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Genetic Improvement and Crossbreeding in Meat Goats
Lessons in Animal Breeding for Goats Bred and Raised for Meat
Will R. Getz
Fort Valley State University
Appendix B. Fundamental Genetics for Understanding Breeding Stock
Change Genes, Chromosomes, Alleles, Nucleus, DNA
Appendix Contents
•
•
Introduction
o Genotype
o Expected outcomes in the formation of new offspring
o Randomness – a key to understanding this stuff
o It ain’t all dominance and recessive
The Concept of Gene Frequency
o Factors that change it and implications for meat goat breeders
o Role of selection
o Simply-inherited versus polygenic traits
o Threshold traits
Introduction
The basic unit of inheritance is called a gene. Genes that are located at the same site in the
chromosome and which affect the same trait but in different ways, are called alleles. Genes are
segments (pieces) of deoxyribonucleic acid (DNA). The whole strand of DNA is referred to as a
chromosome. DNA is a very complex molecule that forms the genetic code for all living things.
Chromosomes occur in pairs; one chromosome of a pair inherited from the sire (father) and the
other chromosome inherited from the dam (mother). These pairs are known as “homologs”
because while each one may contain different genetic information, they each affect the same
traits and are of the same shape and composition.
The number of chromosome pairs is species-specific. All domestic goats have 60 (30 pairs)
chromosomes in each body cell. When cells divide by mitosis (cell division) the number of
chromosomes remains constant (60), and the genes associated with each chromosome regulate
all aspects of the body biology. The major exception to this applies to the sex cells or gametes
(sperm and eggs) which have half the number of chromosomes (30), or one of each pair. This is
necessary so that at fertilization, when the sperm and eggs join, the outcome is the proper
number of chromosomes. The half number “n” is produced through meiosis (reduction division)
whereby the cells divide twice and reduce the number of chromosomes to half.
The location of any pair of genes on the strand of DNA (chromosome) is known as its “locus”; it
is the site of a particular gene. At each locus there will be a pair of genes --- one inherited from
the sire and the other inherited from the dam. The genes at a particular locus are designated by a
letter or a letter/number combination. Different “forms” of the same gene, creating different
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effects for the same trait, are known as “alleles”. These alternative forms of the gene are
designated by the same letter but with different numbers or uppercase/lower case. Examples
include:
•
•
A1 and A2 for the alleles at one locus (the “A” locus), and perhaps B1 and B2 for the
genes (alleles) at another locus.
A and a for the alleles at one locus, and perhaps B and b for the genes or alleles at
another locus affecting another trait or characteristic.
B
B
(For those of you who are inclined to jump ahead or to come to premature conclusions, the upper
and lower case as used here have nothing to do with type of gene action, e.g.
dominance/recessive. Here they are just used to designate alternate form of the gene)
Although there will be only two genes at a particular locus for each individual, those two may be
a subset of a larger series of alternative forms of a gene. There may be “multiple alleles”
involved such that A1, A2, A3 and A4 may exist even though only two of the four will be paired
in the same individual, e.g., A1 A3. As an example, the extension of color pigment in the
Labrador Retriever breed of dogs, is influenced by three alleles and the various combinations of
two of them will suggest the amount of pigment in the hair coat. What little research evidence is
available on “simple” traits such color or color pattern in goats suggests that even that
inheritance involves several alleles.
Genotype
The combination of genes at a particular locus is referred to as a genotype, or a one-locus
genotype. A one-locus genotype is considered homozygous if both genes at that locus are
functionally the same, e.g. A1A1 or BB. One-locus genotypes containing functionally different
genes are considered heterozygous, e. g. A1A2 or Bb. The terms occur because of the root
“homo” meaning alike and “hetero” meaning different and “zygous” making reference to the
zygote or living organism at time of conception.
The Basics in Biology - Mendelian Inheritance. What Mendel knew about meat goat
genetics/what he did not, or, it ain’t all dominance and recessive. Coming back to our friend
Gregor Mendel; there are two fundamental biological laws that he identified that apply to
inheritance. The first was the law of segregation which states that in the formation of a gamete
(sperm or egg), the two genes at a locus in the parent cell are separated and only one of the two
becomes a part of each new germ cell or gamete. That process is quite complicated and today we
know the process as meiosis. The process involves a number of intricate steps during which not
only genes but entire homologous chromosomes are separated. Mendel knew nothing about the
details but had the process reasonable right based on observation — that is, that sperm and eggs
contain only one gene of each pair in the cell.
Mendel’s second law was the law of independent assortment. This means that during meiosis
genes assort independently --- they can assort with any other genes so long as all possible
gametes are formed in equal proportions. For this to happen a given gene from one locus must
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have an equal probability of being present in the same germ cell with either of the two genes
from some other locus.
