Question

T1 generation plants for Cry 1Ac gene showed 3:1 segregation for the selected trait and gene. When the 3 plants with the gene were selfed which one of the following statements explain the results

• A. Two of three plants produced all the progeny plants with the gene
• B. All three plants produced all the progeny plants with the gene
• C. Only one out of 3 plants produced all the progeny plants with the gene
• D. All the three plants produced progeny plants which showed segregation for the gene

Hint:

Among the offspring displaying a dominant phenotype in a monohybrid cross, the ratio of homozygotes to heterozygotes is always 1:2.
Only the homozygote (\(AA\), or 1 out of 3) will breed true to produce 100% dominant progeny.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
In genetic engineering, primary transformants (\(\text{T}_0\) generation) are typically hemizygous for the inserted transgene.
When selfed, a single-locus transgene behaves like a single Mendelian locus, yielding a 3:1 phenotypic segregation ratio in the \(\text{T}_1\) generation.

Step 2: Detailed Explanation:
Let the functional transgene be represented by the allele \(A\), and the absence of the transgene by the allele \(a\).
The transgenic \(\text{T}_0\) parent is hemizygous (or heterozygous), represented as \(Aa\).
When selfed (\(Aa \times Aa\)), the offspring in the \(\text{T}_1\) generation segregate in a classic Mendelian genotypic ratio of: \[ 1 \text{ AA } : 2 \text{ Aa } : 1 \text{ aa} \] The plants carrying the transgene (the "3" in the 3:1 phenotypic ratio) consist of the genotypes:
- \(1/3\) homozygous dominant (\(AA\))
- \(2/3\) heterozygous (\(Aa\))
When these three transgenic \(\text{T}_1\) plants are self-pollinated:
1. The single homozygous (\(AA\)) plant (1 out of the 3 plants with the gene) will breed true, producing 100% progeny that carry the transgene.
2. The two heterozygous (\(Aa\)) plants (2 out of the 3 plants with the gene) will segregate again, producing progeny that show a 3:1 segregation ratio (with some non-transgenic \(aa\) offspring).
Therefore, only one out of the three plants with the gene will produce progeny that all carry the transgene.

Step 3: Final Answer:
This corresponds to option (C).