(a) Describe different types of heritability based on genetic variance components. 10 (b) Explain in detail about the different…

(a) Types of Heritability and Genetic Variance Components

Phenotypic variance (V_P) decomposes into genetic (V_G), environmental (V_E), and genotype-environment interaction (V_GE) components, represented as V_P = V_G + V_E + V_GE. Total genetic variance further partitions into additive (V_A), dominance (V_D), and epistatic interaction (V_I) variances, where V_G = V_A + V_D + V_I.

Heritability is classified into two distinct forms based on these variance components. Broad-sense heritability (H² = V_G / V_P) measures the proportion of total phenotypic variation attributable to all genetic effects combined. Narrow-sense heritability (h² = V_A / V_P) quantifies the proportion of phenotypic variation determined solely by additive genetic variance. In animal breeding, V_A represents the average effects of individual gene substitutions, directly constituting the individual's breeding value and determining the expected response to directional selection. Conversely, non-additive components (V_D arising from intra-locus interactions and V_I arising from inter-locus interactions) break down during meiosis; they do not transmit reliably across generations and are instead exploited through designed crossbreeding programs to capture heterosis.

(b) Methods of Semen Collection in Livestock and Poultry

Artificial Vagina Method: The principal technique for bulls, rams, and bucks, yielding natural ejaculate volume and concentration. The device consists of a rigid outer casing and an inner rubber liner forming a water jacket. Water is added at 45–50°C to achieve an internal working temperature of 40–42°C at the time of collection. Adequate pneumatic pressure is maintained by air insufflation, and the anterior third of the liner is lubricated with sterile, non-spermicidal white petroleum jelly or lubricating jelly. Teaser males, phantom dummies, or restrained females serve as mounts.

Electroejaculation Method: Employed for untamed, injured, or disabled bulls and rams incapable of mounting. A lubricated multipolar rectal probe placed over the ampullae and seminal vesicles delivers rhythmic, low-amperage, increasing alternating voltage (0–15 V) to stimulate the pelvic sympathetic and parasympathetic innervation, inducing erection and emission without natural mounting.

Massage Method: Used in docile or physically disabled bulls and bucks. The operator manually massages the ampullae and vesicular glands per-rectum through gentle, rhythmic backward stroking, causing passive voiding of semen into a collection cone, though the ejaculate is often more dilute.

Poultry Methods: The standard technique in cocks, toms, and drakes is the abdominal massage method (Burrows and Quinn method). The handler restrains the bird, massages the dorso-lateral lumbar-pelvic region to stimulate phallic engorgement, and applies gentle digital pressure on the lateral margins of the cloaca to express semen into a collecting vessel. Collection using dummy females and teaser males equipped with cloacal interceptive funnels is also practiced.

(c) Methods to Estimate Heritability in Animals

ANOVA-Based Classical Methods: These utilize phenotypic resemblance among relatives derived from Falconer’s biometrical expectations:

1. Parent-Offspring Regression: Single parent-offspring regression yields a slope of b_OP = 1/2 h², giving h² = 2 b_OP. Mid-parent to offspring regression yields b_P̄O = h². This method is free from dominance variance bias, though single-dam regression may contain maternal environmental covariance.
2. Paternal Half-Sib Correlation: ANOVA isolates between-sire (σ²ₛ) and within-sire (σ²_w) variance components. The intra-class correlation (t = σ²ₛ / σ²_P) reflects one-fourth additive variance, calculating narrow-sense heritability as h² = 4t. It excludes dominance and common maternal environment bias, making it the standard field estimation method.
3. Full-Sib Analysis: Intra-class correlation (t_FS) estimates heritability as h² = 2t_FS. However, it inflates h² because the covariance among full sibs contains 1/2V_A + 1/4V_D + V_Ec (common maternal environment).

Advanced Statistical Approaches: Restricted Maximum Likelihood (REML) under the Individual Animal Model has superseded basic ANOVA for unbalanced, complex pedigree structures. By using all known pedigree relationships, REML largely accounts for bias from selection and culling (provided the data on which selection was based are included) and for non-random mating. Bayesian methods implemented via Gibbs sampling provide full posterior distributions and credible intervals for heritability, outperforming ANOVA in precision for non-orthogonal field data. Realized heritability is estimated from the response to selection (h2 = R/S) in purpose-designed selection experiments.

In Indian livestock improvement programs, published estimates of first-lactation milk yield heritability from paternal half-sib and REML animal models commonly fall around h² ≈ 0.2–0.3 in indigenous Sahiwal cattle and around 0.3 in synthetic Karan-Fries (Tharparkar × Holstein-Friesian) cattle, varying with herd and period. Standardized semen collection protocols executed at ICAR-NDRI Karnal and ICAR-CIRC Meerut provide high-quality germplasm, ensuring that reliable heritability parameters translate into precise breeding value predictions under the Rashtriya Gokul Mission.