Genetics of PTC Tasting

Overview

This lecture explains how the ability to taste PTC is inherited genetically, demonstrating dominant and recessive traits, alleles, and genotypes.

PTC Tasting and Genetics

• PTC (phenylthiocarbamide) is a chemical that tastes bitter to some people, but tasteless to others.
• The ability or inability to taste PTC is a genetic trait, often used in genetics education.
• Genes are segments of DNA that code for traits such as PTC tasting.
• Most traits, like eye color, involve interactions of multiple genes, but PTC tasting is mainly tied to a single gene.
• The gene for PTC tasting codes for taste receptors on the tongue.

Chromosomes, Alleles, and Inheritance

• Humans have 46 chromosomes (23 pairs), inheriting one set from each parent.
• Each chromosome pair contains the same gene at the same locus (location) from both parents.
• Alleles are different versions of a gene; you inherit one allele from each parent.
• The combination of both alleles (genotype) determines your trait (phenotype).

Dominant and Recessive Traits

• Dominant alleles are represented with capital letters (e.g., T) and recessive with lowercase (e.g., t).
• If at least one dominant allele is present (TT or Tt), the dominant trait (tasting PTC) is expressed.
• Only individuals with two recessive alleles (tt) cannot taste PTC.

Genotypes and Phenotypes

• Genotype TT or Tt results in the phenotype of being able to taste PTC.
• Genotype tt results in the phenotype of not being able to taste PTC.
• Parents who can taste PTC but have a child who cannot must both have the Tt genotype.

Dominant Traits in Populations

• Dominant traits are not always the most common in a population; the dominant allele can be rare.
• Polydactyly (extra fingers) is an example of a rare dominant trait.

Key Terms & Definitions

• Gene — a segment of DNA that codes for a specific trait.
• Allele — a variant form of a gene.
• Genotype — the genetic makeup (combination of alleles) for a trait.
• Phenotype — the observable trait or characteristic.
• Dominant allele — an allele that is expressed if present (capital letter).
• Recessive allele — an allele expressed only if both alleles are recessive (lowercase letter).
• Chromosome — a structure made of DNA and protein, containing genes.
• Locus — the specific location of a gene on a chromosome.

Action Items / Next Steps

• Review Punnett squares for understanding genotype and phenotype probabilities.
• Stay curious and consider exploring more about Mendelian and non-Mendelian genetics.