Mating Systems

Mating systems is one of two strategies to bring about Genetic variation the first being selection.

There are two categories within mating systems:
  • Random Mating: It uses Hardy-Weinberg, Genotypic frequencies is dependent upon allelic frequencies
  • Non-Random Mating : alternative strategies for genetic change.

Strategies for Genetic Change

  • 2 powerful strategies for available genes to breeders to bring about genetic change in a population of animals
  • Selection

Mating systems

Selection

  • Decide which individuals to retain as parents
  • Contribute genes to subsequent generations

Goal: Increase the frequency of desired trait, decrease the frequency of undesired traits

Mating Systems

  • Which males are mated to which females
  • Application of mating system leads to no further changes in allelic frequency beyond those accomplished by selection
  • Alters genotypic frequencies: Proportion of homozygous vs. heterozygous individuals in offspring generation

Kinds of Mating Systems

  • Random or Non-Random
  • Random: H-W (Hardy- Weinberg) any male can mate with any female

  No attempt by animal breeder to pair specific mates

  • Non-Random: Expected proportion of homozygous & heterozygous individuals deviate from H-W expectations

Non-Random Mating

Assignment of mates based on:
  • Genetic Relationship
  • Phenotypic similarity

Genetic Relationship

Based on Pedigree

Male & female mated together because:
  • More or less closely related: Inbreeding
  • Less closely related: outbreeding
Vs. Being chosen from population at random

Inbreeding

  • Attempt to make individuals more homozygous for superior genes of ancestor
Inbreeding Coefficient:
  • P that alleles at a locus in an individual are identical by descent
  • Reflection of the increased proportion of homozygous loci in an individual

Outbreeding (1)

  • Increasing Heterozygosity
  • Commercial meat animal production
  • Hybrid vigor

Phenotypic Similarity

  • Based on performance
Mate male & female because they:
  • Resemble one another more closely (for a trait) than if chosen randomly from the population: Positive assortative mating
  • Resemble one another less closely: Disassortative or negative assortative mating

Mating systems: List

  • Do not Alter allelic frequencies
  • Goal of Non-Random mating systems is to: Alter genetic frequencies
  • Inbreeding and assortative mating: Increase homozygosity
  • Outbreeding and negative assortative mating: Increase heterozygosity

Inbreeding

  • Mating of individuals more closely related than the average of population: Sire and Dam are genetically related
  • The More closely related: The more severe the inbreeding
  • Degree of inbreeding is quantified by Fx: which is the Inbreeding Coefficient
  • Fx measures the percent increase in homozygous gene pairs in an individual compared to the average of the breed
  • Question: If an individual has an Fx of 0.25 what does this tell you?
  • The Fx value for most herd of livestock won’t be more than 0.5
Factors of Inbreeding
  • It increases the frequency of homozygous genotypes
  • Can be traced by converting pedigrees to arrow diagrams: In arrow diagram individuals only appear once
  • Arrows run from ancestors to descendents showing the flow of genes through time
  • When an arrow heads away from an individual, it represents Mendelian segregation of genes: A sample of half of parents genes being transmitted to offspring
  • Individual X is inbred because the Sire and Dam are half siblings
Common ancestor factors
  • In General- the ancestor common to more than one individual
  • Inbreeding, refers to an ancestor common of an inbred individual
  • It can be tracked by following the flow of genes from A to X
  • The Sire and Dam (S & D) can inherit copies of a gene from the common ancestor (A)
  • X (the offspring) can inherit that gene from S & D, so X would be homozygous for that gene
  • This makes the gene identical by decent
  • The Results of Inbreeding increase homozygosity, beyond what would be found in a regular/randomly mating population

In these population can be alleles that are “alike in state” (HH), but the traits are not traceable to a common ancestor

Inbreeding Types & Effects

  • Linebreeding: A less intense form of inbreeding

  • It concentrates genes from a single common ancestor

  • Mates individuals within a particular line

  • It’s designed to maintain a substantial degree of relationship to a highly regarded ancestor

  • This method is common in the horse industry

  • (1) Prepotency: States that the performance of offspring is especially like \n that of the parent, esp. uniform

  • “likeness” referring to variation within the offspring

  • It attempts to make individuals more homozygous for superior genes

  • It is the degree to which an animal will pass on their characteristics consistency

  • (2) Expression of deleterious recessive alleles

  • This gives inbreeding a bad reputation

  • This doesn’t increase the amount of deleterious genes, only increases their expression

  • The frequency of detrimental alleles isn’t increased either

  • (3) Inbreeding Depression

  • Refers to the quantitative traits

  • It is a decrease in performance of inbreds for traits like fertility & survivability

  • This is caused by an unfavorable gene combination value

  • Known as Polygenic traits: when the Individual effects of genes are small, but taken \n together, can decrease performance

  • Inbreeding depression also causes: Poor gene combination value which can happen as a result of: increased homozygosity, and a change in genotypic frequency

  • Not all quantitative traits equally affected:

  • Repro & health – seriously affected \n Production traits – moderately affected \n Product quality traits –little affected

  • Another factor of Inbreeding:

  • Tends to “fix” characteristics (traits) in a population

  • Concentrates genes can be good or bad

  • Increases P of getting similar genes to offspring from ancestor

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Outbreeding

Outbreeding

Outbreeding

Outbreeding (2)

  • Mating of individuals that are less closely related genetically than the average of the population

This method is used extensively in commercial meat animal production

Within Breed

  • Crossing inbred line: Poultry industry

  • Several strains/lines of one breed \n • Within each line – intense selection for \n economically important traits \n • Over time – become genetically distinct \n

  • Grading up (also called Top-crossing) \n Mating purebred males (that are superior) \n to non-purebred females (grade) \n Attempt to create a purebred population

