Overview
This lecture explains how to use Punnett squares to solve genetics problems, including monohybrid and dihybrid crosses, and how to interpret genotype and phenotype ratios.
Genetics Basics
- Alleles are different forms of a gene; dominant alleles are represented by capital letters, recessive alleles by lowercase.
- Homozygous dominant means having two dominant alleles (e.g., BB); heterozygous means having one dominant and one recessive allele (e.g., Bb); homozygous recessive means two recessive alleles (e.g., bb).
- Genotype refers to the genetic makeup (allele combination) of a trait; phenotype is the observable physical characteristic.
Monohybrid Crosses & Punnett Squares
- A monohybrid cross examines inheritance of a single trait using a 2x2 Punnett square.
- Example: A homozygous blue-eyed wolf (bb) crossed with a heterozygous brown-eyed wolf (Bb) produces offspring with a 50% chance of blue eyes and a 1:1 genotype and phenotype ratio.
- Two heterozygous brown-eyed cats (Bb x Bb) have a 75% probability of producing a brown-eyed kitten, with a 3:1 phenotype ratio (brown:blue) and a 1:2:1 genotype ratio (BB:Bb:bb).
Incomplete Dominance
- Incomplete dominance occurs when heterozygotes show a blend of traits (e.g., RW genotype gives pink flowers from red (RR) and white (WW) parents).
- Crossing two pink flowers (RW x RW) yields a 25% chance for red, 50% for pink, and 25% for white flowers.
- Crossing red (RR) and pink (RW) flowers yields a 50% chance for pink and 50% for red offspring.
Dihybrid Crosses
- Dihybrid crosses track two traits at once using a 4x4 Punnett square (16 outcomes).
- Example: A black fur, blue-eyed bear (Ffbb) crossed with a white fur, brown-eyed bear (ffBb) results in:
- 25% probability for black fur & brown eyes (F?B?)
- 25% for white fur & blue eyes (ffbb)
- 25% for heterozygous for both traits (FfBb)
- 0% chance for homozygous dominant for any trait
- Genotype and phenotype ratios are 1:1:1:1 for all trait combinations.
Key Terms & Definitions
- Allele — Different versions of a gene, dominant or recessive.
- Homozygous dominant — Two identical dominant alleles (e.g., BB).
- Heterozygous — One dominant and one recessive allele (e.g., Bb).
- Homozygous recessive — Two identical recessive alleles (e.g., bb).
- Genotype — The genetic makeup (allele combination) for a trait.
- Phenotype — The observable physical trait.
- Monohybrid cross — Cross between individuals for a single trait.
- Dihybrid cross — Cross considering two traits.
- Incomplete dominance — Heterozygous phenotype is a blend between two traits.
Action Items / Next Steps
- Practice drawing and completing Punnett squares for both monohybrid and dihybrid crosses.
- Memorize key definitions and understand how to determine genotype and phenotype ratios.
- Complete any assigned genetics problems using Punnett squares.