(a) Impact of genetic drift on the conservation of endangered species (10 marks) (b) Selection for threshold traits is often…

(a) Impact of genetic drift on the conservation of endangered species (10 marks)

Genetic drift is the random change of allele frequencies from one generation to the next, caused by chance sampling of gametes in a finite population. Its size is about p(1 − p)/2N_e per generation, so it is strongest in small populations, which is the case for endangered species.

Impact:

• Loss of alleles and of heterozygosity at a rate of about 1/(2N_e) per generation, so genetic variation and adaptability fall.
• Fixation of alleles, including mildly harmful ones, and accumulation of deleterious mutations (drift load).
• Inbreeding depression, lower fertility and survival, and poorer disease resistance, which can lead to an extinction vortex.
• Bottleneck and founder effects: a sudden fall in numbers, or a few founders, permanently reduces diversity even if numbers recover.

Conservation measures: keep a large effective population size (an even sex ratio helps); equalise family sizes; minimise kinship in mating; translocate animals for gene flow (genetic rescue); keep studbooks; and freeze semen, embryos and cells.

(b) Selection for threshold traits is often frequency-dependent (10 marks)

A threshold trait is one that is expressed as a discrete category (present or absent, for example twinning, disease susceptibility, calving difficulty), but is controlled by an underlying continuous variable, the liability, which is polygenic and affected by the environment. The trait appears when the liability exceeds a threshold.

Justification: the heritability observed on the 0/1 scale depends on the incidence (frequency) p: h²(obs) = h²(liability) × z²/[p(1 − p)], where z is the height of the normal curve at the threshold. It is highest when p is near 50%: at p = 0.5 it is about 0.64 × h², but at p = 0.05 only about 0.22 × h². So the response to selection depends on how frequent the trait is; selection is efficient at intermediate frequencies, and slows when the trait becomes rare or nearly universal. Since selection itself changes the incidence, the heritability on the observed scale changes across generations. Selection should therefore use threshold models on the liability scale.

(c) Salient features of socio-economic considerations in livestock farming (10 marks)

• Livelihood and employment: livestock supports millions of small and marginal farmers and landless families, and gives regular, year-round income and work, with milk providing daily cash.
• Contribution to the economy: about 30% of agricultural gross value added and about 5% of the national total, and India is the world's largest milk producer.
• Women's role: a large share of the work in feeding, milking and care is done by women, so livestock empowers them.
• Nutrition security: milk, eggs, meat and fish supply protein and micronutrients.
• Risk cover and assets: animals are savings and insurance against crop failure, and give draught power, manure and fuel.
• Social and cultural: cooperative models such as Amul, social status, and religious and cultural sentiments about some species.
• Constraints: fodder shortage, disease, poor market, credit and insurance, and low productivity of animals.
• Policy support: dairy and animal-husbandry credit, insurance, vaccination and breed-improvement missions.

(d) Factors affecting production efficiency of dairy animals (10 marks)

• Genetic: breed, sire and dam merit, and crossbreeding and selection.
• Nutrition: a balanced ration with sufficient green and dry fodder, concentrate and minerals, and clean water; feed is 60 to 70% of the cost of milk.
• Reproduction: age at first calving, calving interval (ideally 12 to 13 months), service period and conception rate, since fertility decides the milk per day of life.
• Health: mastitis, foot-and-mouth disease, metabolic disorders and parasites, with vaccination and prompt treatment.
• Management: housing, hygiene, correct milking practice and frequency, a dry period of about 60 days, and handling.
• Environment: heat stress and humidity, season and shelter.
• Physiological: stage and number of lactation, and age.
• Economics and records: cost control, and record-keeping to cull poor producers.

Together they determine milk yield per lactation, lifetime production and feed conversion.

(e) Steps in the production of recombinant DNA (10 marks)

1. Isolation of the gene of interest: by restriction digestion of genomic DNA, by making cDNA from mRNA, by PCR amplification, or by chemical synthesis.
2. Choice of vector: a plasmid (or phage, cosmid, BAC) with an origin of replication, a selectable marker such as an antibiotic-resistance gene, and a multiple cloning site.
3. Cutting: the gene and the vector are cut with the same restriction endonuclease, giving complementary sticky ends.
4. Ligation: DNA ligase joins the gene into the vector, giving recombinant DNA.
5. Transformation: the recombinant DNA is put into a host, commonly Escherichia coli, by calcium chloride and heat shock or electroporation.
6. Selection and screening: transformed cells are selected on antibiotic medium, and recombinants identified by blue-white screening (lacZ), colony hybridisation or PCR.
7. Expression and scale-up: the clones are grown in a fermenter for expression, and the product harvested and purified.

Examples: insulin, bovine somatotropin, hepatitis B vaccine and chymosin.