Genetics & Plant Breeding - ICAR JRF/SRF 2024

• Herbicide-Tolerant Rice Mutant 'Robin': Developed via gamma-ray induced mutagenesis of cv. Jyothi at Kerala Agricultural University, possessing an altered acetolactate synthase (ALS) target site conferring resistance to the imidazolinone herbicide imazethapyr. (Correct answer: Option 3)

• Statement (I) is True: Single cross hybrids ($A imes B$) combine two completely inbred, homozygous lines, maximizing specific combining ability (SCA) and total genetic homogeneity; hence, they achieve the highest theoretical yield potential and absolute uniformity among all hybrid types.
• Statement (II) is True: Double cross hybrids ($(A imes B) imes (C imes D)$) contain broader genetic heterogeneity across individuals, conferring superior developmental homeostasis, wider climatic and edaphic adaptability, and greater buffering capacity against biotic and abiotic stresses compared to uniform single crosses.
• Conclusion: Both Statement (I) and Statement (II) are true (Option 1).

Correct option: (1) Both Statement (I) and Statement (I) are correct

• Secondary Trisomics in Polyploids: A secondary trisomic is an aneuploid having an extra isochromosome ($2n + 1\text{ isochromosome}$).
• Calculation in Durum Wheat (Triticum durum):
• Durum wheat is an allotetraploid ($2n = 4x = 28$), with haploid chromosome complement $n = 14$.
• Because each of the 14 chromosomes consists of two genetically distinct arms, each arm can generate one unique isochromosome.

• Conclusion: Exactly 28 secondary trisomic lines are possible. (Correct answer: Option 2)