- DNA, genes and chromosomes
- How are genetic disorders inherited?
- Autosomal dominant inheritance
- Autosomal recessive inheritance
- X-linked recessive inheritance
- X-linked dominant inheritance
- Mitochondrial inheritance
DNA, genes and chromosomes
DNA (deoxyribonucleic acid) is the hereditary material in humans and most other organisms. It is composed of four chemical bases/letters called adenine (A), guanine (G), cytosine (C) and thymine (T). The human genome is made of over 3 million letters of DNA. These letters are arranged in various sequences and length to make up genes.
Genes are the basic physical and functional units of heredity. We have more than 20,000 genes, some of which provide instructions to cells in to make specific proteins that control the structure and function of our bodies. However, most genes do not code for any proteins and we are still trying to learn about their functions. Genes are located in small thread-like structures called chromosomes, which in turn are located inside the nucleus of our cells.
Most cells in our body contain 23 pairs of chromosomes. In each pair, we inherit one chromosome from each parent. We therefore have two copies of most of our genes. In contrast, our germ cells (egg and sperm cells) only have one set of chromosomes (and therefore one set of genes) due to a division process called meiosis. When the egg is fertilised by a sperm cell, the fertilised egg will contain two sets of chromosomes, one from each parent.
22 of these pairs are identical in males and females and these are called autosomes. The 23rd pair are the sex chromosomes and they determine gender. Females have two copies of X chromosome (one from each parent) while males have one X chromosome (from the mother) and one Y chromosome (from the father).
How are genetic disorders inherited?
A genetic disorder is usually due to changes (mutations) or “spelling mistakes” in a single gene. These changes can either be inherited from parent(s) or they can be “de novo” (i.e. due to a new change in that person). The faulty gene can be located in one of the 22 autosomes, in the X chromosome or even in a structure called the mitochondria. Mitochondria are small structures present in most cells in our body, providing energy for these cells to function normally (much like a battery).
Genetic conditions can be inherited in different ways depending on where the faulty gene is located and how many copies of the faulty gene are required to cause disease. These include:
- Autosomal dominant
- Autosomal recessive
Autosomal dominant inheritance
In autosomal dominant conditions, the faulty gene is in one of the autosomes (chromosomes 1-22) and only one copy of the faulty gene is required to cause disease. This can be inherited from either parent.
There is a 50% chance of an affected individual passing the faulty gene on to a child and a 50% chance of passing on their normal gene. The risk is the same whether the child is male or female. This is a random process that occurs for each pregnancy.
Occasionally, conditions inherited in this manner have variable expressivity and incomplete penetrance. Variable expressivity means a range of severity from very mild to very severe while incomplete penetrance means that some people with the faulty gene have symptoms and other people with the same exact genetic change have no symptoms at all.
Autosomal recessive inheritance
In autosomal recessive conditions, two copies of the faulty gene are required to cause disease, one from each parent. A parent who has one faulty gene copy and one normal gene copy is known as a heterozygote or a carrier and they usually do not have any symptoms. Carriers are usually not aware of their status until they have an affected child or have undergone genetic testing. For a child to be affected, both parents would need to be carriers of a mutation in the same gene.
If both parents are carriers (most likely scenario):
- 25% chance (one in four) that a child will be affected
- 25% chance (one in four) that a child will be unaffected and not a carrier
- 50% chance (one in two) that a child will be an unaffected carrier
If one parent is affected and the other is not a carrier:
- All children will be carriers
If one parent is affected and the other is a carrier (common with inter-marriages or in small isolated populations):
- 50% chance that a child is affected
- 50% chance that a child is a carrier
This is a random process and these risks apply to each new pregnancy.
X-linked recessive inheritance
The faulty gene is located in the X chromosome in such instances. If a female has a faulty gene on one of her two X chromosomes, she is considered a heterozygote carrier and usually does not show any symptoms. This is because of a process called X-inactivation which ensures that females, like males, only have one functional copy of the X chromosome in each cell. X-inactivation is a random process and therefore, the X chromosome inherited from the mother may be active in some cells while in others the active X chromosome is inherited from the father.
On the other hand, if a male has a faulty gene on his single X chromosome, he is termed “hemizygous” and will show symptoms as he does not have another X chromosome to compensate. Generally, X-linked recessive conditions only manifest in males but a minority of female carriers can be affected (usually to a milder extent) due to non-random X-inactivation.
If the mother is a carrier and the father is healthy:
- Each son has a 50% chance of being affected
- Each daughter has a 50% chance of being a carrier like the mother
If the father is affected and the mother is healthy:
- None of his sons will be affected
- All of his daughters will be carriers
This is because an affected male cannot transmit the disorder to his male offspring who inherit his Y chromosome but all his daughters will receive his X chromosome.
To summarise, in X-linked recessive inheritance:
- The disorder mostly affects males
- Healthy carrier females transmit the condition to their sons
- There is no male-to-male transmission
X-linked dominant inheritance
- Less common than X-linked recessive inheritance
- Both males and females can be affected as only one faulty gene copy on the X chromosome is required to cause disease
- Females are usually less severely affected than males because of random X-inactivation
- Affected females can pass the condition on to their daughters as well as their sons
- Affected males will still only transmit the condition to their daughters but not to their sons
Mitochondria are energy producing structures inside our cells which have their own DNA with 37 genes. Mutations in mitochondrial genes are passed on to the next generation through the mother (maternal inheritance). This is because only the egg cells but not the sperm cells contribute mitochondria to the developing embryo. Both males and females can be affected with mitochondrial inheritance.