(a) Define gene frequency. Describe the method of estimation of gene frequency from genotypic frequency with the help of a…

(a) Gene frequency and its estimation Definition: gene (allele) frequency is the proportion of a particular allele among all the alleles at a locus in a population. For a locus with alleles A and a, their frequencies p and q add up to 1 (p + q = 1). Genotypic frequency, by contrast, is the proportion of individuals of each genotype (AA, Aa, aa), and these add up to 1 as well. Gene frequencies describe the genetic make-up of a breed, and selection, migration, mutation and drift act by changing them.

Method 1: gene counting (when all genotypes can be identified). This applies to codominant or incompletely dominant traits, blood groups and DNA markers. Let D, H and R be the frequencies of AA, Aa and aa. Each AA animal carries two A alleles and each Aa carries one, so:

• p (frequency of A) = (2 × number of AA + number of Aa) / (2 × total animals) = D + ½H
• q (frequency of a) = (2 × number of aa + number of Aa) / (2 × total animals) = R + ½H

Example (coat colour in Shorthorn cattle, R = red, r = white, Rr = roan): a herd of 1,000 animals has 500 red (RR), 400 roan (Rr) and 100 white (rr).

• Total alleles = 2 × 1,000 = 2,000.
• Number of R alleles = 2 × 500 + 400 = 1,400, so p = 1,400 / 2,000 = 0.7.
• Number of r alleles = 2 × 100 + 400 = 600, so q = 600 / 2,000 = 0.3 (check: p + q = 1).
• Equivalently, D = 0.5, H = 0.4 and R = 0.1, so p = 0.5 + 0.2 = 0.7 and q = 0.1 + 0.2 = 0.3.

Testing Hardy–Weinberg equilibrium: the expected numbers are p² × 1,000 = 490 red, 2pq × 1,000 = 420 roan and q² × 1,000 = 90 white. Then χ² = (10²/490) + (20²/420) + (10²/90) = 0.20 + 0.95 + 1.11 = 2.27. This is below 3.84 (1 degree of freedom, 5% level), so the herd is in equilibrium.

Method 2: square-root method (complete dominance). When the heterozygote looks like the dominant homozygote, only the recessive class can be counted, so assuming Hardy–Weinberg equilibrium, q = √(frequency of recessives).

Example (red coat in Holstein-Friesian, recessive to black): among 400 calves, 16 are red. Then q² = 16/400 = 0.04, so q = 0.2 and p = 0.8. The expected proportion of carriers (2pq) is 2 × 0.8 × 0.2 = 0.32, so about 128 of the 400 black calves carry the red gene. This method is used to estimate carriers of harmful recessives such as BLAD or complex vertebral malformation.

(b) Role of NDDB in rural development The National Dairy Development Board was set up in 1965 at Anand, Gujarat, at the initiative of Prime Minister Lal Bahadur Shastri, with Dr Verghese Kurien as its first chairman, to replicate the Anand (Amul) pattern of cooperatives across India. It became a statutory body of national importance under the NDDB Act, 1987.

• Operation Flood (1970–1996): in three phases it linked about 70,000 village milk cooperatives to urban markets through a "national milk grid", financed partly by the sale of donated milk powder and butter oil. This made India the world's largest milk producer by 1998.
• The Anand pattern: a three-tier structure of village society, district union and state federation, owned and controlled by producers. It gives small, marginal and landless farmers an assured market, fair price based on fat and SNF, and year-round income, since dairy pays every 10–15 days.
• Inputs and services: artificial insemination, bull production and progeny testing; cattle feed plants and bypass protein and mineral mixtures; veterinary services; and FMD and other vaccines through Indian Immunologicals Ltd.
• National Dairy Plan Phase I (2012–2019): with World Bank support, it improved breeding through progeny testing and pedigree selection, ran the ration balancing programme, and strengthened village milk procurement.
• Digital tools: the Information Network for Animal Productivity and Health (INAPH) and the e-GOPALA app; NDDB is also a technical partner in the National Digital Livestock Mission.
• Women and new institutions: it promotes women-led milk producer companies (such as Paayas, Maahi and Baani) and women's participation in cooperative boards.
• Livelihood diversification: the SuDhan manure-value-chain model (biogas and slurry marketing, first at Zakariyapura in Gujarat), fruit, vegetable and edible oil businesses (Mother Dairy, Safal, Dhara), and dairy plant engineering through IDMC.
• Impact: dairying gives regular cash income to more than 8 crore rural households, most of them small holders and women, and reduces distress migration.

(c) Short notes

(i) Methane inhibitors Methane inhibitors are feed additives or treatments that reduce the production of methane by methanogenic archaea (such as Methanobrevibacter ruminantium) in the rumen, which reduce CO₂ with hydrogen. Methane carries away about 2–12% of the gross energy of feed and is a potent greenhouse gas.

