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E2

Understanding Inheritance

  • Andrew James, "Bathampton", QLD.

It is necessary to understand something about the way traits are inherited before planning a breeding program.

Kinds of traits

There are basically two kinds of traits. One kind is simply inherited and the other is more complex or polygenic. The way an animal appears is called the phenotype, whereas the genetic makeup for that animal is called the genotype.

Simply inherited traits

Simply inherited traits are conditioned by a small number of genes acting at one or two loci. Genes on a chromosome may be thought of as beads on a string. Each chromosome is generally paired by another of the same length, gene for gene along their entire length. The position on the chromosome where the gene resides is called a locus. There is one gene at the same locus on each chromosome. Either gene may generate the necessary message to ensure that the trait occurs. However, genes are not necessarily equal in their activity. If a dominant gene occurs at a locus matched with a recessive gene on the other chromosome, the dominant gene will override the activity of the recessive gene and the animal will appear as if only the dominant gene were present. That is the phenotype of the animal will be that conditioned by the dominant gene, whilst the genotype will be heterozygous. Heterozygous means mixed. If an animal has two recessive genes, both the phenotype and the genotype of the animal will match the trait conditioned by the recessive gene. If additive genes occur at the locus, each gene will have a measured but additive effect.

    Table 1. Simply inherited traits
    Trait Inheritance Number of loci
    Polled/ horned Polled is dominant 1
    Normal/ hermaphrodite Hermaphrodite is recessive and only expressed in females. Homozygous polled females are nearly always hermaphrodite, that is they appear intermediate between male and female and are sterile. The gene for hermaphrodite is either the same gene as for polled or is very closely linked. 1
    Wattled neck/ smooth Presence of wattles on the neck is dominant. Common in some strains of feral and milking goat. 1
    Normal/ cryptorchid Cryptorchid, one or both testicles not descended is recessive but not always expressed. Some males may be homozygous for cryptorchid but appear normal. Common in mohair goats before recent introductions from Texas and South Africa. 1
    Normal/ aetresia ani Aetresia or blocked lower bowel is recessive and fatal. Common in mohair goats before recent introductions from Texas and South Africa. 1
    Normal/ Beta mannosisosis B Mannosisosis is a recessively inherited abnormality. Kids are unable to stand from birth due to fixed position of the limbs. Found in some strains of the Anglo Nubian breed. 1
    Medium/ short/ long guard hair length Medium appears dominant to both long and short. 1
    Short hair/ Long Britch In otherwise short haired goats, Long hair on the back legs appears dominant to short. In entire males long hair appears to extend further forward than in females. 1
    White/ coloured White is dominant and includes animals with biscuit colour or what the cashmere industry would class as WC. 1
    White down/ pigmented In coloured animals white down appears to be dominant to pigmented down. 1
    Grey/ black Among coloured animals Grey is dominant to black 1
    Light grey/ dark grey Grey pigment appears to be conditioned by a gene series where Light is dominant or additive to dark. Entire males tend to be darker than females of the same genotype. 1?
    Alpine/ solid colour Alpine markings appear dominant to solid colour 1
    Saddle/ solid colour A white band or saddle appears dominant to solid colour 1
    Solid colour/ coloured head. Solid colour appears dominant to coloured head (ie Boer) colouring 1
    Horn bead/ smooth Beaded edge to horn dominant to smooth in profile 1
    Tan skin/ pink skin In white goats Tan skin pigment appears dominant to white 1
    Small ears/ large Ear size appears to be conditioned by a gene series where Small ears are dominant or additive to large ears. 1?
    Nervous/calm temperament Among goats which are otherwise similarly treated, Nervous temperament appears dominant to calm. 1

Some simply inherited genes are listed in the Table 1. Some of these genes are documented in scientific literature and others are best-bet estimated by the author.

Although conditioned mainly by single genes, inheritance of colour within a mob of animals of mixed origin is likely to be complex. However if we consider the practicality of what the breeder may be interested in, the breeder may attempt to produce an all white herd or an all black herd. In the all black instance, this should be successful and become true breeding, that is when black animals are mated to black they should produce all black progeny. This is because the gene for black pigment is recessive to all lighter shades and solid colour is generally recessive to broken colours. A black animal should be homozygous for both black pigment and for solid colour and will therefore breed true. However, breeding a true breeding white herd is more difficult. This is because the white gene is dominant to the coloured gene and can therefore mask its effect. It is therefore possible and indeed common in some herds for coloured progeny to appear from matings of white parents. This even occurs occasionally in stud Mohair or Saanen goats with a long history of breeding white to white. A possible solution for this is for bucks used in the herd to be test mated to coloured does before being used in the main herd. A buck that produces coloured progeny will obviously be a carrier of the coloured gene.