As an example, consider a genotype like AaBb (two pairs of genes; two loci “A” and “B”). Meat
goats with an AaBb genotype can produce four possible gametes (sperm or eggs) – AB, Ab, aB,
and aa. If all four gametes occur in equal proportions then these genes have assorted
independently. Mendel was lucky. Genes on different chromosomes controlled the traits he chose
to observe, in his pea plants. He did not experience and knew nothing about linkage.
Chromosomes assort independently so the genes on those chromosomes assort independently
too. Because all the genes Mendel was studying did in fact assort independently, he came to
believe that all genes assort independently.
We know now that there are exceptions to the law of independent assortment --- these exceptions
occur now and then, and Mendel knew nothing about them:
•
Linkage – Two or more loci (the genes at two or more loci) are linked if they occur on
the same chromosome. They are inherited together rather than independently. This is one
basis of current efforts in genetic marker-assisted selection. During meiosis genes on the
same chromosome tend to end up in the same gamete. This is not complete, because of
the mechanism of crossing over.
•
Crossing over – Involves a reciprocal exchange of chromosome segments between
homologs, and occurs during meiosis prior to the time the chromosomes are separated to
form gametes. In the crossover process mutual breaks occur at identical sites on each
chromosome, and chromosome fragments are exchanged between the two homologs.
These exchanges prevent linkage from being complete.
If linkage occurs then the actual number of gametes of each type will not occur in equal
proportions, as would be the case under independent assortment, but at some level whereby two
of the four possibilities in our example will occur at a much higher frequency as compared to the
other two. Crossing-over may occur at only one point along the chromosome, but it is more
common for multiple crossover events to occur. The longer the chromosome the more likely that
multiple crossover will occur.
Expected outcomes in the formation of new offspring
When new offspring (zygotes) are formed by the joining of a male gamete and a female gamete,
they have the normal number of genes and chromosomes for that species. Half the chromosomes
will be contributed by the sire and half from the gamete contributed by the dam. Determination
of which sperm will join with which egg, is a random process. Nearly all gametes have an equal
chance of contributing to a zygote.
A useful device for visualizing how this process works is the Punnett Square. Although an oversimplification for most traits, this tool can help you come to an understanding of how individual
gene pairs contribute to the next generation in going from gametes to zygotes. In this example an
A1,A2 B1,B2 male is mated to an A1,A2 B1,B2 female. Each parent can produce four distinct
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gametes (sperm or eggs), one from each pair, so there are four rows and four columns in the
Punnett Square which then results in 16 cells as listed in the table, when they are combined.
Example No. 1
Female gametes (eggs)
A1 B1
A1 B2
A2 B1
A2 B2
Male gametes
A1 B1
A1A1 B1B1 A1A1 B1B2 A1A2 B1B1 A1A2 B1B2
A1 B2
A1A1 B1B2 A1A1 B2B2 A1A2 B1B2 A1A2 B2B2
A2 B1
A1A2 B1B1 A1A2 B1B2 A2A2 B1B1 A2A2 B1B2
A2 B2
A1A2 B1B2 A1A2 B2B2 A2A2 B1B2 A2A2 B2B2
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
B
By looking closely at the cells it becomes obvious that not all cells are unique. Some of the
genotypes are repeated. In this example there are nine distinct types of zygotes. Further, the order
of listing of alleles within a locus has no importance, e.g., there is no difference between B1B2
and B2B1. With the information from the Square, it is possible to determine the likelihood or
probability of any particular offspring genotype occurring by noting the frequency of the cells
that contain that genotype for “simply-inherited” traits (traits affected by only a one or a few
pairs of genes) you can also determine the expected proportions of offspring phenotypes.
B
B
This would not be the case for more complex polygenic traits. Polygenic traits are controlled by
many genes, where no single gene has an overriding influence. In considering issues of
likelihood and probability, there are two fundamental rules that must be assumed:
•
The probability of two independent events occurring together is the product of the
probabilities of their individual occurrences. For example if the combination at the “A”
locus in the above table, is independent of what is going on at the “B” locus, then if the
probability of having an A1A1 genotype is 0.25, and if the probability of having a B2B2
genotype is 0.25 then the probability of having an A1A1B2B2 genotype out of all the
possibilities is 0.25 x 0.25 = 0.0625; or we could say 6.25% or 1/16th.
B
•
B
The probability of one or the other of two mutually exclusive events occurring is equal to
the sum of the probabilities of their individual occurrences. The probability of tossing a
die twice and getting a one or a six each time is equal to the probability of the sum of the
single probabilities. That is 1/6 + 1/6 = 2/6 = 1/3.
