Ever wondered why you have your father’s height but your mother’s eye color? Or why two tall pea plants can sometimes produce a short one? The answer lies in the Monohybrid Cross—the foundation of modern genetics. 

If you are a Class 10 student preparing for your CBSE, ICSE, or State Board exams, this is one of the most important topics in the “Heredity” chapter. In this guide, we’ll break down the concepts, ratios, and diagrams into simple, bite-sized pieces that will help you score full marks. 

 

What is a Monohybrid Cross? (Simple Definition) 

A monohybrid cross is a genetic experiment that focuses on the inheritance of one single trait at a time. For example, if we only look at the height of a plant (Tall vs. Dwarf) while ignoring its seed color or shape, we are performing a monohybrid cross. 

Gregor Mendel, the “Father of Genetics,” used garden pea plants (Pisum sativum) to show how traits are passed from parents to offspring through these crosses. 

Key Term: A “Monohybrid” is an offspring produced by parents that differ in only one trait. 

 

The Step-by-Step Process of a Monohybrid Cross 

Mendel’s classic experiment involved crossing a pure-bred Tall plant with a pure-bred Dwarf plant. Here is how it works: 

1. The Parental (P) Generation

He took a homozygous tall plant (TT) and crossed it with a homozygous dwarf plant (tt). 

• Gametes from TT: All will be ‘T’. 

• Gametes from tt: All will be ‘t’. 

2. The First Filial (F1) Generation

When these gametes fuse, all the offspring in the F1 generation are Tt. 

• The Result: Even though they have a ‘t’ gene, all plants are Tall. This is because ‘T’ (Tallness) is the Dominant trait and masks ‘t’ (Dwarfness), which is Recessive. 

3. The Second Filial (F2) Generation

When the F1 plants (Tt) are allowed to self-pollinate, they produce the F2 generation. This is where the magic happens! The dwarf trait, which “disappeared” in F1, suddenly reappears.

Understanding the Genotypic vs. Phenotypic Ratio 

This is the most “exam-sensitive” part of the topic. You must memorize these two ratios for the F2 generation: 

1. Phenotypic Ratio (The “Look”)

This ratio describes what the plants look like on the outside. 

• Ratio: 3 : 1 

• Meaning: 3 Tall plants for every 1 Dwarf plant. 

2. Genotypic Ratio (The “Genes”)

This ratio describes the actual genetic makeup of the plants. 

• Ratio: 1 : 2 : 1 

• Meaning: 1 Pure Tall (TT) : 2 Hybrid Tall (Tt) : 1 Pure Dwarf (tt). 

Why Is This Important for Your Exams? 

In Class 10 Biology, examiners love to ask “Reasoning” questions based on this cross. They often test your understanding of Mendel’s Laws: 

• Law of Dominance: Why was the F1 generation all tall? Because the dominant allele expresses itself. 

• Law of Segregation: How did the dwarf trait reappear? Because the alleles (T and t) separated during gamete formation and came back together in the F2 generation. 

 FAQs: What Students Often Get Mixed Up 

Q: Can a monohybrid cross happen in humans? A: Yes! Simple traits like attached vs. free earlobes or the ability to roll your tongue follow the same monohybrid pattern. 

Q: What is a Test Cross? A: If you have a Tall plant but don’t know if it’s TT or Tt, you cross it with a dwarf plant (tt). If any dwarf offspring appear, your parent plant was Tt. 

Q: Why did Mendel choose pea plants? A: They grow fast, are easy to cross-pollinate, and have very clear “either-or” traits (like Purple or White flowers). 

Q: What is the difference between an allele and a gene? A: A gene is the section of DNA that controls height. An allele is the version of that gene (like ‘T’ for tall or ‘t’ for dwarf). 

 

 

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