Plant Genetics and Breeding

A locus is the specific fixed position on a chromosome where a particular gene is located. Chiasma refers to crossover points, while cistron is a functional unit of gene action.

In tissue culture, auxin promotes root formation while cytokinin stimulates shoot development. The balance between these two hormones determines whether roots or shoots will form.

Fraenkel-Conrat and Singer separately conducted experiments with Tobacco Mosaic Virus (TMV) that demonstrated RNA carries genetic information by reconstituting infectious virus from its RNA and protein components.

Hardening is the acclimatization process where tissue-cultured plantlets are gradually exposed to normal environmental conditions to develop tolerance before transfer to soil.

Germinal mutations occur in reproductive cells and are heritable, which can alter expected Mendelian ratios like the 9:3:3:1 dihybrid ratio in the F2 generation. Somatic mutations only affect body cells and are not inherited.

In DNA, adenine pairs with thymine through 2 hydrogen bonds, while guanine pairs with cytosine through 3 hydrogen bonds. This difference contributes to DNA stability and the melting temperature of the double helix.

A valuable tool for plant breeding and improvement of crop plants is hybridisation.

Mirabilis jalapa is commonly called the 4 o'clock plant because its flowers typically open in the late afternoon around 4 PM. It is also known for its variegated flower colors, which helped demonstrate cytoplasmic inheritance.

DNA is a polymer consisting of many nucleotide monomers linked together in a chain. The prefix 'poly-' means many, referring to the long chain structure of DNA molecules where individual nucleotides are connected by phosphodiester bonds.

Ribosomes are cellular organelles composed of ribosomal RNA (rRNA) and proteins. The rRNA provides the structural framework and catalytic activity, while proteins help in assembly and function. This combination makes ribosomes ribonucleoprotein complexes essential for protein synthesis.

Spontaneous mutations occur naturally without any external artificial intervention. They arise from random errors during DNA replication, spontaneous chemical changes in DNA, or natural background radiation. This distinguishes them from induced mutations caused by mutagens like X-rays or chemicals.

The 4'O clock plant (Mirabilis jalapa) is the classic example of incomplete dominance where heterozygotes show an intermediate phenotype between red and white flower colors. When red and white-flowered plants are crossed, the offspring have pink flowers, demonstrating that neither allele is completely dominant.

Mirabilis jalapa (4 O'clock plant) is the standard example of incomplete dominance in genetics. When plants with red (RR) and white (rr) flowers are crossed, the heterozygous offspring (Rr) produce pink flowers - an intermediate phenotype. This clearly demonstrates incomplete dominance where neither allele is completely dominant.

tRNA has a characteristic clover leaf secondary structure formed by internal hydrogen bonding between complementary bases. This structure contains the anticodon loop, amino acid attachment site, and other loops essential for its function in translation. The 3D structure is actually L-shaped, but clover leaf is the correct description of the 2D structure.

Messenger RNA (mRNA) typically constitutes 3-5% of total cellular RNA in most cells. It has the shortest half-life of all RNA types because it's continuously synthesized and degraded during protein synthesis. rRNA makes up about 80-85% while tRNA constitutes about 10-15% of total cellular RNA.

A large seeded mutant variety of Bengal gram is Cicer arietinum.

Cupid is a dwarf variety of sweet pea (Lathyrus odoratus) commonly used in genetic studies. Sweet peas have been important in genetics research, particularly in studying flower color inheritance patterns. The Cupid variety is specifically known for its dwarf growth habit compared to standard sweet pea varieties.

Tissue culture, specifically meristem culture, is the technique used to produce virus-free plants. The meristem (apical meristem) is typically free of viruses, and when cultured in vitro, it develops into a virus-free plant. This method has been successfully used for potatoes, tulips, sugarcane, orchids, and many other crops to eliminate viral infections.

X-rays, UV rays, mustard gas, nitrous acid, and formaldehyde are all mutagens - agents that cause mutations in DNA. They can induce various types of DNA damage including base modifications, cross-linking, and strand breaks, which lead to mutations during repair or replication. These are examples of induced mutations (as opposed to spontaneous).

MacLeod and McCarty (with Avery) identified DNA as the transforming substance in their famous 1944 experiment on bacteria. They demonstrated that purified DNA from virulent bacteria could transform non-virulent bacteria, providing strong evidence that DNA was the genetic material. This was a landmark discovery in molecular biology.