Polygenic traits

Most production traits are inherited polygenically and are affected by the environment in which an animal exists. That is the phenotype is conditioned by the activity of multiple genes and by the environment in which the genes act. The effect of individual genes can generally not be identified, but are considered to act in additive, dominant or recessive manners. For animal breeding, the genes acting in an additive fashion are the most easily used. A simple way of looking at the variability conditioned by the relationship between the phenotype and the genotype is as follows:

    VPhenotype = VGenotype + VEnvironment

Animal breeders often wish to estimate the proportion of the variance in phenotype which is conditioned by the genotype. This measure is called the heritability (h2) and is calculated as:

    Heritability (h2) = VGenotype / VPhenotype

The heritability gives a measure of the accuracy with which a trait may be measured. For example with excellent record keeping, a highly variable flock and making adjustments for if an animal was a male or female, born a single, twin or triplet and date of birth and by removing any obvious errors in fleece testing, the cashmere breeding project at Wollongbar was able to achieve an heritability of 61 percent for down weight, which is considered quite high (Table 2). However, if for example a flock with a narrow genetic base was used and only visual estimates of fleece production made, then heritability of down production may be close to zero and little genetic progress would be possible.

    Table 2. Heritabilities with standard errors of liveweight and fleece characteristics of Australian goats (Pattie and Restall, 1989)
    Liveweight 0.29 0.10
    Fleece weight 0.29 0.10
    Yield 0.90 0.23
    Down weight 0.61 0.18
    Down diameter 0.47 0.15
    Down length 0.70 0.19

When considering using an animal for breeding it is necessary to consider more than just their performance for a single trait. Some traits are correlated with other traits (Table 3). There is a weak negative genetic correlation (-0.18) between liveweight and down weight of cashmere in a fleece. This means that animals which are genetically larger will also have a slight genetic tendency to produce less down weight. The size of this correlation is relatively small so it should be possible to make simultaneous gain in both down weight and body weight. Of greater importance is the very strong correlation (0.62) between down weight and fibre diameter. This means that animals with a genetic predisposition toward producing more down will have a strong tendency toward higher fibre diameter.

    Table 3 Genetic correlations with standard errors in brackets above the diagonal, phenotypic correlations below the diagonal among traits in Australian goats (Pattie and Restall, 1989)
    Live weight Fleece weight Yield Down weight Down diameter Down length
    Liveweight 0.17 (0.18) -0.24 (0.17) -0.18 (0.17) -0.06 (0.18) -0.31 (0.16)
    Fleece weight 0.21 -0.13 (0.05) 0.39 (0.04) 0.14 (0.05) 0.05 (0.18)
    Yield -0.07 -0.11 0.84 (0.02) 0.57 (0.04) 0.78 (0.02)
    Down weight 0.07 0.41 0.80 0.62 (0.03) 0.88 (0.02)
    Down diameter 0.12 0.18 0.36 0.46 0.52(0.04)
    Down length -0.05 0.24 0.57 0.65 0.35

In practice, if traits are correlated and one trait is selected for then the other trait will be affected to the degree that traits are correlated. For example, a breeder may choose to select for increased down weight by selecting animals with the longest down length. This is an easy and cheap measure to perform and since down weight and length are highly correlated (0.88), this should result in good progress in down weight. However, it will also have the effect of increasing fibre diameter, though to a lessor extent as it is not so tightly correlated with length (0.52). It is however necessary to take measures to control fibre diameter otherwise, the breeder may find that fibre diameter falls outside the cashmere range. There is no good surrogate measure for fibre diameter, so in order for it to be maintained within a reasonable range, it would need to be measured directly.

Estimated Breeding Values

Another important measure of the genetic worth of an animal is called the breeding value. At its simplest, this is a measure of the difference of an animal from the average of its peers. For example if an animal has a down weight of 250 grams and its peers average 200 grams, the variance from average would be +50 grams. However as this variance includes the effect of environment, the true breeding value would be less than this.

