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

To predict the results of monohybrid crosses and calculate phenotypic ratios, we can utilize genetic diagrams, specifically Punnett squares. Here, we will focus on two common phenotypic ratios: 1:1 and 3:1.

Monohybrid Cross Overview

A monohybrid cross involves the breeding of two organisms that differ in a single trait controlled by one gene. The alleles for this gene can be dominant or recessive.

1. 1:1 Phenotypic Ratio

Scenario: This ratio occurs when a heterozygous individual is crossed with a homozygous recessive individual.Example:

• Let’s consider a trait where “T” (tall) is dominant over “t” (short).
• A heterozygous tall plant (Tt) is crossed with a homozygous short plant (tt).

Punnett Square:

Results:

• Offspring Genotypes: 2 Tt (tall), 2 tt (short)
• Phenotypic Ratio: 2 tall : 2 short or simplified to 1:1.

2. 3:1 Phenotypic Ratio

Scenario: This ratio occurs when two heterozygous individuals are crossed.Example:

• Using the same trait where “T” is dominant over “t”, we cross two heterozygous tall plants (Tt x Tt).

Punnett Square:

Results:

• Offspring Genotypes: 1 TT (homozygous dominant), 2 Tt (heterozygous), 1 tt (homozygous recessive)
• Phenotypes: 3 tall (TT and Tt) : 1 short (tt)
• Phenotypic Ratio: 3:1.