Mating Methods and Genetics Help
Mating Methods—Random Mating
Once the selected individuals have been chosen, they may be mated in various ways. The process known as "breeding" includes the judicious selection and mating of individuals for particular purposes.
Random Mating (Panmixis)
If the breeder places no mating restraints upon the selected individuals, their gametes are likely to randomly unite by chance alone. This is commonly the case with outcrossing (non-self-fertilizing) plants.Wind or insects carry pollen from one plant to another in essentially a random manner. Even livestock such as sheep and range cattle are usually bred panmicticly. The males locate females as they come into heat and inseminate them without any artificial restrictions. Most of the food that reaches our table is produced by random mating because it is the most economical mating method. This mating method is most likely to generate the greatest genetic diversity among the progeny.
Positive Assortative Mating
This method involves mating individuals that are more alike, either phenotypically or genotypically, than the average of the selected group.
- Based on Genetic Relatedness. Inbreeding is the mating of individuals that aremore closely related than the average of the population to which they belong. Figure 8-10(a) shows a pedigree in which no inbreeding is evident because there is nocommon ancestral pathway from B to C (D, E,F, and G all being unrelated). In the inbred pedigree of Fig. 8-10(b), B and C have the same parents and thus are full sibs (brothers/sisters). In the standard pedigree form shown in Fig. 8-10(b), sires appear on the upper lines and dams on the lower lines. Thus, B and D are males; C and E are females. It is desirable to convert a standard pedigree into an arrowdiagram for analysis [Fig. 8-10(c)]. The coefficient of relationship (R) estimates the percentage of genes held in common by two individuals because of their common ancestry. Since one transmits only a sample half of one's genotype to one's offspring, each arrow in the diagram represents a probability of 2. The sum (Σ) of all pathways between two individuals through common ancestors is the coefficient of relationship.
- On an individual basis, the coefficient of inbreeding indicates the probability that the two alleles at any locus are identical by descent, i.e., they are both replication products of a gene present in a common ancestor.
- On a population basis, the coefficient of inbreeding indicates the percentage of all loci that were heterozygous in the base population that now have probably become homozygous due to the effects of inbreeding.The base population is that point in the history of the population from which we desire to begin a calculation of the effects of inbreeding. Many loci are probably homozygous at the time the base population is established. The inbreeding coefficient then measures the additional increase in homozygosity due to matings between closely related individuals.
- If the common ancestor is not inbred, the inbreeding coefficient of an individual (Fx) is half the coefficient of relationship between the sire and dam (RSD):
- If the common ancestors are not inbred, the inbreeding coefficient is given by
- If the common ancestors are inbred (FA), the inbreeding coefficient of the individual must be corrected for this factor:
- The coefficient of inbreeding of an individual may be calculated by counting the number of arrows (n) that connect the individual through one parent back to the common ancestor and back again to the other parent, and applying the formula
- Based on Phenotypic Similarity. Positive phenotypic assortative mating is seldom practiced in its purest form among the selected individuals, i.e., mating only "look-alikes" or those with nearly the same selection indices. However, it can be used in conjunction with random mating; a few of the best among the selected group are "hand-coupled," artificially cross-pollinated, or otherwise forced to breed.
EXAMPLE 8.17 In the arrow diagram of Fig. 8-10(c), there are two pathways connecting B and C. The coefficient of relationship between individuals B and C (RBC) = Σ(1/2)s, where s is the number of steps (arrows) from B to the common ancestor and back to C.
B and C probably contain (1/2) (1/2) = 1/4 of their genes in common through ancestor D.
Similarly, B and C probably contain 1/4 of their genes in common through ancestor E.
The sum of these two pathways is the coefficient of relationship between the full sibs B and C: RBC = 1/41/4 = 1/2 or 50%.
When matings occur only between closely related individuals (inbreeding), the genetic effect is an increase in homozygosity. The most intense form of inbreeding is self-fertilization. If we start with a population containing 100 heterozygous individuals (Aa) as shown in Table 8-2, the expected number of homozygous genotypes is increased by 50% due to selfing in each generation.
Other less intense forms of inbreeding produce a less rapid approach to homozygosity, as shown graphically in Fig. 8-11.
As homozygosity increases in a population, due to either inbreeding or selection, the genetic variability of the population decreases. Since heritability depends upon the relative amount of genetic variability, it also decreases, so that in the limiting case (pure line) heritability becomes zero, meaning there is no genetic variation for selection to act upon.
When population size is reduced to a small isolated unit containing less than about 50 individuals, inbreeding very likely will result in a detectable increase in genetic uniformity. The coefficient of inbreeding (symbolized by F) is a useful indicator of inbreeding at two levels.
The coefficient of inbreeding (F) can be determined for an individual in a pedigree by several similar methods.
Fx = (1/2) RSD
where p1 is the number of generations (arrows) from one parent back to the common ancestor and p2 is the number of generations from the other parent back to the same ancestor.
The following table will be helpful in calculating F:
Linebreeding is a special form of inbreeding utilized for the purpose of maintaining a high genetic relationship to a desirable ancestor. Figure 8-12 shows a pedigree in which close linebreeding to B has been practiced so that A possesses more than 50% of B's genes. Individual D possesses 50% of B's genes and transmits 25% to C. Individual B also contributes 50% of his genes to C. Hence, C contains 50% + 25% = 75% B genes and transmits half of them (37.5%) to A. Individual B also contributes 50% of his genes to A. Therefore, A has 50% + 37:5% = 87:5% of B's genes.
EXAMPLE 8.18 A beef cattle rancher may maintain a small "show string" in addition to a commercial herd. The few show animals would be closest to the ideal breed type (conformation of body parts, size for age, color markings, shape of horns, etc.) and would be mated like-to-like in hopes of generating more of the same for displaying at fairs and livestock expositions. The rest of the herd would be randomly mated to produce slaughter beef. Some of the cows from the commercial herd might eventually be selected for the show string; some of the young bulls or cows of the show string might not prove to be good enough to save for show and yet perform adequately as members of the commercial herd.
Both inbreeding and positive phenotypic assortative mating tend to reduce genetic heterozygosity, but the theoretical end results are quite different.
EXAMPLE 8.19 As a model, consider a quantitative trait governed by two loci, each with a pair of alleles both additive and equal in effect. Inbreeding among the five phenotypes would ultimately fix one of four homozygous lines (AABB, AAbb, aaBB, aabb). Positive phenotypic assortative mating would fix one of two lines (AABB and aabb).
The rate at which heterozygous loci can be fixed (brought to homozygosity) in a population can be greatly accelerated by combining a system of close inbreeding with the additional restriction of positive phenotypic assortative mating; in other words, they must also "look" alike.
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