• A gene can affect a?phenotype?of an organism
  • A gene codes for a single protein
  • The protein affects the phenotype through a particular mechanism
• The phenotype of an individual can also be affected by the?environment

Genes code for mRNA which gets translated into a polypeptide (protein) that affects the phenotype of the organism

TYR?gene & albinism

• Humans with albinism?lack the pigment melanin?in their skin, hair and eyes
• This causes them to have very pale skin, very pale hair and pale blue or pink irises in the eyes
• There is a metabolic pathway for producing melanin:

1. The amino acid tyrosine is converted to DOPA by the?enzyme tyrosinase
2. DOPA is converted to dopaquinone again by the?enzyme tyrosinase
3. Dopaquinone is converted to melanin

tyrosine → DOPA → dopaquinone → melanin

• A?gene called?TYR?located on chromosome 11 codes for the enzyme tyrosinase
• There is a?recessive allele?for the gene?TYR?that causes a lack of enzyme tyrosinase or the presence of inactive tyrosinase
• Without the tyrosinase enzyme tyrosine can not be converted into melanin

HBB?gene & sickle cell anaemia

• Sickle cell anaemia?is a condition that causes individuals to have frequent infections, episodes of pain and anaemia
• Humans with sickle cell anaemia have?abnormal haemoglobin?in their red blood cells
• β-globin is a polypeptide found in haemoglobin that is coded for by the?gene?HBB?which is found on chromosome 11
• There is an?abnormal allele?for the gene?HBB?which produces a slightly different amino acid sequence to the normal allele
  • The change of a single base in the DNA of the abnormal allele results in an amino acid substitution (the base sequence CTC is replaced by CAC)
  • This?change in amino acid sequence?(the amino acid Glu is replaced with Val) results in an?abnormal β-globin?polypeptide
• The abnormal β-globin in haemoglobin affects the structure and?shape of the red blood cells
  • They are pulled into a half moon shape
  • They are?unable to transport oxygen?around the body
  • They stick to each other and clump together blocking capillaries
• A homozygous individual that has two abnormal alleles for the?HBB?gene produces only sickle cell haemoglobin
  • They have sickle cell anaemia and suffer from the associated symptoms
• A heterozygous individual that has one normal allele and one abnormal allele for the?HBB?gene will produce some normal haemoglobin and some sickle cell haemoglobin
  • They are a carrier of the allele
  • They may have no symptoms

Normal red blood cells and Sickle cell blood cells - The sickle cells cause a blockage in the capillary and restrict blood flow

F8?gene & haemophilia

• Factor VIII?is a coagulating agent that plays an essential role in blood clotting
• The gene?F8?codes for the Factor VIII protein
• There are?abnormal alleles?of the?F8?gene that result in:
  • Production of abnormal forms of factor VIII
  • Less production of normal factor VIII
  • No production of factor VIII
• A?lack of normal factor VIII prevents normal blood clotting?and causes the condition?haemophilia
• The?F8?gene is located on the X chromosome
  • This means?F8?is a?sex-linked gene
  • Haemophilia is a sex-linked condition
  • If males have an abnormal allele they will have the condition as they have only one copy of the gene
  • Females can be heterozygous for the?F8?gene and not suffer from the condition but act as a carrier

HTT?gene & Huntington's disease

• Huntington’s disease is a genetic condition that develops as a person ages
• Usually a person with the disease will not show symptoms until they are 30 years old +
• An individual with the condition experiences?neurological degeneration; they lose their ability to walk, talk and think
• The disease is ultimately fatal
• It has been found that individuals with Huntington's disease have?abnormal alleles of the?HTT?gene
  • The?HTT?gene codes for the protein?huntingtin?which is involved in neuronal development
  • People that have a large number (>40) of?repeated CAG triplets?present in the nucleotide sequence of their?HTT?gene suffer from the disease
• The?abnormal allele is dominant?over the normal allele
  • If an individual has one abnormal allele present they will suffer from the disease

Gene, protein & phenotype summary table