Family selection methods

Family selection methods are characterized by three general steps:

Creation of a family structure.

Evaluation of families and selection of superior ones by progeny testing.

Recombination of selected families or plants within families to create a new base population for the next cycle of selection. Generally, the duration of each step is one generation, but variations exist.

1 Half-sib family selection methods

The basic feature of this group of methods is that half-sib families are created for evaluation and recombination, both steps occurring in one generation. The populations are created by random pollination of selected female plants in generation 1. The seed from generation 1 families are evaluated in replicated trials and in different environments for selection. There are different kinds of half-sib family selection methods, the simplest one being ear-to-row selection.

This method is applicable to cross-pollinated species.

Steps

_ Season 1. Grow source population and select desirable plants based on phenotype according to the traits of interest. Harvest plants individually. Keep remnant seed of each plant.

_ Season 2. Grow replicated half-sib progenies  from selected individuals in one environment. Select best progenies and bulk to create progenies of the next cycle. The bulk is grown inisolation and random mated.

_ Season 3. The seed is harvested and used to grown the next cycle. Alternatively, the breeder may bulk the remnant seed of S0 plants whose progeny have been selected and used that to initiate the next cycle.

Genetic issues

The expected genetic gain from half-sib selection is given by:

where sPHS is the standard deviation of the phenotypic variance among half-sibs. Other components are as before. The tester is the parental population, and hence selection or control is over only one sex. The genetic gain is hence reduced by half.

Genetic gain can be doubled by selfing each parent to obtain S1, then crossing to obtain half-sibs.

Application

Half-sib selection is widely used for breeding perennial forage grasses and legumes. A polycross mating system is used to generate the half-sib families from selected vegetatively maintained clones. The families are evaluated in replicated rows for 2–3 years. Selecting traits of high is effective. Modification The basic or traditional ear-to-row selection method did not show much gain over mass selection. An improvement was proposed by J.H. Lohnquist in which creation of family structure, evaluation and

recombination are conducted in one generation. The half-sib families are evaluated in replicated trials in many environments. The approach was to better manage the environmental and G _ E interactions.

Steps

_ Season 1. Select desirable plants from the source population. Harvest these open-pollinated individually.

_ Season 2. Grow progeny rows of selected plants at multiple locations and evaluate for yield performance. Plant female rows within seed from individual half-sib families alternating with male rows planted with bulked seed from the entire population. Select desirable plants from each progeny separately. Bulk the seed to start next cycle.

Genetic issues

The genetic gain has two components – among earrows across environments and

within families.The total genetic gain is given by: where swe is the square root of the plant-to-plant within plot variance. Others components are as before.