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• The goal of a breeding program is to create new genetic combinations.
• Deciding who should be parents of the next generation is the most important breeding decision.
• New genetic combinations occur from independent assortment, mutations and cross-overs.

How can we produce the perfect cow? Yes, luck is involved in breeding the perfect cow. Although a resemblance is expected between parent and offspring, only one-half of the genes in the offspring came from a parent. The other half of the genes came from the other parent. The challenge in breeding better dairy cattle is to decide who should be parents of the next generation.

The goal of a breeding program is create new genetic combinations, which will lead us to improved genetic merit toward increased economic value in the shortest amount of time. New genetic combinations occur in three ways.

INDEPENDENT ASSORTMENT

The principle of independent assortment put forth by Gregor Mendel is the most common way to generate new combinations of genetic material. Independent assortment leads to gametes that will carry the genetic material (DNA) to the next generation. Each parent transmits a sample half of their genes to any one of their offspring. Genes in the parents are said to be located at loci on the 30 pairs of chromosomes in cattle, humans have 23 pairs of chromosomes. One chromosome of each pair is transmitted to an animal's offspring. Independent assortment defines the principle that the chromosome chosen at one chromosome pair is not affected by what happens at another chromosome pair. This is one of the reasons that breeding dairy cattle is considered an art. We can not predict which chromosome of each pair will be transmitted. In the rare event that the parent is homozygous (has two identical chromosomes), prediction of outcome is possible for that pair of chromosomes but not for the other 29 chromosomes. When you get the perfect cow, you hope that it will be homozygous so that you can get perfect offspring. Reproducing the perfect cow is where the technology of cloning may have a contribution. Cloning duplicates the genetic material but does not create any new genetic combinations.

MUTATIONS

Another way to generate new genetic combinations is through mutations. Mutations occur in the DNA of an animal. Some of the genes in the parents may undergo a change which could lead to altered genetic material. At a locus on these chromosomes we have two alleles. If the two alleles are the same, they are homozygous. If the two alleles are different, they are described as heterozygous. These alleles will segregate independently. Something could go wrong in the process and some of the DNA could be altered. One allele at a locus could be changed to something else. The changed allele is called a mutation. A mutation may be responsible for an improved or detrimental phenotype. Depending on the mode of inheritance, a mutation could be transmitted for several generations without being detected. If the mutation is dominant it will be expressed in the first generation after occurrence. If the mutation is recessive, two copies of the mutation (one transmitted through each parent) would be necessary before expression of the trait. Most abnormalities that result from mutations are recessive. Mutations that are favorable and dominant are easily incorporated into the population, although it could take several generations.

CROSSING-OVER

One other way to create new genetic combinations is crossing-over. Again as with the other methods to create new genetic material, we can not force crossing-over to happen. Crossing-over occurs naturally. What happens is that some genetic material is exchanged between homologous pairs of chromosomes. If the parent is homozygous, we will not be able to detect the cross-over and we will not have a new genetic combination. If the parent is heterozygous at several loci, then crossing-over may create new genetic material. The following diagram of a portion of one pair of chromosomes will illustrate this principle. Letters represent alleles. Upper case letters are dominant alleles and lower case letters are recessive. Four loci are shown in the diagram.

Mating together two animals represented by the genetic material illustrated in Figure 1a) with no crossing-over, the offspring could have alleles of AabbCCDD, aaBBCCdd or AaBbCCDd. If a cross-over between loci B and C occurred (Figure 1b)) in one of the parents in the formation of their gametes (Figure 1c)), the offspring could have new genetic combinations of AabbCCDd, AaBbCCDD, AaBbCCdd or aaBBCCDd.

One cross-over doubled the number of possible genetic combinations in the offspring. Ability to predict whether the offspring will be the perfect cow is more difficult but the chance to get something different (perhaps the perfect cow) has increased.

Independent assortment, mutations and cross-overs will contribute to the success of a breeder in producing the perfect cow.