Monohybrid Inheritance - Traits Passed Through Generations

 

What is Monohybrid Inheritance?

Monohybrid inheritance focuses on the inheritance of a single characteristic controlled by one gene with two alleles. It helps us predict how traits (like eye color or flower height) pass from one generation to the next.

 

Key Terms to Remember

Term	Definition
Inheritance	The passing of genetic information from parents to offspring.
Gene	A section of DNA that codes for a specific trait (e.g., flower color).
Allele	A version of a gene (e.g., blue eyes or brown eyes).
Genotype	The genetic makeup of an organism (e.g., $\text{BB}, \text{Bb}, \text{bb}$ ).
Phenotype	The observable traits of an organism (e.g., tall plant or short plant).
Homozygous	Having two identical alleles for a gene (e.g., $\text{BB}$ or $\text{bb}$ ).
Heterozygous	Having two different alleles for a gene (e.g., $\text{Bb}$ ).
Dominant Allele	An allele that always shows in the phenotype if present (e.g., $\text{B}$).
Recessive Allele	An allele that only shows in the phenotype if both alleles are recessive (e.g., $\text{b}$).

 

How Does Monohybrid Inheritance Work?

Monohybrid inheritance involves one gene with two alleles. Each parent gives one allele to their offspring.

• If one allele is dominant, it will mask the recessive allele in the phenotype.
• If two recessive alleles are present, the recessive trait will show.
• When referring to alleles we usually show the dominant allele with a capital letter and the recessive is shown as a lower case letter
• If two parents have identical alleles for a specific gene/characteristic (identically homozyous) they will produce an offspring with the same phenotype and genotype of the parents
• This is referred to as pure breeding as the offspring with definitely have a specific characteristic 

 

Genetic Diagrams

A Punnett square is used to predict the possible genotypes and phenotypes of offspring.

Example:

Trait: Tall (T) is dominant, and short (t) is recessive.
Parents: Heterozygous ($\text{Tt}$) crossed with Homozygous Recessive ($\text{tt}$).

	$\text{T}$	$\text{t}$
$\text{t}$	$\text{Tt}$	$\text{tt}$
$\text{t}$	$\text{Tt}$	$\text{tt}$

Results:

• Genotype: $50 \%$ $\text{Tt}$, $50 \%$  $\text{tt}$.
• Phenotype: $50 \%$ tall, $50 \%$ short.

 

Steps for Solving a Monohybrid Cross

1. Identify the alleles and their dominance:

   • Example: $\text{T}$ (Tall, dominant), $\text{t}$ (Short, recessive).

2. Determine parent genotypes:

   • Example: Parent 1 ($\text{Tt}$), Parent 2 ($\text{tt}$).

3. Create a Punnett square:

   • Write the alleles of each parent on the top and side.

4. Fill in the grid:

   • Combine the alleles to find all possible offspring genotypes.

5. Analyze the results:

   • Count each genotype and phenotype.

 

Phenotypic Ratios

The phenotypic ratio shows the proportion of observable traits in the offspring.

Common Ratios:

• 3:1: Found when both parents are heterozygous ($\text{Tt} \times \text{Tt}$).
• 1:1: Found when one parent is heterozygous and the other is homozygous recessive ($\text{Tt} \times \text{tt}$).