is concerned. It all made sense to every one of us and we thought highly of ourselves as geneticists - not until we started treating topics relating to codominance, incomplete dominance, linkage, and gene interactions such as pleiotropy and epistasis.

From there, we also dabbled into sex-limited inheritance, cytoplasmic inheritance, Y-limited traits, and so on. Some aspects of the subject initially came as too complex to understand but with continuous practice and reading, many of us were able to grab it. Still some aspects of human inheritance exist that seem not to conform to any of the patterns that have been researched and established in Genetics.

It all started with the work of Gregor Mendel and everyone thought the codes of inheritance have been cracked. Then, a few exceptions to the laws of Mendel were first discovered. Later, scientists found out that the laws of Mendel were actually the exceptions to inheritance, and not the norm. Only a handful of traits in humans follow a simple dominance/recessive inheritance pattern and the interaction of multiple genes is often behind many of the traits we see expressed morphologically.

The truth is, even with the advancement in technology and the availability of sophisticated equipment to study chromosomes and inheritance, many things remain uncomprehended when it comes to humans. According to the Human Genome Project, an average human has between 20,000 and 25,000 genes. With the findings that genes on one the same or even separate chromosomes have the capacity to interact to produce a physical trait, one can only imagine the complexity of interactions that can happen with thousands of genes. Definitely not something that can be fully understood by scientists for a very long time - if ever.

Science has gone far in trying to understand nature. But in reality, the more we know as humans, the more the unknown increases.

If you have this far, I sincerely thank you for your time.