Bacillus subtilis is a prokaryotic bacterium commonly used as a host in genetic engineering. It's a Gram-positive, non-pathogenic bacterium that can be easily transformed and has well-characterized genetics. B. subtilis is particularly useful for expressing proteins, studying gene regulation, and as a model organism for prokaryotic research.

The 7:1:1:7 ratio is a classic example of gene linkage discovered by Bateson and Punnett in sweet pea (Lathyrus odoratus). This dihybrid ratio deviates from Mendel's expected 9:3:3:1 ratio because the two genes are linked on the same chromosome and do not assort independently. Options 12:3:1, 1:1:1:1, and 9:3:4 represent other genetic interactions (epistasis, test cross, and modified epistasis respectively) but are not the Bateson-Punnett linkage ratio.

Complementary genes (like in sweet pea flower color) produce a 9:7 phenotypic ratio, while various forms of epistasis produce ratios like 12:3:1, 9:3:4, or 15:1. All these ratios differ from Mendel's standard 9:3:3:1 dihybrid ratio, but the genes involved still assort independently and follow Mendel's second law. The deviation occurs due to gene interactions at the phenotypic level, not because genes fail to assort independently.

Interchange of chromosome segments between non-homologous chromosomes is called translocation. Translocations can create reproductive barriers and lead to speciation because individuals with different translocation arrangements may have reduced fertility when mating. The other options describe chromosomal duplications, inversions, and deletions, which are less likely to cause species differentiation on their own.

Griffith effect is also known as bacterial transformation. Transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material (exogenous DNA) from its surroundings and taken up through the cell membrane.

Mendel's monohybrid cross produces a 3:1 phenotypic ratio (dominant:recessive) and a 1:2:1 genotypic ratio (homozygous dominant:heterozygous:homozygous recessive). The dihybrid cross produces a 9:3:3:1 phenotypic ratio when genes assort independently. These ratios are fundamental to Mendelian genetics and result from the segregation and independent assortment of alleles during gamete formation.

In sweet peas, flower color is controlled by complementary gene interaction where both genes C (chromogen production) and P (enzyme to convert chromogen to anthocyanin) must be present in dominant form (C_P_) for purple color. The genotype CcPp has one dominant allele of each gene, so it produces purple flowers due to anthocyanin pigment. White flowers result when either gene is homozygous recessive.

Pisum sativum (garden pea), the plant Mendel used in his experiments, has a diploid chromosome number of 2n=14. This means somatic cells contain 14 chromosomes (7 pairs of homologous chromosomes). Option C (7) represents the haploid number (n) found in gametes, not somatic cells.

The question is poorly framed as all mutation types can be genetically significant depending on context. However, the intended answer is likely substitution (point mutation) as it directly alters gene sequences at the molecular level. Somatic mutations (A) are not heritable; translocation (B) and inversion (D) are chromosomal-level changes. Substitution mutations can change protein function and are heritable if they occur in germ cells, making them 'genetically significant' in evolutionary terms.

The assertion states that reciprocal crosses (tall male × dwarf female or dwarf male × tall female) produce identical F1 offspring (all tall). This demonstrates that inheritance is not influenced by the sex of the parent plant. The reason correctly explains this by stating that sex does not influence character inheritance - meaning the genetic contribution is equal regardless of which parent is male or female. This is true for Mendelian traits in plants like peas.

The assertion is true: recessive traits only express phenotypically in homozygous recessive conditions (rr) because the dominant allele masks their effect. The reason is also true: alleles do segregate during gamete formation (Mendel's first law). However, the reason does NOT explain the assertion - segregation is about allele separation, not about why recessive traits are masked. The explanation for recessive expression relates to dominance relationships and protein function/dosage, not segregation mechanics.

A test cross involves crossing a hybrid (heterozygous) with a recessive (homozygous recessive) parent to determine the hybrid's genotype. A back cross involves crossing a hybrid with a pure breeding (homozygous) parent - which could be either dominant or recessive. Therefore, hybrid × recessive = test cross, and hybrid × pure breeding = back cross. Note that back cross is broader - test cross is a specific type of back cross.