    Vindividual x Heritability = Breeding value

In this case the heritability of down weight is reported to be about 61 percent. So this animals breeding value would be 50 x 0.61 or +30.5 grams. However because this is only an estimate of an animals breeding value it is called an estimated breeding value or an EBV for short. When used as a parent in breeding, the contribution to the progeny from one parent is half the EBV of that parent because the progeny acquire an equal genetic contribution from both parents. EBVs can be improved in their accuracy by taking into account the EBVs of all known relatives and their relationship to an individual animal. This is called a Best Linear Unbiassed Predictor or BLUP for short. The mathematics required to calculate a BLUP is too complex to present here, but it is easily calculated by a number of computer programs. The more relatives of an animal that are recorded the more accurate the EBV. This is because, for traits that are inherited, an animal will be more like its close relatives than other animals within the herd, and the more relatives an animal has the more accurate the knowledge of its genetic background will be. For example an EBV on a buck with 50 recorded progeny will be much more accurate than on a kid with no known relatives and only its own performance. An enormous benefit of EBVs is that they can be used to compare animals from different environments, independent of the effect of environment, provided that there are related animals in the different environments.

An example of the advantage of using EBVs is presented in Table 4. In this example, the performance of a buck like "Gladiator" with his very high down weight (653 grams at 22 months of age) may lead a breeder to use this buck extensively. However inspection of his EBV data shows that his breeding value is only slightly above average (+12.0 grams) for adult down weight, whilst his breeding value for fibre diameter as an adult (+ 0.67 um) was much stronger than average. As a comparison, the buck "636" has EBVs of similar magnitude, but only slightly more than half the down weight of Gladiator, yet the average performance of their progeny was similar.

    Table 4. Performance figures and estimated breeding values for some well-known industry sires from the 1985-90 era and two bucks from a single herd. Buck "636" was a contemporary of the industry sires, and "P147" a more recent sire. Data is based on analysis of 1329 fleece tests on 786 progeny born within the herd over an eleven-year period commencing in 1988.
    Sire and number of progeny recorded Age at test (months) Down weight (g) Fibre Diameter (um) Yield

    (%)

    Down weight (EBV) Fibre Diameter (EBV) Yield (EBV)
    Gladiator (40) 10 259 16.5 52.0 + 9.5 + 0.68 + 2.4
    22 653 17.1 52.2 + 12.0 + 0.67 + 1.5
    Emir (18) 10 174 13.1 40.2 - 35.8 - 0.94 - 4.2
    22 190 14.9 26.7 - 49.0 - 0.72 - 3.9
    Ghenghis Khan (86) 10 186 14.7 64.7 - 4.5 0.00 - 3.5
    22 410 17.0 34.0 + 11.1 + 0.41 - 4.3
    "636" (13) 10 211 16.4 40.5 + 7.6 + 0.06 - 2.3
    22 361 17.69 40.6 + 22.9 + 0.52 - 2.8
    P147 (57) 10 161 15.8 41.4 + 48.0 + 0.25 + 10.2
    22 543 15.96 55.1 + 104.9 + 0.11 + 10.3
    Long term average 10 110 14.98 37.3 0.0 0.00 0.0
    22 165 16.12 34.93 0.0 0.00 0.0

EBVs for down weight for the recent sire "P147" are high compared with those of the earlier bucks. When joined to average does he would be expected to produce progeny averaging 217 grams of down at 16.17 um as two year old animals, compared with Emir"s progeny which would be expected to produce 140 grams at 15.76 um. At around $100 per kilo for fine white, the difference in value of production would be expected to be $7.70 per animal. In this example the effect of using a different buck over average does was worth an additional $7.70 per animal in a single year. Gains of this magnitude are certainly worthwhile. Because the difference in performance is genetic, gains are permanent and cumulative within a herd. Greater gains are possible if larger numbers of animals are recorded either within herds or across herds, then the elite animals used widely in the national flock.

Acknowledgment. I am grateful for analysis of data leading to generation of EBVs conducted by the Advanced Breeding Services group within NSW Ag.

Referances

Pattie, W.A., and Restall, B.J. (1989) The inheritance of Cashmere in Australian Goats. 2. Genetic Parameters and Breeding Values. Livestock Production Science 21 251-261.

© 2000 ACGA