16.2.7 Gene, Proteins & Phenotype

• A gene can affect aphenotypeof 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 theenvironment

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

TYRgene & albinism

• Humans with albinismlack the pigment melaninin 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 theenzyme tyrosinase
2. DOPA is converted to dopaquinone again by theenzyme tyrosinase
3. Dopaquinone is converted to melanin

tyrosine → DOPA → dopaquinone → melanin

• Agene calledTYRlocated on chromosome 11 codes for the enzyme tyrosinase
• There is arecessive allelefor the geneTYRthat causes a lack of enzyme tyrosinase or the presence of inactive tyrosinase
• Without the tyrosinase enzyme tyrosine can not be converted into melanin

HBBgene & sickle cell anaemia

• Sickle cell anaemiais a condition that causes individuals to have frequent infections, episodes of pain and anaemia
• Humans with sickle cell anaemia haveabnormal haemoglobinin their red blood cells
• β-globin is a polypeptide found in haemoglobin that is coded for by thegeneHBBwhich is found on chromosome 11
• There is anabnormal allelefor the geneHBBwhich produces a slightly different amino acid sequence to the normal allele
  • The change of asingle basein the DNA of the abnormal allele results in anamino acid substitution
    • The DNA base sequence GAG is replaced by GTG
      • This means that CTC is replaced by CAC on the complementary DNAtemplate strand, meaning that GAG is replaced by GUG in the resulting mRNA
  • Thischange in amino acid sequenceresults in anabnormal β-globinpolypeptide
    • The amino acid Glu is replaced with Val
• The abnormal β-globin in haemoglobin affects the structure andshape of the red blood cells
  • They are pulled into a half moon shape
  • They areunable to transport oxygenaround the body
  • They stick to each other and clump together blocking capillaries
• A homozygous individual that has two abnormal alleles for theHBBgene 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 theHBBgene 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

F8gene & haemophilia

• Factor VIIIis a coagulating agent that plays an essential role in blood clotting
• The geneF8codes for the Factor VIII protein
• There areabnormal allelesof theF8gene that result in:
  • Production of abnormal forms of factor VIII
  • Less production of normal factor VIII
  • No production of factor VIII

• Alack of normal factor VIII prevents normal blood clottingand causes the conditionhaemophilia
• TheF8gene is located on the X chromosome
  • This meansF8is asex-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 theF8gene and not suffer from the condition but act as a carrier

HTTgene & 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 experiencesneurological 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 haveabnormal alleles of theHTTgene
  • TheHTTgene codes for the proteinhuntingtinwhich is involved in neuronal development
  • People that have a large number (>40) ofrepeated CAG tripletspresent in the nucleotide sequence of theirHTTgene suffer from the disease

• Theabnormal allele is dominantover the normal allele
  • If an individual has one abnormal allele present they will suffer from the disease

Gene, protein & phenotype summary table