Question

What would be the minimum possibility of a dominant trait being expressed in an offspring after a monohybrid test cross ?

• A. 25%
• B. 50%
• C. 75%
• D. 100%

Hint:

A test cross with a heterozygous parent gives 1 dominant : 1 recessive, which is the lowest possible dominant share.

Answer 1

The Correct Option is B

Step 1: Understand the test cross:
A test cross crosses an individual showing the dominant trait with a homozygous recessive parent.
Its aim is to find whether the dominant individual is homozygous or heterozygous.
The recessive parent is always \( tt \), so it gives only the recessive allele \( t \) in every gamete.

Step 2: Case 1: dominant parent is heterozygous:
Cross \( Tt \times tt \). The gametes are \( T \) and \( t \) from one parent and only \( t \) from the other.
The offspring are \( Tt \) and \( tt \) in equal numbers.
So half the offspring (50%) show the dominant trait and half show the recessive trait.

Step 3: Case 2: dominant parent is homozygous:
Cross \( TT \times tt \). Every offspring is \( Tt \).
So all the offspring (100%) show the dominant trait.

Step 4: Find the minimum:
The two possible results are 50% and 100%. The smaller value is 50%.
So the minimum chance of the dominant trait is 50%.

Step 5: Check the other options:
25% is wrong, because a test cross never gives only one quarter dominant offspring.
75% is the dominant share in a monohybrid \( F_2 \) cross \( Tt \times Tt \), not in a test cross.
100% is the maximum value (homozygous dominant parent), not the minimum.

Final Answer:
The minimum expression of the dominant trait in a monohybrid test cross is 50%, which is option (B).