Unit 6 Mating Systems

Random Mating (Panmixia):

• Every male has equal chance of mating with every female
  • All individuals are fertile
  • All individuals contribute an equal number of progeny to the next generation
  • Simplest of all mating systems

Non-random Mating:

1. Based on phenotypic resemblance: Mating of either similar or dissimilar individuals – assortative mating

1. Positive assortative mating:
   • Like to like mating
   • Phenotypically similar animals are mated with each other
   • Increases homozygosity in the population, reduces heterozygosity
   • Increases genetic variance (distribution of genes) in the population
   • Might lead to fixing of certain genes and creation of strains within breeds
   • Leads to creation of extreme phenotypes in the population

1. Negative assortative mating:
   • Mating of phenotypically dissimilar individuals
   • Loss in homozygosity and gradual increase in heterozygosity
   • The whole population starts showing the intermediate phenotype
   • Leads to a decrease in genetic variance
   • Maintains polymorphism in the population by maintaining different alleles in their heterozygous states

• Based on genetic resemblance

Introduced after the discovery of a measure of genetic relationship (coefficient of relationship)

2.1 Genetic Assortative Mating (Inbreeding)

• Mating of closely related animals
  • Homozygosity increases
  • Genetic variability reduces
  • Expression of deleterious recessive alleles is prominenet

2.2 Outbreeding:

• Mating of unrelated individuals
• Increases heterozygosity
• Increases genetic variability
• Offers more opportunity for effective selection

Inbreeding:

• Mating of related individuals
  • At least one or more common ancestors upto 4-6 generations in the pedigree
  • Classification of inbreeding:
    • Close inbreeding
    • Line breeding
    • Strain (breeding) formation

1. Close inbreeding
   1. Matings between sibs/parents and progenyProduces inbred lines with relatively high degree of homozygosityMost commonly used method – ‘full-sib mating’Same effect if continuous back crossing is done to the younger parent
   1. Purposes of close breeding:
      a) Developing highly inbred lines
      b) Discover undesirable recessive genes
      c) To get more uniform progeny

1. Line breeding:
   1. Milder form of inbreedingRelationship of individuals is kept close to an outstanding ancestor (generally a male) in the pedigreeParents chosen – have high relationship to the admired ancestor but not much related to each otherReason: Concentrate the inheritance of one ancestor in the line bred offspringPracticed in a purebred population of a high degree of excellence after identifying outstanding individualsApplication: Creation of families or lines within breeds
   1. Two ways:
      1. Half-sib mating/Cousin mating – lower rate of inbreeding
      1. Descendants mated to the outstanding ancestor directly for 3-4 generations – high rate of inbreeding

Consider the following pedigree:

Here, the pedigree has only one common ancestor between the sire and dam of ‘X’ – that is ‘1’

If there were no common ancestor between ‘S’ and ‘D’, ‘X’ would receive only 12.5% of genes of ‘1’. Instead, due to practice of line breeding, genes from ‘1’ are kept in the line-bred individuals and are passed on to ‘X’, which will have 50% inheritance from ‘1’, which is equivalent to what it would get from a single parent. Therefore, the inheritance of the outstanding ancestor ‘1’ remains concentrated in this pedigree or line.

*Strain formation: Breeding in a population without entry of new animals for atleast 3-5 generations; mildest form of inbreeding

Inbreeding Depression:

• Expression of unfavourable recessive alleles influencing polygenic traits
• Opposite to hybrid vigour
• Unfavourable gene combination value
• Performance level of inbred animals is low, and susceptibility to stress is high
• Noticeable in traits like fertility and survivability
• Inbreeding depression is not heritable

Outbreeding:

• Mating of unrelated individuals
  • Increase in heterozygosity and variability of the population

Heterosis/Hybrid Vigour (H)

• Increased phenotypic value of the progeny over the average of its parents
  • For polygenic traits, influenced by dominance
  • Outbreeding – gain in gene combination value (due to non-additive dominance & interaction)
  • The new combinations create high productivity
  • Genetic basis of heterosis : Dominance theory, Over dominance theory, Epistatic theory

Different types of outbreeding systems:

1. Outcrossing:
   1. Mating of unrelated individuals within the same breed (generally purebred animals).No relationship between mates for 4-6 generations
   1. Exploitation of intra-herd or intra-breed variability

