Genetics Basics and Punnett Squares

Overview

This lecture covers the basics of alleles, genotypes, phenotypes, and how to solve genetics problems using Punnett squares, including both monohybrid and dihybrid crosses.

Alleles, Genotypes, and Phenotypes

• Alleles are different versions of a gene; dominant (e.g., B) or recessive (e.g., b).
• Homozygous dominant: two dominant alleles (e.g., BB).
• Heterozygous: one dominant and one recessive allele (e.g., Bb).
• Homozygous recessive: two recessive alleles (e.g., bb).
• Genotype refers to the genetic makeup (the allele combination).
• Phenotype refers to the physical trait that results (e.g., brown or blue eyes).
• In heterozygotes, the dominant allele determines the phenotype.

Monohybrid Crosses and Probability

• Punnett squares predict offspring genotypes and phenotypes for one trait.
• Example 1: Homozygous blue-eyed wolf (bb) × heterozygous brown-eyed wolf (Bb)
  • Offspring genotypes: Bb (brown eyes) and bb (blue eyes).
  • Probability of blue-eyed offspring: 50% (1:1 ratio).
  • Phenotype ratio: 1 brown : 1 blue.
  • Genotype ratio: 1 Bb : 1 bb.
• Example 2: Two heterozygous brown-eyed cats (Bb × Bb)
  • Probability offspring is brown-eyed: 75%.
  • Probability offspring is homozygous (BB or bb): 50%.
  • Phenotype ratio: 3 brown : 1 blue.
  • Genotype ratio: 1 BB : 2 Bb : 1 bb.

Incomplete Dominance Example

• Incomplete dominance: heterozygote displays a blend (e.g., pink from red [RR] and white [WW]).
• Pink flower × pink flower (RW × RW):
  • Red (RR) = 25%, Pink (RW) = 50%, White (WW) = 25%.
• Red flower (RR) × pink flower (RW):
  • 50% Red (RR), 50% Pink (RW).

Dihybrid Crosses

• Dihybrid cross analyzes inheritance of two different traits simultaneously.
• Example: Bear with black fur (Ff) and blue eyes (bb) × bear with white fur (ff) and brown eyes (Bb).
• Use the FOIL method to determine gametes for a 4x4 Punnett square (16 outcomes).
• Probability baby has black fur and brown eyes: 25%.
• Probability baby has white fur and blue eyes: 25%.
• Probability baby is homozygous dominant for at least one trait: 0%.
• Probability baby is heterozygous for both traits: 25%.
• Genotype and phenotype ratios: 1:1:1:1.

Key Terms & Definitions

• Allele — Variant form of a gene (dominant or recessive).
• Genotype — The allele combination for a specific gene.
• Phenotype — Observable physical trait determined by genotype.
• Homozygous — Two identical alleles for a trait (BB or bb).
• Heterozygous — Two different alleles for a trait (Bb).
• Monohybrid Cross — Genetic cross focusing on one trait.
• Dihybrid Cross — Genetic cross focusing on two traits.
• Incomplete Dominance — Heterozygotes show a blend of traits.

Action Items / Next Steps

• Practice drawing Punnett squares for both monohybrid and dihybrid crosses.
• Review definitions of genotype, phenotype, homozygous, and heterozygous.
• Complete any assigned genetics problems or textbook readings on Punnett squares.