Genetics 101: To BB or Not To bb
Maxine Kinne

 

Heredity - it gave me my mother's eyes and my father's chest. Maybe that's funny to you, but it's a good way to illustrate the inheritance of traits we see every day in people, plants and animals.

How can we get the kind of animals we want? Genetics. Progress can be fast or slow, depending on your degree of determination, and there are no absolute guarantees. All we can do on the road to improvement is to increase the chances of getting what we want by knowing where we've been, where we are, and where we want to go. And be prepared for a few surprises along the way.

Genetic terminology is descriptive, so you will learn some new words and what they mean. Some portions of this article are over-simplified for beginners, but they are based on sound principle and a place to start learning..

It takes more than breeding. Genetics is only one part of a 3-pronged improvement program. Equal amounts of correct feeding and management are required to see genetic improvement. These environmental factors, over which you have complete control, combine to allow animals to develop to their maximum genetic potential. Disregard for aspect of the breeding/feeding/gentics tio affects the others. For example, poor diet, parasites, neglected feet, crowding and many other problems can ruin the physical development of a genetically superior animal. If a genetically good goat suffers neglect, especially during its growth period, it may look less than desirable, but it will still have the genetics and be able to produce good qualities in its offspring.

Genetics is the science that deals with inheritance and its variations. You change the frequency of certain genes in your herd when you select (keep) offspring. For example, if you keep kids with narrow chests, you will fix (make permanent) that trait in your herd. To get wider chests, use a buck who has a wide chest, and whose immediate ancestors have wide chests. It's rare to get a better trait in the offspring than either parent has. It is also very important to use a buck who has the correct trait you are trying to improve rather than an exaggerated one.

Another way to manipulate the frequency of genes is through culling. Selling, neutering, butchering and euthanizing are all ways of eliminating what you don't want from your gene pool. Culling methods should relate directly to the severity of the deficit. For instance, kids of inferior quality can be sold, and those with congenital defects (present at birth) can be euthanized so they don't live to pass the trait. It seems harsh, but we can't afford to lose sight of what is best for the pygmy breed in the long run.

It's been said time and again, "The buck is half your herd." Each kid has half of his genes. When neatly planned breedings are foiled by an escaped buck who breeds the whole herd, you will get half his genes in every kid, whether you like it or not. The buck's contribution adds or subtracts quality in the kids. His impact on your herd is enormous because all his kids have half of his genes.

"Almost any buck can improve a poor herd, but it takes a superior buck to improve a good herd," is also an old adage, and true. Once we get the traits we think we want, breeding choices get more difficult as we try to refine smaller and smaller differences, keeping good traits and avoiding bad ones.

Phenotype (appearance) is determined by genotype (genetic makeup), provided that nutrition and management are both up to snuff. You keep the best looking goats. If they perform well, you breed and keep more like them. If perform badly, it makes sense to change your breeding program. Such changes are usually made by adding a new buck who embodies the type you want.

Genotype can be sort of guessed at by looking at an animal's ancestors. Mammals are 50% of each parent, 25% of each grandparent, 12.5% of each great-grandparent, and so on. If all the parents and grandparents uniformly have a certain trait, the individual stands a good chance of having it, too. Where ancestry is largely unknown, selective breeding needs to be practiced long enough for traits become evident and well-established in a herd. There will always be huge gaps in what we know about genotype because most traits are controlled by many genes, and each fertilization is a random event.

 

Genes are sequential chemical units within the DNA which forms the chromosomes. Each goat sperm and egg contain 30 randomly divided chromosomes. At fertilization, the chromosome halves join in zipperlike fashion to become 60 chromosomes, giving the offspring half the genes from each parent. Genes for the same traits are at the same location on the chromosomes, so they match.

Most traits are the product of many genes and have a continuous range of values from one extreme to the other, for example, short-to-tall and narrow-to-wide. Few traits are controlled by single genes, but single gene inheritance (Mendelian theory) is the easiest to understand, so that's what I'll use for purposes of illustration.

Genes are either dominant or recessive. Dominant genes are expressed in phenotype, what the animal looks like. Recessive genes are masked by dominant ones, unless they are homozygous.

Genes occur in pairs on the chromosomes. When a pair matches, like two dominants or two recessives, it is homozygous (homo = same), or genetically pure. A gene pair having one dominant and one recessive is heterozygous (hetero = different), or genetically impure. In breeding, we try to get two dominants or two recessive together to eliminate the variation caused by heterozygosity. Repeatability in inheritance is much greater with dominant genes than recessives, especially when they are homozygous.

