(a) Define heritability. What do you mean by broad sense and narrow sense heritability ? Explain its applications in crop…

(a) Heritability (20 marks)

Heritability is the proportion of the observed (phenotypic) variation in a trait, in a given population and environment, that is due to genetic differences. It measures how far the phenotype of parents is passed on to offspring, and it is a property of a population, not of an individual. Since phenotypic variance V_P = V_G + V_E (genetic plus environmental), and V_G = V_A + V_D + V_I (additive, dominance and epistatic), the two meanings are:

1. Broad-sense heritability, h²(b) = V_G/V_P, the fraction of phenotypic variance due to all genetic effects. It is estimated from inbred lines or clones, where the variance among genotypes is separated from the environmental variance, and it is useful for self-pollinated and vegetatively propagated crops, where the whole genotype is fixed.
2. Narrow-sense heritability, h²(n) = V_A/V_P, the fraction due to additive gene effects alone. It is the fraction that is transmitted reliably to progeny, so it predicts the response to selection. It is estimated by parent-offspring regression, or from half-sib and diallel or North Carolina analyses. Always h²(n) ≤ h²(b).

Values range from 0 to 1 (0 to 100%); above about 60% is usually called high.

Applications in crop improvement:

• Predicting genetic advance: the response to selection is R = h² × S (S is the selection differential), and genetic advance GA = k × h² × σ_P (k is the standardised selection intensity). High heritability with high GA shows additive gene action, and selection is effective.
• Choosing the selection method: a trait with high heritability, such as plant height, days to flowering or grain weight, can be selected in early generations by mass or pedigree selection; a low-heritability trait, such as yield, needs replicated testing and selection is deferred to later generations, or is done by recurrent selection.
• Choosing the breeding method: simple selection where additive variance is large, and hybridisation with recurrent selection, or heterosis breeding, where non-additive variance is large.
• Deciding the number of replications and locations, and the sample size, for a target reliability.
• Comparing genotypes and environments, and judging the trustworthiness of the observed differences.
• Indirect selection through correlated traits.

Cautions: heritability is not fixed, since it changes with population, environment, generation and the method of estimation, and a high value does not mean the trait is unaffected by the environment.

(b) Principles and practices of seed production in self-pollinated and cross-pollinated crops (20 marks)

The aim of seed production is to multiply seed of a released variety or hybrid in large quantity while maintaining its genetic purity, identity, physical quality and viability. Seed moves through the classes of breeder seed (produced by the originating breeder), foundation seed (from breeder seed) and certified seed (from foundation seed), and is regulated in India under the Seeds Act, 1966 and the Seeds Rules, 1968.

Principles:

1. Genetic principles: keep the variety pure by preventing genetic deterioration, from mechanical mixture, natural crossing, mutation, residual variability and selection pressure. Limit the number of generations of multiplication.
2. Agronomic principles: grow seed on a suitable, well-drained soil with known cropping history, in a region and season with favourable weather for seed setting and maturity, and give the best crop management.
3. Isolation to prevent contamination, and rogueing to remove off-types.
4. Seed quality standards: adequate physical purity, germination, moisture and freedom from disease.

Practices in self-pollinated crops (wheat, rice, groundnut, pulses): a small isolation distance is enough because natural crossing is low, for example about 3 m for rice and wheat. The pure seed is sown in a field free of volunteer plants, off-types are removed at the vegetative, flowering and pre-harvest stages, harvest is made at maturity, and the crop is threshed and cleaned separately.

Practices in cross-pollinated crops (maize, pearl millet, sunflower, cotton hybrids): the risk of pollen contamination is high, so isolation is larger, for example about 400 m for foundation and 200 m for certified seed in maize. For an open-pollinated variety, the whole field is kept isolated and rogued. For a hybrid, the seed parent (female) and pollen parent (male) are sown in fixed row ratios, with detasseling of the female (in maize) or the use of cytoplasmic male sterility, and staggered sowing so that flowering coincides. Rogueing of off-types and shedders is done before pollen shedding. Hand pollination or bagging is used for parental lines and for cotton.

Common to both: a field inspection by the certification agency, harvest at physiological maturity, drying to safe moisture (about 12% or below for cereals), threshing, cleaning and grading, seed treatment, testing, tagging, and storage in moisture-proof packing, with the requirement of about 98% physical purity and at least 80 to 85% germination for cereals.

(c) Cytoplasmic inheritance; sex-influenced and sex-limited traits (10 marks)

Cytoplasmic inheritance is the transmission of characters through genes located in cytoplasmic organelles, the mitochondria and plastids (and plasmids), and not in nuclear chromosomes. Because the egg supplies almost all the cytoplasm, the trait is usually inherited from the female parent alone, so reciprocal crosses differ and it does not follow Mendelian ratios. Examples are the variegation in four-o'clock (Mirabilis jalapa), plastid-inherited streptomycin resistance in Chlamydomonas, and cytoplasmic male sterility in maize and sorghum.

Sex-influenced traits are governed by autosomal genes present in both sexes, but the dominance relation depends on the sex. A heterozygote shows the trait in one sex and not in the other, under the influence of sex hormones. Examples: pattern baldness in humans, dominant in men and recessive in women, and horns in Dorset sheep, dominant in males.

Sex-limited traits are expressed in only one sex, though the genes are present in both, because the trait needs the sex organs or hormones of that sex. Examples: milk yield in cattle, egg production in poultry, beard growth in men, and cock plumage in fowl. The genes may still be passed on through the sex in which they do not show.