CIE A Level Biology (9700) exams from 2022

Revision Notes

16.2.6 Genes, Proteins & Phenotype

Genes, Proteins & Phenotype

  • 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

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 CTT is replaced by CAT)
    • 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

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

Gene, Protein and Phenotype Summary Table, downloadable AS & A Level Biology revision notes

Exam 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.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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