Each person has a unique set of chemical blueprints, which is contained in our DNA inherited from both parents at the time of conception. The segments of DNA that contain the instructions for making specific body proteins are called genes. These affect how we look and how our body functions. Genes are neatly packaged in chromosomes, which are duplicated as the foetus develops, copying the same genetic information.
Genetic disorders explained
These occur when there is a ‘misprint’ in the DNA, which causes an altered instruction to the body. For example, short, broad bones in people with achondroplasia or dwarfism.
Single gene defects
These are inherited either as ‘autosomal recessive’ (as in cystic fibrosis) or ‘autosomal dominant’ (as in dwarfism), depending upon whether the mutated gene is dominant over the corresponding normal gene or not.
For a recessive gene to have an effect, the child has to inherit the mutated gene from both parents who have no defects but are carriers of the altered gene.
Trisomy is when there are too many chromosomes (three instead of two of a particular chromosome in every cell). Trisomy almost always results in mental retardation and physical abnormalities because of the confusion caused by an extra set of genetic instructions. For example, the result of having three number 21 chromosomes is Down’s syndrome. Monosomy is when only one copy of a particular chromosome is produced. Most of these cause a miscarriage.
Other chromosome defects include:
- Deletions, where part of the chromosome is lost
- Translocations of sections between the chromosomes
- Inversions where a small portion of the chromosome is printed back to front or upside down.
Balanced translocations and inversions don’t cause malformations or abnormalities but people who have them, male or female, maybe at risk of miscarriage or having children with genetic disorders. For example, if a man’s number 15 and number 21 chromosomes are stuck together, he might pass on these two joined chromosomes as well as his other 21. The baby also inherits a 15 and 21 from the mother resulting in Down’s syndrome. It is important to recognise this as the cause of Down’s syndrome, as the risk of recurrence is high.
Males have an X and a Y chromosome. They inherit an X chromosome from their mother and a Y chromosome from their father. This is why it is the father who determines the sex of a baby, as only he can pass on the male Y chromosome.
X chromosomes are large and carry many genes. Y chromosomes are smaller (their main function being to make the developing embryo male). Females have two X chromosomes, inheriting one from each parent. If a female inherits only one X chromosome she will have only 45 chromosomes which results in Turner’s syndrome and sterility. On the other hand, if the male inherits an extra sex chromosome (perhaps two from the mother and a Y from the father), he will be an XXY male with 47 chromosomes, which results in Klinefelter’s syndrome, the most common cause of male sterility.
These are mostly recessive. If one of a female’s X chromosomes contains a faulty gene, it is usually dominated by the good gene in the other X chromosome.
Males only have one X chromosome, which they inherit from their mother and the tiny Y chromosome, which they inherit from their father, and which doesn’t have all the genes found in an X chromosome. Therefore if a boy gets a faulty X from his mother he will be affected. Although a girl is protected from the same disorder by the good gene in her other X, she will be a carrier like her mother.
Examples of X-linked recessive inheritance affecting boys are haemophilia, Duchenne muscular dystrophy and colour blindness.