Randomness - a key to understanding this stuff
There is a significant amount of randomness in what goes on with meat goat, and other livestock,
inheritance. Mathematical rules, guidelines, and expectations do exist, but outcomes are often
measured in terms of probabilities rather than absolute outcomes. This is partially because of the
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tremendous genetic variability that exists, and the many genes that influence most traits of
economic importance. The randomness of inheritance is critically important from an
evolutionary standpoint and you will see that it is also vitally important to the success of artificial
selection. Nonetheless it does create problems for breeding improved meat goats because it
reduces our ability to control the outcomes of matings. There is no control over the Mendelian
sampling of genes which determines the genetic makeup of the offspring. An individual superior
offspring or an individual inferior offspring from a particular mating does not mean that worse or
better results will occur in the future. Genetics of goat breeding involves chance, and to some
degree some luck or good fortune. When you begin to think that you are entirely in control as a
breeder, you are in serious danger of overestimating your abilities in biology.
It ain’t all dominance and recessive
Gene action is the term that is used to describe the expression of genes in the phenotype. There
are two general categories and several additional specific categories which will be visited briefly.
The two general categories are additive and nonadditive gene action. Nonadditive gene action is
the expression of most simply-inherited traits — those that are easily observed. Two general
categories exist for nonadditive gene action:
•
Dominance – defined as an interaction between genes at a single locus. There are four
degrees of dominance.
o
Complete dominance – defined as when the expression of the heterozygote is
identical to the expression of the homozygous dominant genotype. The mode of
gene expression at the “height” locus in Mendel’s experiments was complete
dominance. It was the only type of gene action he observed.
o
Partial dominance occurs when the expression of the heterozygote is intermediate
to the expressions of the homozygous genotypes and more closely resembles the
expression of the homozygous dominant genotype. An actual, if not meat goat,
example is the condition in horses, particularly show and pleasure horses, known
as HYPP (hyperkalemic periodic paralysis) where in there are episodes of muscle
tremors ranging from shaking to complete collapse. It is also an example of a trait
that could be easily eliminated because the condition is expressed in both the
homozygous dominant and heterozygous individuals, i.e. it is visible. However
because these individuals are often show winners, members of the show horse
fraternity often lack the will to do so.
o
No dominance exists if the expression of the heterozygote is exactly midway
between the expressions of the homozygous genotypes. Both alleles appear to
have equal expression. A hypothetical example might exist when a homozygous
individual carrying two resistant genes would survive 100% of the time when
exposed to a communicable pathogen; the homozygous individual carrying two
susceptible genes would survive only 40% of the time, and the heterozygous
population would survive 70% of the time.
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o
•
Overdominance is the most extreme form of dominance. The expression of the
heterozygote is outside the range defined by the expressions of the homozygous
genotypes and most closely resembles the expression of the homozygous
dominant genotype. Overdominance is often characterized as having an extreme
genotype. An instructive example involves survivability in wild rats in regard to
the anticoagulant poison warfarin. In places where warfarin is used, rats without
the resistance gene die from warfarin poisoning, rats homozygous for the
resistance gene suffer from vitamin K deficiency because the poison increases the
need for vitamin K, and the heterozygote rats remain healthy. The heterozygote
rats are healthier than either of the two homozygous populations.
Epistasis – another form of nonadditive gene action in which there is interaction among
genes at different loci such that the expression of genes at one locus depends on the
alleles present at one or more other loci. Color or color pattern in several species is
governed by epistasis, especially in regard to dilution genes.
Illustration of types of dominance on a line of comparative results at the “A” locus
No dominance =
________AA________________Aa________________aa______
Partial dominance =
_______ AA________Aa________________________aa______
Complete dominance =
_______AA__________________________________aa______
Aa
Overdominance =
_Aa_ _AA_ _____________________________ aa_____
Additive or independent gene action generally occurs when traits are influenced by many pairs of
genes (polygenic) and each gene has small but additive effects that accumulate. Years of
breeding research has indicated that most of the performance traits of economic importance are
governed by additive gene action. This is related to breeding value.
The Concept of Gene Frequency
Factors that change it and implications for meat goat breeders
Mendelian principles again explain genetic mechanisms operating in individual goats. However
the charge to meat goat breeders is not to change individuals, but populations of individuals
(herds or breeds). In describing an individual for some simply-inherited trait, reference might be
made to the specific genes which that individual possesses or the one-locus or two locus
genotype might be described.
To describe populations genetically the answer is to use gene and genotypic frequencies. A gene
frequency or allelic frequency is the relative frequency of a particular allele in a population. It is
a measure of how common that allele is relative to other alleles that occur at that locus. Relative
frequencies range from zero to one. If an allele does not exist in a population then its gene
frequency is zero. As an example in a herd of black Angus cattle, if there are no “red” alleles, the
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gene frequency for red is zero. Likewise in that same herd the gene frequency for the “black”
allele is one. In a black Angus herd in which an occasional red calf is born, the frequency of the
red allele will be something greater than zero. The sum of the frequencies of the various alleles
(two or multiple) at a single locus in a population must equal one. Not all alleles are of equal
merit or desirability.
