Question

The T1 Plants showed 3:1 segregation for the selected trait and gene. When the 3 T1 plants with the target 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:

To identify homozygous lines in transgenic breeding, researchers select multiple plants from the segregating generation, self-pollinate them individually, and screen their offspring. The lines that show no segregation (100% survival on selection media) are confirmed homozygotes.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
In plant genetic engineering, transgenic plants of the first transformed generation (T${}_0$) are typically hemizygous for the inserted transgene.
Upon self-pollination, the transgene segregates in the next generation (T${}_1$) according to Mendelian genetics.

Step 2: Detailed Explanation:
Let us analyze the Mendelian segregation pattern:
1. The T${}_0$ plant is hemizygous ($T/-$), containing a single copy of the dominant transgene.
2. Self-pollination of this T${}_0$ plant ($T/- \times T/-$) yields a T${}_1$ generation displaying a classic Mendelian monohybrid segregation ratio:
- $1/4$ homozygous dominant ($T/T$) $\rightarrow$ Contains the gene (transgenic)
- $2/4$ heterozygous ($T/-$) $\rightarrow$ Contains the gene (transgenic)
- $1/4$ homozygous recessive ($-/-$) $\rightarrow$ Lacks the gene (non-transgenic)
This produces a phenotypic segregation ratio of 3 (transgenic) : 1 (non-transgenic).
3. Among the T${}_1$ plants that contain the target gene (the $3$ parts representing the $T/T$ and $T/-$ genotypes):
- One-third ($1/3$) are homozygous dominant ($T/T$).
- Two-thirds ($2/3$) are heterozygous ($T/-$).
4. When these three plants containing the gene are selected and selfed:
- The single homozygous dominant plant ($T/T$, representing 1 out of the 3 plants) will breed true. It will produce $100\%$ transgenic progeny containing the gene ($T/T \times T/T \rightarrow$ all $T/T$).
- The two heterozygous plants ($T/-$, representing 2 out of the 3 plants) will segregate again in a 3:1 ratio, meaning some of their progeny will lack the gene.
Therefore, only one out of the three selected T${}_1$ plants will produce progeny that all contain the target gene.

Step 3: Final Answer:
The result is explained by the statement that only one out of 3 plants produced all the progeny plants with the gene, corresponding to option (C).