Prepotence is the ability of a parent to transmit good qualities to its offspring with a high degree of repetition. Homozygosity, or having many matching gene pairs, is a big factor in being able to pass traits consistently. Ideally, you want use a buck for each doe who shares her good traits, plus adding good traits to overcome the doe's weaknesses. The more traits you breed for, the less progress you can make in any single area, so it's a good idea to concentrate on the most important traits first.

Dominant traits are written as capital letters, and recessive traits are written as small letters. For easier explanation in this article, I will use only one gene for barrel with a capital B for round barrel and small b for flat barrel. Remember that few traits are this simple, and barrel width and depth is the result of many genes.

In our first example, the buck has 2 dominant genes for round barrel, BB, meaning that he has a round barrel and can pass it. The doe has 2 recessive genes for flat barrel, bb, so she is flat-sided and does not carry genes for a round barrel. Both of these conditions are homozygous (same), but one goat is genetically dominant and one is genetically recessive. There is only one possible genetic combination in all offspring, as you can see from Figure 1. The BB buck is represented horizontally on the top line, and the bb doe is represented vertically on the left side.
 

Figure 1

homozygous dominant buck
X
homozygous recessive doe 

Buck
BB
Buck
BB
Doe
bb
Bb Bb
Doe
bb
Bb Bb

 

As you can see, one gene is contributed to each kid by the buck and by the doe. The buck gives his dominant B to each kid, and the doe contributes her recessive b. Even though each parent is homozygous for its own trait, all four kids are heterozygous because they carry both a dominant and a recessive genes. They will all have a full barrel because that is the dominant gene in this example.

Using the round and flat barrels as simple examples again, Figure 2 shows the results of heterozygous matings. The parents are both heterozygotes and have unmatching gene pairs. One kid is homozygous dominant BB, two are Bb heterozygotes, and one is homozygous recessive bb (flat barrel).

 

Figure 2

Heterozygous buck
X
Heterozygous doe

Buck
B
Buck
b
Doe
B
BB Bb
Doe
b
Bb bb

  

After enough kids, you will be able to see which traits are are more dominant in your herd.
 

Mating Systems

Outcrossing, or mating unrelated animals, results in a high degree of variation because gene frequencies for dominant and recessive traits vary in different lines of goats. There are many pros and cons for outcrossing that I will talk about in a future article.

Linebreeding, or mating somewhat related animals, makes up to 12.5% of the genes homozygous beyond those that are normally the same, depending on the relationship of the animals, because fewer "outside" (heterozygous) genes are introduced. This is usually done when there are outstanding ancestors in the pedigree - the more good ancestores there are, the better your chances for fixing good genes in your herd. Linebreeding nondescript parents is pointless - you are certain to get kids just like them and fix their traits more firmly in your herd.

Inbreeding is mating directly related animals: mother/son, father/daughter, and full brother/full sister. This makes an additional 25% of the genes homozygous.

 

Breeding closely related goats (inbreeding) can have negative effects. It may reduce size, early life vigor, and fertility, as it forces undesirable recessives that have a greater chance of remaining recessive in outcrosses. If neither parent has a certain detrimental trait and at least one of their kids has it, both parents are carriers (heterozygotes). Even if you eliminate the defective offspring, either parent can continue to pass it. There are a number of single-gene diseases that can be forced out, some showing up later in life, so be prepared to deal with them, especially in inbreeding. Successful linebreeding is based on the merit of the foundation stock, the rate of inbreeding, culling, and selection for fitness traits and structural soundness.

The trick to reproducing desirable qualities with great regularity is to pair dominant genes by the method of breeding you choose - outcrossing or linebreeding.

 

There are no shortcuts in animal breeding - you have to be devoted to it for the long haul. Doe kids born today won't reproduce for a couple of years - their daughters will be two years later, and theirs, and theirs. Bucks are used at much younger ages and are easier to prove. Use bucks that produce the most consistently good results.

Do you like each generation better than the last? Are your kids consistently improved over their parents? Do they have the qualities you are breeding for? Each year, you should be able to say that your kids are better than last years'.

When you stop seeing progress, you're not just standing still - you're falling behind.
 

Related Reading

Mendelian inheritance

 


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