D3.2 Inheritance Expression of phenotypes

D3. Continuity and change: Organisms

D3.2 Inheritance and Expression of Phenotypes

D3.2.3: Genotype

D3.2.4: Phenotype

D3.2.5: Effects of Dominant and Recessive Alleles on Phenotype

Genetics of Tasting (PTC)

• Genes can influence taste perception.

• Example: Tasting phenylthiocarbamide (PTC).

• Bitter taste perception is determined by the gene TAS2R38.

• This gene codes for a taste receptor protein on the tongue.

• Two common alleles: tasting and non-tasting.

• Heterozygous individuals (both alleles) can taste PTC.

• 'Tasting allele' is dominant over the 'non-tasting' allele.

• Recessive allele: The effect is masked by the dominant allele.

• Homozygous dominant individuals can also taste the chemical.

• Only homozygous recessive individuals (two 'non-tasting' alleles) cannot taste PTC.

• Similar to tongue rolling, where only homozygous recessive individuals cannot roll their tongue.

• Tongue rollers: homozygous dominant or heterozygous.

• PTC's structure is similar to toxic compounds in poisonous plants.

• Ability to taste bitter chemicals is important for avoiding toxic compounds.

D3.2.6: Phenotypic Plasticity

• Phenotype is influenced by genotype and environment.

• Phenotypic plasticity: Ability of a genotype to produce different phenotypes in different environments.

• Allows individuals with the same genome to adapt to different conditions.

• Changes can involve behavior, physiology, or morphology.

• Changes are not caused by alterations in the genotype.

• Changes may be reversible.

• Important for survival in changing global climate.

• Example: Seasonal polyphenism of the butterfly Bicyclus anynana.

  • Dry-season adults: longer lifespan, breed at the end of the season, wing patterns resemble dry foliage.

  • Wet-season adults: shorter lifespan, breed many times, wing patterns have distinctive eye spots to prevent predation.

D3.2.7: Phenylketonuria (PKU)

• Recessive genetic disorder.

• Individuals need to be homozygous recessive to express the trait.

• Affects approximately 1 in 24000 live births globally.

• Caused by a mutation in a gene on chromosome 12.

• The gene codes for phenylalanine hydroxylase (PAH).

• PAH converts phenylalanine (Phe) into tyrosine (Tyr).

• One correct allele is sufficient for a healthy individual.

• In homozygous recessive individuals, Phe is not broken down, leading to toxic levels.

• Symptoms: musty odor from skin and urine, fair skin, eczema, seizures, tremors, hyperactivity.

• Babies are tested for PKU at 1–2 days of age via a heel stick.

• A high level of Phe indicates the need for further testing.

• If untreated, brain damage can occur.

• Managed with a life-long low-protein diet and frequent blood tests to monitor Phe:Tyr ratio.

• Creativity, activity, service: Rare Disease Day is a global awareness day.

• Possible activities for Rare Disease Day include:

  • Infomercial to share at your school.

  • Fun quiz to share at your local supermarket.

  • Invite members of your community and be willing to speak

  • Raise funds to go towards a local charity or foundation.

Activity

• Jigsaw activity in groups of four to research recessive disorders.

• Recessive disorders: PKU, sickle-cell disease, cystic fibrosis, Tay-Sachs disease.

• Each group researches a different disorder.

• Students regroup to share their knowledge.

Theory of Knowledge

• Ethical considerations related to genetic testing and potential discrimination.

Equations/Formulas

Phe + O2 + H^+ \rightarrow Tyr + H2O