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

Sourav PanNovember 15, 2024

Answered

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.