Gene + Allele - A sequence of bases on a DNA molecule that codes for a protein (polypeptide) is a gene whislt alleles are different versions of the same gene.
Genotype - The alleless an organism has, e.g. Bb, BB or bb for eye colour (b= blue eyes whilst B= Brown eyes
Phenotype - The Characteristics the alleles produce, e.g. brown eyes
Codominant - Alleles that are both expressed in the phenotype - neither one is recessive, e.g. the alleles for haemoglobin
Homozygote - An organism that carries two copies of the same allele, e.g. BB or bb
Heterozygote - An organism that carries two different alleles, e.g. Bb
Sickle cell is an example of a point mutation.
How does sickle cell anaemia occur?
- The beta-strands of haemoglobin differ by one amino acid at position 6.
- In normal haemoglobin, glutamic acid is at position 6, but in sickle-cell haemoglobin, valine is present instead.
- When this abnormal haemoglobin is deoxygenated it is not soluable and becomes crystalline and aggregates into more linear and less globuar structures.
- This deforms the red blood cells, making the, inflexible (often sickle shaped) and unable to squeeze through capillaries.
- After many cycles of oxygenation and deoxgynation some of the cells become irreversibly sickled and some destroyed.
- If enough sickle-cells become lodged in capillaries blod flow is impeded. Organs, particularly bones, do not recieve enough oxygen, leading to a painful crisis
- Eventually organs, especially heart, lungs and kidneys become damaged