Gas A: The gas being referred to is likely ozone (O3).

Formation at higher levels of the atmosphere:
Ozone is primarily formed in the stratosphere, the second major atmospheric layer, through the photochemical reaction involving oxygen molecules:

O2 + UVradiation → 2O
O + O2 → O3

In simpler terms, high-energy ultraviolet (UV) radiation from the Sun causes the dissociation of oxygen molecules (O2) into individual oxygen atoms (O). These oxygen atoms then combine with oxygen molecules to form ozone (O3).

b. Importance for all living beings:
Ozone plays a crucial role in protecting life on Earth by absorbing the majority of the Sun’s harmful ultraviolet (UV) radiation. High-energy UV radiation can cause damage to living tissues, including DNA mutations that may lead to skin cancer. Ozone acts as a shield, preventing a significant portion of these harmful UV rays from reaching the Earth’s surface.

Cause for depletion:
The depletion of ozone in the upper atmosphere, particularly in the ozone layer, is mainly attributed to the release of human-made chemicals called ozone-depleting substances (ODS). The most notable of these substances are chlorofluorocarbons (CFCs), halons, carbon tetrachloride, and methyl chloroform.

Once released into the atmosphere, these ODS can reach the stratosphere, where they undergo photochemical reactions that release chlorine and bromine atoms. These atoms catalytically destroy ozone molecules:
Cl + O3 → ClO + O2
ClO + O → Cl + O2

The net result is the depletion of ozone, leading to the formation of the so-called ozone hole, particularly over Antarctica. The decrease in ozone concentration in the upper atmosphere allows more harmful UV radiation to reach the Earth’s surface, posing significant risks to human health and the environment. The 1987 Montreal Protocol is an international agreement aimed at phasing out the production and consumption of ozone-depleting substances to address this issue.

When the F1 plants (RrYy) are self-pollinated, the possible combinations of alleles segregating during gamete formation can be determined using the principles of Mendelian genetics. The key is to consider the segregation of alleles during both the formation of gametes and their subsequent combination during fertilization.
The possible combinations of alleles in the F2 generation can be obtained through the multiplication of the individual allele combinations for each gene. The alleles segregate independently during gamete formation, following Mendel’s law of independent assortment.

The genotype of the F1 plants (RrYy) can produce gametes with the following combinations:
• RY
• Ry
• rY
• ry

These gametes can then combine in various ways during fertilization. The possible genotypes in the F2 generation, along with their phenotypic expressions, are as follows:
1. RRYY (round yellow)
2. RRYy (round yellow)
3. RrYY (round yellow)
4. RrYy (round yellow)
5. RRyy (round green)
6. Rryy (round green)
7. rrYY (wrinkled yellow)
8. rrYy (wrinkled yellow)
9. rryy (wrinkled green)

So, there are 9 possible combinations of characters in the F2 generation.
To determine the ratio of these combinations, you can use a Punnett square or the multiplication rule. If you cross RrYy x RrYy, you get a 9:3:3:1 ratio for the phenotypes (round yellow: round green: wrinkled yellow: wrinkled green). This is based on the fact that each gene segregates independently, and the combination of alleles for one gene does not influence the combination for the other gene.
Now, if a total of 160 seeds are produced in the F2 generation, you can calculate the expected number of seeds for each phenotype by multiplying the ratio by the total number of seeds:
• Round Yellow (RY): 9/16 * 160 = 90 seeds
• Round Green (Ry): 3/16 * 160 = 30 seeds
• Wrinkled Yellow (rY): 3/16 * 160 = 30 seeds
• Wrinkled Green (ry): 1/16 * 160 = 10 seeds
So, you would expect 90 round yellow seeds, 30 round green seeds, 30 wrinkled yellow seeds, and 10 wrinkled green seeds in the F2 generation.

The use of DDT (dichlorodiphenyltrichloroethane) to control mosquito breeding in a lake can have profound effects on the trophic levels in the associated food chain. DDT is a pesticide that can bioaccumulate and biomagnify through food chains, impacting various organisms.

Let’s analyze the potential effects on each trophic level in the given food chain:

1. Plankton: DDT can enter the aquatic environment and affect plankton, which are primary producers. Plankton may absorb DDT from the water, leading to changes in their populations.
2. Small Fish: Small fish consume plankton and may accumulate DDT as they feed. The bioaccumulation of DDT in smaller organisms can lead to higher concentrations in organisms at higher trophic levels.
3.Large Fish: Large fish that prey on smaller fish can accumulate even higher levels of DDT due to biomagnification. DDT is known to persist in fatty tissues, and as larger predators consume numerous smaller organisms, the concentration of DDT can increase significantly.
4. Hawk: If the lake supports a population of fish that are contaminated with DDT, hawks (or other birds of prey) that feed on these fish may be exposed to high levels of the pesticide. Birds, especially raptors, can be particularly sensitive to DDT and its breakdown products.
5. Aquatic Environment: DDT can have detrimental effects on the overall aquatic environment. It can disrupt the balance of the ecosystem by affecting non-target organisms and reducing biodiversity. The long-term use of DDT can have cascading effects on various trophic levels, potentially leading to imbalances in the ecosystem.

Justification:
• Bioaccumulation: DDT tends to accumulate in the fatty tissues of organisms. As smaller organisms with lower trophic levels absorb DDT, the concentration increases in their tissues.
• Biomagnification: The process of biomagnification occurs as DDT moves up the food chain. Predators at higher trophic levels accumulate higher concentrations of the pesticide because they consume multiple contaminated organisms.
• Impact on Birds of Prey: DDT is notorious for its role in thinning eggshells of birds, particularly raptors like hawks. This thinning can lead to reproductive failures and population decline in these species.

In summary, the use of DDT in a lake can have far-reaching consequences on the trophic levels, potentially disrupting the balance of the ecosystem and posing risks to the health of organisms at higher trophic levels, especially birds of prey.

The greenish-black powdery mass observed on the stale piece of bread is likely to be a type of mold. Molds are fungi that commonly grow on organic matter under suitable conditions.

a. Organism and Asexual Reproduction:

The organism responsible for this could be a common mold like Rhizopus stolonifer, commonly known as black bread mold. The specific mode of asexual reproduction in Rhizopus is called sporulation.

b. Vegetative and Reproductive Parts:

In Rhizopus stolonifer, the vegetative part is the mycelium, which consists of a network of thread-like structures called hyphae. The mycelium grows on and within the bread, secreting enzymes to digest and absorb nutrients.
The reproductive part involved in asexual reproduction is the sporangium. The sporangium is a specialized structure at the tip of a vertical stalk called a sporangiophore. Within the sporangium, spores (called sporangiospores) are produced through a process called sporulation.
The sporangiospores are released when the sporangium bursts, and these spores can then disperse to new areas, where they can germinate and grow into new mycelia under suitable conditions.

So, in summary:
• Vegetative Part: Mycelium
• Reproductive Part: Sporangium (containing sporangiospores)
This asexual reproductive strategy allows molds like Rhizopus to produce numerous spores for dispersal and colonization of new substrates.