MOLECULAR BREEDING
Not very long ago all the animal breeding techniques were done based on the phenotype (animals were selected based on their
performance). What happens is that a lot of traits of interest, such as muscle mass or milk production, are quantitative.
Quantitative traits are not ruled by one single gene, but many different loci, which makes it more difficult to control that the animal passes
to the next generation the traits that we are interested in.
Nowadays, there are genomic techniques that help us to do that. Breeding values are still used to know the performance of a determined
animal, but the molecular DNA techniques give us extra information that allows us to select our animals better.
MOLECULAR MARKERS
These are the 3 most used molecular markers in molecular breeding. All of these methods are based on the differences that can be found
within our genomes.
1. Restriction fragment length polymorphisms (RFLPs): RFLPs were the first molecular markers that were used for molecular
breeding. They are still used nowadays for human disease detection and in forensics.
2. They are sequences that are recognised by restriction enzymes that are able to cut DNA. When we digest a DNA fragment with
this enzymes, we obtain different band patterns that can be observed in a gel.
3. For example, there is a disease we are interested in. The wildtype animal has a certain number of RFLPs in a fragment of DNA.
The animal that suffers from the disease has a mutation that eliminates one of these RFLPs.
4. When we digest the DNA of this animals with a restriction enzyme and run a gel, we observe different band patterns so we can
easily distinguish between a healthy and a sick animal.
5. The RFLPs are inherited. We know the band patterns that result from an animal depending on their genotype (dominant
homozygote, heterozygote or recessive homozygote). Basically the heterozygote has the bands from both the homozygotes.
6. When we cross two animals and have an offspring with diverse genotypes, we can use this technique to identify which animals
are affected by the disease (or any other trait) and which are not.
2. Microsatellites or Short Tandem Repeats (STR): Are repeated sequences found throughout the genome. Organisms vary in the
number of copies of these repeats at each of the loci, so we can use them to follow loci that we are interested in.
3. Most of the STRs are found in the non-coding region, so they can be independent of the selection and only give us a fingerprint
of the genome of the animal that expresses the trait that we are interested in.
4. The repeats are usually 1-10 bp long and the mutation rate is high than the base rate, because in the non-coding region the
mutation rate is normally higher than in the rest of the genome.
