Genetic conditions
On the previous page, you learned about ways in which genes combine to determine inherited characteristics such as eye colour, hair colour and height.
Occasionally, a faulty chromosome, gene or allele may be inherited and this can cause the child to develop a genetic condition or disorder. Let’s take a look at some examples of genetic conditions and how they are caused – click the buttons below to learn more.
Cystic fibrosis
This is an inherited disorder, caused by a faulty recessive allele on the 7th chromosome. It affects the cell membranes in the lungs and digestive system; too much mucus is produced and this can cause the lungs to become clogged and damaged (eventually, the lungs may stop working properly).
A child will have to inherit two copies of the faulty recessive gene to be born with cystic fibrosis – one from each parent.
So, if the recessive allele for cystic fibrosis is represented with a lower case ‘f’, and the dominant allele is represented with an upper case ‘F’, we can see that...
- If a child is homozygous, with two recessive alleles (ff), they will be born with cystic fibrosis.
- If a child is homozygous, with two dominant alleles (FF), they will not be born with cystic fibrosis.
- If a child is heterozygous, with one recessive allele and one dominant allele (Ff), they will not be born with cystic fibrosis but they will be a carrier of the faulty recessive allele. This means that they could pass this faulty allele onto their own children in future.
Down’s syndrome
This is a genetic condition which is caused by the presence of an extra chromosome when the baby develops in the womb. The baby has 47 chromosomes, rather than 46. There are still 23 pairs of chromosomes, but the 21st chromosome has three copies rather than two, like all the others.
The presence of this extra chromosome causes certain genes to be ‘over expressed’. This means that babies born with this condition may have:
- Reduced muscle tone
- Slower rate of learning
- Slower rate of growth
- Slightly different facial features
- Development delays (they may reach milestones such as sitting, crawling and walking at a later stage, compared with other children their age)
- Delays in developing self-care skills (such as feeding, dressing and toilet training)
- Speech impairments
- Heart problems
Polydactyly
This is an inherited condition which causes a baby to be born with extra fingers or toes. It is caused by a dominant allele, which means it can be passed on and inherited through just one allele from one parent.
Key point
As well as conditions caused by faulty genes/alleles, a child’s genetics can also increase their risk of developing certain health conditions. These conditions are known as hereditary, as they are passed down through families. Examples of hereditary health conditions include certain cancers, diabetes, heart disease and Alzheimer’s disease. Many eye diseases are also caused by hereditary factors, such as colour blindness, glaucoma and age-related macular degeneration (AMD). Glaucoma and AMD are two leading causes of sight loss in adults.