  • Crossbreeding → crossing different breeds \n • i.e. Duroc ♂ X Landrace ♀

  • There are also Species crosses

  • Closely related species: \n Horse (64) chromosomes) X donkey (62 \n chromosomes = mule (63 chromosomes) – \n can’t mate because chromosomes can’t \n pair normally at meiosis

  • Domestic cattle X American bison = \n “beefalo”

  • Bos taurus X Bos indicus: Important for beef production in Southern US

  • Angus (European) X Brahman (Indian) = Brangus

  • By combining growth rate, carcass attributes of \n Angus w/hardiness & adaptability to hot, \n humid conditions of Brahman \n 1st bred by USDA in coastal Louisiana

Effects of Outbreeding

  • It’s the opposite of inbreeding

  • Genetic = increased heterozygosity

  • Keeps most deleterious recessives in heterozygous form, so they are not \n expressed:AKA masking their expression

  • This does not not eliminate deleterious recessive alleles they are perpetuated via masking

  • Phenotypic = heterosis or hybrid vigor \n  Opposite of inbreeding depression

  • Heterosis can be defined as: the superiority, for many traits, of the offspring from outbred matings, in comparison to the average phenotypic merit of offspring from random matings within the population contributing to the cross (esp fertility and survivability)

  • By Observing heterosis, the net affect of influences at the loci can be determined

  • In essence, Heterosis is the degree of superiority of the outbred offspring for a particular trait as compared to the average of the parental line or breed

  • It is the difference between the average of all crossbreds, in comparison to the average of all purebreds contributing to those crosses

  • If crossbreeding is the mating system of choice, this should be the case for overall efficiency of production: \n Sometimes it is not: example \n  Dairy cattle production in US

  • No breed, when crossed with the Holstein, \n will produce F1 generation crossbred cow \n whose fluid milk yield exceeds that of \n straightbred Holsteins

  • (Although crossbred cows do produce \n more milk than the average of straightbreds contributing to the cross) HxH JxJ average purebreds \n 7000 kg 5000 kg 6000 kg \n H x J cross = 6300 kg

  • Meat Animal Production

  • crossbreeding used by most \n commercial producers (beef cattle, \n sheep, hogs)

  • Often crossbreds will not exceed the \n better purebred parent for individual \n traits such as survival, growth rate, feed \n efficiency, & carcass merit

  • However, heterosis for all the traits contributing to overall production efficiency will cumulate & result in crossbreds with greater net economic merit that the best average straightbred \n

Effects of Outbreeding

  • Calves of Angus ancestry sell for higher prices than cows with Angus ancestry

  • \n Sire breeds to compliment Angus \n  Those who would increase growth rate and muscling of \n crossbred calves

  • Many producers use Charolais or Simmental

  • So breed complementation & heterosis both \n contribute importantly to efficiency in herds using \n these systems

The degree of heterosis and complementarity depends on:

  1. Genetic relationship of parents
  2. Trait - h2 & ability to measure trait     h2 = heritability

 \n Measures the strength of the relationship \n between performance ( The phenotypic values) and \n breeding values (The genotype) for a trait in a \n population

1.Genetic Relationship of Parents \n  The more genetically diverse the background \n of the parents, the greater the expression of \n heterosis and complementation

Example 1) B. taurus x B. indicus tend to be \n superior to B. taurus x B. taurus \n Example 2)crossing highly inbred lines of 2 \n breeds of swine express greater heterosis \n than non-inbred lines of the same breed

In essence outbreeding takes advantage of \n heterosis & complimentarity from low h2 \n traits \n

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Breed complementation
  • Combining the desirable characteristic \n (traits) of 2 or more breeds (or lines)
  • Combine desirable aspects of two or more \n breeds into the same offspring
  • One breed in a cross can be chosen \n partially to compensate for deficiencies of \n another breed
  • EX: \n Commercial beef cattle producers in VA \n  Maintain herd of Angus (or Angus \n crossbred cows) \n  Angus characteristics: \n  Moderate mature size \n  Good fertility \n Milk production, somewhat higher than Hereford \n  Muscle is more highly marbled after a standard \n time on feed vs other breeds
  • Marblings, or intramuscular fat, is necessary \n for carcass to grade choice under USDA \n grading standards \n  Genetic potential for marbling is impt in \n establishing market value of feeder cattle

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Major uses of Outbreeding in the livestock industry

In General- It’s used to optimize the production of meat offspring

 \n Commercial cow/calf \n • Almost all commercial cows are \n crossbred (black baldy, H x A) \n • Very common beef animal in US \n Swine \n • Extensive crossing of inbred lines & breeds to maximize reproduction & growth (maternal lines)

Reciprocal Recurrent Selection – \n • A system of selection for increasing the combining ability of two (or more) lines (or breeds) that have demonstrated from past crosses that they combine well. \n  Development of superior inbred lines – \n  crossing more genetically diverse lines of two breeds = superior stock

Major Uses of Outbreeding

 \n Parents of best performing offspring are \n selected for next generation \n  Net result \n  Development of genetically diverse inbred \n lines \n  Better performing crossed lines would \n express greater heterosis \n  These inbred lines could then be crossed \n with inbred lines of other breeds

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Parents of The Best performing offspring are selected

Parents of The Best performing offspring are selected:

 \n Development of genetically diverse inbred \n lines \n  Better performing crossed lines would \n express greater heterosis \n  These inbred lines could then be crossed \n with inbred lines of other breeds

 \n Poultry – meat breeds use \n crossbreeding for growth advantages \n white cornish ♂ x white plymouth rock ♀ \n (growth & meat): Meatier, smaller

(maternal characteristics ): Larger plumage