16.7 Genes, proteins and phenotype

Albinism

Gene: TYR

TYR found on long arm of chromosome 11

Faulty recessive allele for tyrosinase gene

1 in 17000 are homozygous recessive and therefore have it

Tyrosinase converts the amino acid tyrosine to DOPA and then dopaquinone, which is converted to melanin

If tyrosinase is absent or inactive, tyrosine is not converted into DOPA and then dopaquinone, so no melanin is formed

Sickle cell anaemia

Gene: HBB

HBB found on short arm of chromosome 11

Single base substitution resulting in faulty codominant allele for beta-globin gene (part of haemoglobin molecule)

When not combined with oxygen, the haemoglobin is much less soluble and sticks together inside the RBC

RBCs form long internal fibres, distorting their shape into a sickle shape which gets stuck in capillaries

Sickle-shaped cells also lose the ability to transport oxygen, causing anaemia (lack of oxygen in cells)

Haemophilia

Gene: F8

F8 found on non-homologous region of X chromosome, so is sex-linked

Faulty recessive allele for coagulation factor VIII

People with haemophilia bleed excessively from even small injuries, as their blood does not clot

Coagulation factor VIII is synthesised in liver cells and secreted into blood plasma

Huntington's disease

Gene: HTT

HTT found on chromosome 4

Faulty dominant allele for huntingtin

If number of CAG triplet repeats (or 'stutters') is over 40, then the person develops Huntington's

Symptoms only develop after around 30 years of age, so people may have children without knowing they have Huntington's and then pass on the disease

Plant dwarfism

Gene/alleles: Le (dominant), le (recessive)

le is a faulty allele for the gene producing the last enzyme in the gibberellin synthesis pathway

Gibberellin stimulates cell elongation and division, so plants homozygous with le which cannot produce gibberellin remain short