About heredity

As every living creature, humans are masterly examples. How we are as a person is the result of a complex combination of factors. One very important factor is our hereditary predisposition. After all, many of our characteristics, features, talents and even faults are to an important degree determined by our genes. They define our possibilities. On the other hand, environmental factors such as education and culture also determine one’s development.

This can be compared with the functioning of a large organisation such as a railway company. On the one hand, we have the intrinsic possibilities of such an organisation. What is its operational budget? Which equipment does it dispose of? How is the quality of the rails and the turnouts? On the other hand, we find environmental factors such as work culture and European legislation. The result is a complex machinery including all train connections, stops, arrival and departure times, degree of punctuality, quality of railway stations, etc.
The smallest unity of our hereditary material is the gene, which can be compared with a turnout. No matter how well a machine is greased, it remains dependant of its smallest parts. A defect of turnouts or an abnormality in a gene…both can have disastrous consequences.

The human body is built out of various types of cells, each with their own typical function and operation: chrotoplasts, hepatocytes, brain cells, etc. The genes are in the nucleus of about every body cell and control the operation thereof. There are approximately some ten thousands of them, divided over 46 chromosomes. A chromosome looks like a small bar with some 1,000s of genes. In humans there are 23 pairs of chromosomes: each pair is composed of one chromosome of the father and one of the mother. This also implies that every gene is double.
Because every cell dies sooner or later, it needs to be replaced by cell division: a cell divides into two identical cells with the same genetic content of the nucleus. During this copying process, however, errors could arise, so-calledmutations, resulting in genetic changes. You could compare it with a sudden breakdown of a turnout through which part of the train traffic is jammed and the normal schedule is disturbed. A mutated gene will contain another instruction or manual indicating what needs to be produced in an organism and at what time. This could for instance result in tumour growth and cancer, but it could also explain hereditary diseases and why children are stillborn. If a turnout breakdown occurs on busy railway lines this certainly results in a train traffic infarction.

Congenital or hereditary?

Not every congenital abnormality or condition is hereditary. One can be born with a certain abnormality, e.g. a facial malformation, without any risk that it is passed onto one’s children. A condition is only hereditary if there is a risk that sperm cells or ovules pass on a genetic abnormality or mutation at conception.

Hereditary

We can distinguish 22 pairs of body-determining chromosomes, called autosomes and a 23rd pair that determines the sex: the sex chromosomes: XX for a female and XY for a man.

As mentioned above, each chromosome has some 1,000s of genes. If a genetic abnormality or mutation occurs in only one single gene we distinguish three different ways of heredity, and thus conditions.

First group: autosomal dominant conditions.
This is characterized by mutation in one gene on only one of the two autosomes of a autosomes pair. The other equivalent gene is not capable of adjusting or neutralising this abnormality. Imagine all turnouts in a railway network being provided twice, then a breakdown in one turnout shall be sufficient to have a train being derailed.
An example of this type is Huntington’s disease. Each child, male or female, of a Huntington patient has a 50% chance to inherit this disease. One inherits either the affected gene, or the normal one. In the first case one gets ill, in the second one doesn’t. One travels either on the good turnout or on the bad one.

Second group: autosomal recessive conditions.
In this case the same mutation occurs on both autosomes of a autosome pair before this results in a disease. Both turnouts must be broken to have the train being derailed. If only one turnout is broken, the other will take over its operation. In that case we talk about ‘carriership’. If a mutation occurs on only one of both autosomes, one is a carrier of this genetic abnormality and the disease will not develop.
An example hereof is mucoviscidosis or Cystic fibrosis. If both parents are carrier of a mutation, all of their children will have a 25% chance of developing the disease, a 50% chance to be a carrier and a 25% chance of not (having) inheriting the mutation.

Third group: sex-linked conditions.
Most frequently, males are affected, but the conditions are passed on by females. The mutation is on the X chromosome. Since males do not have a second X chromosome, a mutation inevitably occurs. The train then travels on one turnout, there is no second.
If a woman is a carrier of such a mutation each of her children, female or male, will have a 50% chance of inheriting it. A father having inherited the abnormal gene, cannot pass it on to his son, but his daughter who is a carrier anyhow, could get it from him.
Examples of such conditions are haemophilia, Duchenne’s syndrome and certain types of mental disabilities.

We just discussed the consequences of an abnormality in only one gene (the so-called monogenic conditions). No matter how drastic the consequences thereof, most of our characteristics and features are determined by a combination of various genes, in interaction with the environment. Also an organisation’s operation is rarely reducible to one single factor. In the railway company’s example, the turnouts are only a cog in the complex machine. Perfectly-made turnouts do not count for much if, for instance, at a higher level coordination is poor.

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Last updated: 10 December 2015 - 10:23
Copyright 2017 Center for Medical Genetics, Gent.