Role of selection
From a population genetics standpoint, the effect of selection is to increase the gene frequency of
favorable alleles. When replacement does are chosen the owner is attempting to retain those
animals with the best sets of genes and reject those with poorer sets of genes. As a result,
offspring in the next generation should have, on average, better sets of genes than the current
generation. Another way of saying better sets of genes is to say better breeding values. Gene
frequencies, average breeding values, and mean (average) performance are really tied together.
Although selection is not the only force that can change gene frequencies in a population, it is the
most powerful one available to most meat goat breeders.
Simply-inherited versus polygenic traits
Simply-inherited traits are affected by only a few genes. Only a single locus or at most, a few
loci are involved in their expression. There are two common secondary characteristics of simplyinherited traits:
•
•
They tend to be “either / or” or categorical in nature.
They are typically affected very little by environmental factors.
Polygenic traits are affected by many genes, and no single gene is thought to have an overriding
influence. Examples include growth rate, feed efficiency, and ribeye area of the carcass.
•
•
•
•
Phenotypes for polygenic traits are usually described by numbers, e.g. 0.45 pounds per
day in gain, 40 pound weaning weight, 1.89 square inch ribeye.
Phenotypes for polygenic traits are typically quantitative or continuous in their
expression rather than either / or.
Most, although not all, polygenic traits are also quantitative traits.
Polygenic traits are clearly affected by environmental factors to varying degrees.
Be aware that there can be some crossover between the secondary characteristics of simplyinherited and polygenic traits. Size is an example wherein mature size is clearly polygenic but
some dwarf body types are simply-inherited and controlled by a major gene as well.
Threshold traits
There is a special category of traits which are polygenic in control but in which the phenotypes
are expressed in discrete categories, not different from simply-inherited traits. Two such traits
that come to mind are dystocia and fertility, as reflected in conception rate (pregnant versus not
pregnant).
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These threshold traits present special challenges for the breeder. Fertility is believed to be
influenced by many genes and is therefore polygenic. But the trait may be measured in only two
phenotypes; pregnant or nonpregnant. Dystocia is measured in only three to five categories
although there is evidence that it is polygenic. Threshold traits are no different from quantitative
polygenic traits in regard to genotype. How do we deal with this? Well think of a threshold trait
as having a continuous but unobservable underlying scale of expression; a liability scale. Think
of an animal’s liability for a threshold trait as the sum of its genetic values for the trait. On the
liability scale there is a point above which animals exhibit one phenotype, and below which they
exhibit another.
Simply-inherited and polygenic traits have a great deal in common as noted below:
•
•
•
Genes affecting both kinds of traits are subject to the same Mendelian mechanisms.
Mendel’s laws of segregation and independent assortment apply equally.
Dominance and epistasis affect gene expression for both kinds of traits.
The basic tools of goat breeding, selection and mating, are the same for both types of
traits. Selection focuses on increasing the frequencies of favorable alleles.
However, different approaches to genetic improvement are taken between the two types of traits.
This different approach is primarily a function of the number of genes involved. The more genes
affecting a trait, the more difficult it is to observe the effects of individual genes, and therefore
the less specific information we have about those genes. The amount of available information
affects the way we characterize genotypes and therefore determines the breeding technology to
be used. To add to this difficulty, consider the fact that relatively few studies have been made on
the specific genetics of meat goats. There is somewhat more for dairy goats, but even so not
abundant amounts. Test matings may be advisable in some cases to more clearly identify or
describe the genotype.
Polygenic traits are affected by so many genes that it is extremely difficult to observe the effects
of specific loci and specific alleles at those loci. It is impossible then to explicitly identify an
individual’s many – locus genotype for a polygenic trait. The logical alternative is to characterize
the net effect of the individual genes influencing the trait. That is, to quantify the individual
performance and breeding value for a trait. This requires statistical tools including concepts such
as heritability and accuracy. The terminology will change from A1A2 or Bb used for simplyinherited traits, to the terminology of polygenic traits, e.g. EBV’s, EPD’s, ACC’s and the like.
________________________________________________________________________________________________________
Information contained in this document is part of a web-based training and certification program for meat goat producers
(http://www2.luresext.edu/goats/training/qa.html) that was developed with funding received by Langston University from
USDA/FSIS/OPHS project #FSIS-C-10-2004 entitled "Development of a Web-based Training and Certification Program for
Meat Goat Producers."
Collaborating institutions/organizations include Alcorn State University, American Boer Goat Association, American Kiko Goat
Association, American Meat Goat Association, Florida A&M University, Fort Valley State University, Kentucky State
University, Langston University, Prairie View A&M University, Southern University, Tennessee Goat Producers Association,
Tennessee State University, Tuskegee University, United States Boer Goat Association, University of Arkansas Pine Bluff, and
Virginia State University.
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