• Crossbreeding
  • Mating of animals from different breedsProgeny produced : crossbredsMost common form of outbreedingWidely practiced in swine, sheep and poultry; lesser in cattle and horsesReasons for crossbreeding:
    • Use of heterosisBreed complementarityIntroduce new genes in a closed populationDevelop synthetic breeds
  • Useful when fertility is high, females can be kept for long, and cost of replacement is low

• Top crossing:
  • Inbred male × non-inbred female of the same breedFemales taken from base population
  • The inbred male is the best or ‘top’ sire in the pedigree

• Line crossing
  • Inbred male of one line × Inbred female of another lineExploits heterosis by crossing of both homozygous linesIncrossing: Mating of inbred lines within the breed
  • Incross breeding: Mating of inbred lines between different breeds

• Grading up
  • Continuous use of purebred sires on females of another breed or non-descript breed to raise them to the level of purebred sire.By 7^th generation, inheritance of the mongrel stock will reach that of the purebred line
  • Normally done for buffaloes in India

• Species hybridisation:
  • Cross between two speciesMost extreme form of outbreedingSurvivability of outbreds is very low
  • Progeny are usually sterile

S. No.	Hybrid	Species Involved
1.	Mule	Male donkey (Jack)	Female horse (Mare)
2.	Hinny	Female ass (Jennet)	Male horse (Stallion)
3.	Zebroid	Male zebra	Female horse
4.	Cattalo	Male American bison	Bos taurus
5.	Pien niu	Male cattle	Female yak
6.	Liger	Male lion	Female tiger
7.	Geep	Male sheep	Female goat

Combining Ability

General combining ability (GCA):

When a particular line is crossed with a number of other lines at random, the mean value of all the F1’s in crosses with the other lines (i.e. the mean performance of the particular line) is known as the general combining ability of that line.

• Definition: The average performance of a parent in hybrid combinations.
• Represents the average value of an inbred line based on its behavior in crosses with other lines.
• Indicates the ability of a parent to transmit desirable genes to its offspring
• GCA effects are primarily due to additive gene action
• Used to identify superior parents for breeding programs

Specific Combining Ability (SCA):

The performance of a particular cross, as deviating from the average general combining ability of the two lines is called the specific/special combining ability (SCA) of the cross.

• Definition: The deviation in performance of a specific cross from what would be expected based on the GCA of the parents.
• Represents the unique combination effects that cannot be accounted for by GCA alone
• SCA effects are primarily due to non-additive gene actions (dominance, overdominance and epistasis)
• Used to identify superior specific cross combinations
• Important for traits showing heterosis or hybrid vigour

Key Points:

• GCA is useful for selecting parents, while SCA is useful for selecting specific crosses
  • High GCA indicates a parent’s ability to produce superior progeny when crossed with a variety of other parents
  • SCA is important for identifying exceptional hybrid combinations that perform better than expected based on parental GCA

Recurrent Selection and Reciprocal Recurrent Selection

Recurrent Selection (RS)

• Highly inbred lines with good GCA employed to test a new line
  • Test cross progeny are evaluated
  • Males and females within the lines are selected based on the progeny testing and used in their own lines to produce the next generation
  • Used for improving single lines
  • Effective for traits with high heritability

Reciprocal recurrent selection (RRS)

• Two highly inbred lines used to produce progeny
  • Each line acts both as a source material for selection and a tester for the other population
  • Say two lines ‘A’ and ‘B’
  • Males of ‘A’ are mated with females of ‘B’ and vice versa
  • Progeny produced are judged for their performance, further selection is done based on it within the lines
  • Next, males of ‘A’ are mated with females of ‘A’ (similarly for ‘B’) to produce the next generation of parents to be tested.
  • The test cross progeny and the individuals not selected as parents are discarded
  • The lines ‘A’ and ‘B’ are assumed to have a high degree of homozygosity at all loci, but in opposite ways, such that application of RRS make the lines even more opposite w.r.t. the homozygous loci. Therefore, the progeny produced will have high degree of heterosis.
  • Widely used in commercial poultry breeding
  • Uses GCA of both lines and SCA of cross to produce superior progeny