Monohybrid Cross – Inheritance of One Gene

Monohybrid cross occurs when two organisms with contrasting variations at a particular genetic locus are crossed. The traits being studied are influenced by multiple variations at a single gene location. The resulting offspring, known as the F1 generation, will all be heterozygous and express the dominant trait, masking the recessive trait. The monohybrid cross demonstrates Mendel’s law of dominance.

Monohybrid Definition

A monohybrid cross involves the inheritance of a single gene or trait by crossing individual with differing alleles for that gene, demonstrating Mendel’s Law of Dominance.

What is Monohybrid Cross?

Monohybrid cross refers to the genetic inheritance that involves a single characteristic or trait like plant height, pod shape, etc. Mendel crosses two homozygous traits and forms a heterozygous trait which is known as a monohybrid cross or the inheritance of a single gene. Each gene has two versions that control a character that is called an allele. Through Punnett square, it can be easily shown.

After the monohybrid cross the heterozygous progeny is obtained which is called Fillial1 (F1) generation. And again cross is done between F1 progeny or heterozygous trait and the resulting progeny is called Fillial2 (F2) generation. Three of the resulting alleles exhibit the dominant trait, while one show the recessive trait, giving rise to a phenotypic ratio of 3:1. The ratio of the dominant allele, a heterozygous and recessive allele is 1:2:1 which is called as genotypic ratio.

How to carry out a Monohybrid cross?

The monohybrid cross is carried out by carrying out following steps:

• Select two parent organisms with differing traits controlled by a single gene.
• Ensure one parent is homozygous dominant, and the other is homozygous recessive for that gene.
• Cross the parents, allowing for controlled pollination or mating.
• Observe the offspring (F1 generation), which will all be heterozygous for that gene.
• Allow the F1 generation to self-pollinate.
• Observe the F2 generation to determine the phenotypic ratios.
• Analyze the data and compare the observed ratios to Mendel’s principles of inheritance.

Example of Monohybrid Cross

Example of a monohybrid cross using pea plants with purple (P) and white (p) flower color as the trait is as follows;

Parental Generation (P):

• One parent has purple flowers (PP, homozygous dominant).
• The other parent has white flowers (pp, homozygous recessive).

F1 Generation:

• Cross the two parents.
• All the offspring in the F1 generation will have purple flowers (Pp, heterozygous).

F2 Generation:

• Allow the F1 generation to self-pollinate or cross-pollinate.
• Observe the offspring in the F2 generation.
• 3:1 phenotypic ratio
• Three-fourths (75%) will have purple flowers (PP or Pp).
• One-fourth (25%) will have white flowers (pp).

This monohybrid cross demonstrates Mendel’s law of dominance, where the dominant trait (purple flowers) masks the expression of the recessive trait (white flowers) in the F1 generation, but the recessive trait reappears in the F2 generation in a 3:1 ratio.

Huntington’s Disease

Huntington’s is a genetic disorder that affect the brain and nervous system. That means the person having a dominant gene of this will be affected by Huntington’s disease. Symptoms include involuntary movements, mood swings, and cognitive decline. It is an autosomal dominant disorder, meaning a child of an affected parent has a 50% chance of inheriting the gene mutation. There is currently no cure for Huntington’s disease.

FAQs on Monohybrid Cross

1. What is a Monohybrid cross and a Dihybrid cross?

Answer: 

A monohybrid cross studies the inheritance of a single gene or trait, while a dihybrid cross examines the inheritance of two different genes or traits simultaneously.

2. What is Law of Mendel in Monohybrid cross?

Answer:

Mendel’s Law of Dominance states that in a heterozygous individual (having one dominant and one recessive allele), the dominant allele will be expressed, and the recessive allele remains masked.

3. What is the Difference between Monohybrid and Dihybrid cross Phenotypic Ratio?

Answer:

The phenotypic ratio in a monohybrid cross is 3:1 (one gene), while in a dihybrid cross, it’s 9:3:3:1 (two genes).

4. What is meant by Dihybrid cross?

Answer:

A dihybrid cross is a genetic cross that examines the inheritance of two different genes or traits simultaneously by crossing individuals with differing alleles for both genes.

5. What are the three Laws of Mendel?

Answer:

Mendel proposed 3 laws after experiments on pea plants with a variety of traits:

• Law of Dominance
• Law of Segregation
• Law of Independent assortment