Use genetic diagrams, including Punnett squares, to predict the results of monohybrid crosses and calculate phenotypic ratios, limited to 1:1 and 3:1 ratios

Sourav PanNovember 4, 2024

Answered

Genetic Diagrams and Punnett Squares for Monohybrid Crosses

Monohybrid crosses are used to study the inheritance of a single trait controlled by one gene with two alleles. Punnett squares are a useful tool for predicting the genotypic and phenotypic ratios of offspring resulting from these crosses. Below, we will explore how to set up Punnett squares for monohybrid crosses and calculate the resulting phenotypic ratios, specifically focusing on 1:1 and 3:1 ratios.

Key Concepts

• Alleles: Different forms of a gene. For example, let’s use:
  • T = dominant allele (tall)
  • t = recessive allele (short)
• Genotypes:
  • Homozygous dominant: TT
  • Heterozygous: Tt
  • Homozygous recessive: tt

Example 1: Monohybrid Cross (1:1 Ratio)

Cross Between Heterozygous and Homozygous Recessive

Parental Genotypes:

• Parent 1 (heterozygous): Tt
• Parent 2 (homozygous recessive): tt

Punnett Square Setup:

Offspring Genotypes:

• Tt (Tall): 2
• tt (Short): 2

Genotypic Ratio:

• 2 Tt : 2 tt or simplified to 1:1

Phenotypic Ratio:

• Tall (Tt) : Short (tt) = 2 : 2 or simplified to 1:1

Example 2: Monohybrid Cross (3:1 Ratio)

Cross Between Two Heterozygous Parents

Parental Genotypes:

• Parent 1 (heterozygous): Tt
• Parent 2 (heterozygous): Tt

Punnett Square Setup:

Offspring Genotypes:

• TT (Tall): 1
• Tt (Tall): 2
• tt (Short): 1

Genotypic Ratio:

• 1 TT : 2 Tt : 1 tt

Phenotypic Ratio:

• Tall (TT + Tt) : Short (tt) = (1 + 2) : 1 = 3:1

Summary of Results

1. Cross between Tt and tt:
   • Phenotypic Ratio: Tall : Short = 1:1
2. Cross between Tt and Tt:
   • Phenotypic Ratio: Tall : Short = 3:1