Breeding of Vegetable, Spice and Tuber Crops

Systems of mating

The breeder has two basic tools to change the genetic composition of a population:

1. Selection, and
2. Mating system.
• We have discussed the effects of selection in some detail. Here we would examine the consequences of various systems of mating.

There are five basic mating schemes

1. Random mating,
2. Genetic assortative mating
3. Genetic disassortative mating,
4. Phenotypic assortative mating, and
5. Phenotypic disassortative mating.

Random Mating

• In random mating, each female gamete is likely to unite with any male gamete equally and the rate of reproduction of each genotype is equal, i.e. there is no selection.

Genetic Assortative Mating

• In genetic assortative mating, the mating is between individuals that are more closely related by ancestry than in random mating.
  
• Thus in this mating system, it is not important that the genotypes of the plants selected for mating be correctly identified.
  
• This mating system is more commonly known as inbreeding.

Genetic Disassortative Mating

• In genetic disassortative mating, such individuals are mated, which are less closely related by ancestry than would be under random mating. Thus in this system, totally unrelated individuals are mated.
  
• These individuals often belong to different population.
  
• Example of such a mating are intervarietal and interspecific crosses. The effects of genetic disassortative mating are similar to those of migration.
• It may be expected that this mating system would reduce homozygosity and increase heterozygosity.

Phenotypic Assortative Mating

• Mating between phenotypically dissimilar individuals belonging to the same population is referred to as phenotypic disassortative mating. The consequences of this mating are as follows.
• It leads to the maintenance of or even some increase in heterozygosity
• There is some reduction in population variance since it tends to produce intermediate phenotypes
• There is a reduction in correlation between relatives or in prepotency due to the increase in heterozygosity.
• This mating system is very useful in making a population stable, i.e., in maintaining variability. Suitable parents may be selected to remove their weaknesses.
  
• The progeny from such a mating would be more desirable than the parents. It is also useful when the desirable type is an intermediate one and the available parents have the extreme phenotypes.
  
• But the most notable use of this mating system is in maintaining variability in relatively smaller populations as it reduces inbreeding.