What is Mendel's First Law of Dominance?

S6-SA5-0019

Grade Level:

Class 10

AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine

Definition

What is it?

Mendel's First Law of Dominance states that when two different pure-breeding parents with contrasting traits are crossed, only one form of the trait (the dominant one) will appear in the first generation. The other trait (the recessive one) remains hidden.

Simple Example

Quick Example

Imagine you have two types of cricket teams: one always wears a bright red jersey (dominant trait) and the other always wears a dull blue jersey (recessive trait). If you combine players from a pure red team and a pure blue team, all the new players in the first generation will wear only bright red jerseys. The blue jersey trait is still there, but it's not seen.

Worked Example

Step-by-Step

Let's consider pea plant height. Tallness (T) is dominant over dwarfness (t).

Step 1: Identify the pure-breeding parents. A pure tall plant has genotype TT. A pure dwarf plant has genotype tt.

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Step 2: Determine the gametes produced by each parent. The pure tall plant (TT) produces only 'T' gametes. The pure dwarf plant (tt) produces only 't' gametes.

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Step 3: Perform the cross for the first generation (F1). When 'T' gametes combine with 't' gametes, all offspring will have the genotype Tt.

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Step 4: Observe the phenotype of the F1 generation. Since 'T' (tall) is dominant over 't' (dwarf), all plants with the genotype Tt will appear tall.

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Answer: All plants in the first generation (F1) will be tall, even though they carry the dwarf trait.

Why It Matters

Understanding dominance helps scientists in Biotechnology predict how traits are inherited in plants and animals. It's crucial for geneticists in Medicine to understand genetic disorders and for breeders in Agriculture to develop new crop varieties. This knowledge can even inspire careers in genetic counseling or pharmaceutical research.

Common Mistakes

MISTAKE: Thinking the recessive trait disappears forever. | CORRECTION: The recessive trait is only hidden in the first generation (F1) but can reappear in later generations (F2) if the dominant trait is not present.

MISTAKE: Confusing phenotype (what you see) with genotype (the genetic makeup). | CORRECTION: The Law of Dominance describes what trait is *expressed* (phenotype), not necessarily the underlying genetic combination (genotype). A tall plant can be TT or Tt.

MISTAKE: Believing that dominant traits are always more common or 'better'. | CORRECTION: Dominance only describes how traits are expressed when both alleles are present. It doesn't mean the dominant trait is more frequent in a population or more advantageous.

Practice Questions

Try It Yourself

QUESTION: In humans, brown eyes (B) are dominant over blue eyes (b). If a pure brown-eyed person marries a pure blue-eyed person, what color eyes will their children (F1 generation) have? | ANSWER: All children will have brown eyes.

QUESTION: A pure-breeding pea plant with yellow seeds (Y) is crossed with a pure-breeding pea plant with green seeds (y). If yellow is dominant, what are the genotypes of the F1 generation? | ANSWER: All F1 plants will have the genotype Yy.

QUESTION: In a certain type of flower, red color (R) is dominant over white color (r). A pure red flower is crossed with a pure white flower. If two F1 generation flowers are then crossed with each other, what percentage of their offspring (F2 generation) would you expect to be white? | ANSWER: 25% (because the F1 generation will all be Rr, and crossing Rr x Rr gives 1 RR, 2 Rr, 1 rr, so 1 out of 4 is white).

MCQ

Quick Quiz

According to Mendel's Law of Dominance, if you cross a pure tall pea plant (TT) with a pure dwarf pea plant (tt), what will be the phenotype of the F1 generation?

All dwarf plants

All tall plants

Half tall, half dwarf plants

Some tall, some medium-height plants

The Correct Answer Is:

Since tallness (T) is dominant over dwarfness (t), all offspring with the genotype Tt will express the dominant tall trait. The recessive dwarf trait will be hidden.

Real World Connection

In the Real World

This concept helps plant breeders in India develop new crop varieties. For example, ICAR scientists use knowledge of dominance to cross high-yielding rice varieties with disease-resistant ones. They can predict which traits will show up in the first generation to create better crops for our farmers, ensuring food security.

Key Vocabulary

Key Terms

DOMINANT TRAIT: A trait that is expressed when two different alleles are present | RECESSIVE TRAIT: A trait that is hidden when a dominant allele is present | ALLELE: A variant form of a gene | PHENOTYPE: The observable physical properties of an organism | GENOTYPE: The genetic makeup of an organism

What's Next

What to Learn Next

Next, you should learn about Mendel's Law of Segregation. It builds on the Law of Dominance by explaining how these hidden recessive traits reappear in later generations, giving us more insights into inheritance patterns.