Causes of Genetic Diseases
- A gene can affect the phenotype of an organism
- A gene codes for a single polypeptide
- The polypeptide affects the phenotype through a particular mechanism
- The phenotype of an individual can also be affected by the environment
- A genetic disease is caused by a gene which results in an abnormal protein that alters the phenotype of the individual
- Most genetic diseases are caused by recessive alleles on autosomal chromosomes
- This means that an individual would need two copies of the recessive allele in order to develop the disease
- Individuals that are heterozygous do not suffer from the disease but are carriers and can pass the recessive allele on to the next generation
- A disease determined by a recessive allele includes cystic fibrosis
- Some diseases are caused by dominant
- This means that only one copy of the allele is required in order to develop the disease and this one copy can also be passed on to the next generation
- Individuals that are homozygous dominant, will suffer from the disease and will also pass the allele on to the next generation with 100% probability
- A disease determined by a dominant allele includes Huntington's disease
- It is also possible, but rare, for a disease to be caused by codominant alleles
- This means that in individuals with heterozygous genotype, both alleles are expressed in the phenotype
- Therefore giving a 3rd phenotype that is different to the homozygous phenotypes
- A disease determined by codominant alleles includes sickle cell anaemia
- The genes which causes some genetic diseases are found on the sex chromosomes
- This means they affect males and females differently
- Examples of sex-linked diseases include haemophilia and colour blindness
Examiner Tip
You may be asked to predict the inheritance of diseases like the ones above. An example question would be:
Max and Jane are trying for a baby but they are concerned about the possibility of their child having haemophilia. Neither Max or Jane have haemophilia themselves but Jane’s father had the condition. What are chances that their child could have haemophilia?
For questions like this, it is very important to gather early on whether the abnormal allele that causes the disease is dominant or recessive and if there is any sex linkage. In this example for haemophilia, the abnormal allele is recessive and the gene is sex-linked. Then the next step would be to work out the genotypes of the parents from the information given and use this to create a genetic diagram.