• Direct inhibitors of methanogenesis: 3-nitrooxypropanol (3-NOP, Bovaer) blocks methyl-coenzyme M reductase and cuts methane by about 30%. Bromoform from the red seaweed Asparagopsis and halogenated compounds such as bromochloromethane act on the same final step.
• Alternative hydrogen sinks: nitrate, fumarate and malate take up hydrogen and leave less for methane.
• Ionophores: monensin suppresses Gram-positive, hydrogen-producing bacteria and protozoa, shifting fermentation towards propionate.
• Plant compounds: tannins, saponins and essential oils, and defaunation (removal of protozoa), which carry methanogens.
• Indian product: ICAR-NIANP's Harit Dhara (commercialised in 2022), made from plant sources, reduces enteric methane by about 17–20%.
• Significance: they cut greenhouse emissions and can save feed energy, but effects may wane as microbes adapt, and residues, cost and safety must be checked.

(ii) Role of the rumen in feed fermentation The rumen is the largest compartment of the ruminant stomach (about 100–150 L in an adult cow), a continuous anaerobic fermentation chamber at 38–40°C and pH 5.5–7.0.

• Microbes: it holds about 10¹⁰–10¹¹ bacteria and 10⁵–10⁶ protozoa per mL, besides anaerobic fungi. Cellulolytic bacteria (Fibrobacter succinogenes, Ruminococcus albus and R. flavefaciens) digest fibre; amylolytic bacteria (Streptococcus bovis) digest starch.
• Carbohydrate fermentation: cellulose, hemicellulose and starch are fermented to volatile fatty acids, roughly 65:20:15 acetate:propionate:butyrate on roughage diets, which supply about 70% of the animal's energy, together with CO₂ and methane.
• Nitrogen: dietary protein and non-protein nitrogen (urea) are broken down to ammonia and built into microbial protein, which is digested in the abomasum and small intestine and supplies most of the animal's absorbed protein.
• Other functions: microbes synthesise B-complex vitamins and vitamin K, hydrogenate unsaturated fats (giving CLA), and detoxify some plant toxins; rumination and rumen contractions mix contents and reduce particle size.
• Significance: this lets ruminants live on fibrous feeds that non-ruminants cannot digest, but imbalance causes acidosis, bloat or ketosis.

(iii) Chemical regulation of respiration Chemical regulation adjusts the rate and depth of breathing to changes in arterial PCO₂, H⁺ and PO₂, sensed by chemoreceptors that signal the respiratory centres in the medulla and pons.

• Central chemoreceptors: these lie near the ventral surface of the medulla and respond to H⁺ in the cerebrospinal fluid. CO₂ crosses the blood–brain barrier and forms H⁺ there, so a rise in arterial CO₂ is the main stimulus to breathing, providing about 70–80% of the CO₂ response.
• Peripheral chemoreceptors: the carotid bodies (at the carotid bifurcation, signalling through the glossopharyngeal nerve) and aortic bodies (through the vagus nerve) respond mainly to low arterial PO₂ (below about 60 mmHg). They also respond to rising H⁺, for example in metabolic acidosis, and to CO₂.
• Interaction: hypoxia and hypercapnia reinforce each other. In chronic lung disease, where central receptors adapt to high CO₂, breathing depends on the hypoxic drive, so excess oxygen can depress respiration.
• Birds also have intrapulmonary CO₂ receptors in the lungs.
• Significance: chemical control keeps blood gases and pH constant during exercise, at high altitude and in acid–base disturbances.

(iv) Blood–brain barrier (BBB) The BBB is the selective barrier between blood and brain tissue that keeps the neural environment stable.

• Structure: brain capillary endothelial cells joined by tight junctions (claudin-5, occludin), with no fenestrations and little pinocytosis, surrounded by basement membrane, pericytes and the end-feet of astrocytes.
• Transport: lipid-soluble substances, O₂ and CO₂ pass freely; glucose enters by the GLUT1 carrier and amino acids by specific transporters; ions, proteins, most bacteria and many drugs (such as penicillin, when the meninges are not inflamed) are kept out.
• Efflux pumps: P-glycoprotein (MDR1) pumps many drugs back into blood. Collies and related breeds with the MDR1 gene mutation lack it and suffer ivermectin toxicity.
• Weak points: circumventricular organs, such as the area postrema (the vomiting chemoreceptor trigger zone) and the posterior pituitary, lack a BBB. The barrier is immature in neonates, as in kernicterus, and opens in meningitis, trauma or hypoxia.
• Significance: it protects neurons from toxins and pathogens but limits drug treatment of brain infections and tumours.