Question

How many chromosomes must be seen in bread wheat karyogram after staining?

• A. 7
• B. 21
• C. 42
• D. 28

Hint:

Wheat is a classic example of polyploidy in exams.
Einkorn wheat (Diploid) = 14 chromosomes.
Durum/Pasta wheat (Tetraploid) = 28 chromosomes.
Bread wheat (Hexaploid) = 42 chromosomes.
The base number \( x = 7 \) is constant across these types.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the total number of chromosomes present in a stained karyogram (visual representation of the full set of chromosomes) of bread wheat.
This requires knowledge of the ploidy level and basic chromosome number of the species Triticum aestivum.
Key Formula or Approach:
The chromosome number in a polyploid species is expressed as:
\[ 2n = nx \]
Where:
\( 2n \) = Somatic (diploid) chromosome number.
\( n \) = Gametic chromosome number.
\( x \) = Basic chromosome number (monoploid set).

Step 2: Detailed Explanation:

• Bread Wheat Genetics:
Bread wheat (Triticum aestivum) is an allohexaploid species.
This means it contains six sets of chromosomes derived from three different ancestral diploid species.
The three genomes are designated as A, B, and D.

• Ploidy Level:
For bread wheat, the basic chromosome number (\( x \)) is 7.
Since it is a hexaploid (\( 6x \)), the total somatic chromosome number (\( 2n \)) is calculated as:
\[ 2n = 6 \times 7 = 42 \]
Each of the three genomes (A, B, and D) contributes 14 chromosomes (\( 7 \text{ pairs each} \)).

• Karyogram Representation:
A karyogram is prepared from cells during metaphase of mitosis, where chromosomes are most condensed.
After staining (e.g., Giemsa staining), all somatic chromosomes are visible.
In a healthy bread wheat cell, exactly 42 individual chromosomes will be observed.

Step 3: Final Answer:
The somatic chromosome number of bread wheat is 42.