Friday, March 12, 2010

Science post: Genetic throwbacks

Last year we bred our cow herd to three different bulls. The main brood herd was serviced by two black Angus bulls (solid black) while our replacement heifers were bred to a red Angus bull. The results have been an interesting science flashback to junior high and the use of Punnett squares to predict genetic outcomes.

We refer to the genetics of an animal in two ways. The genotype refers to all the hereditary information in the genetics (BB, Br, rr) - essentially the unseen part. While the phenotype refers to how that information is expressed - the type of genetic information we can tell simply by looking at the animal (black cow, black cow, red cow).

With the black Angus, black is the dominant color so if you breed a red cow (rr) to a purebred black bull (BB) you will always get a black calf.

r r
B Br Br
B Br Br

In this scenario the bull will always throw the dominant gene for black color and since you need two recessive genes to express that coloring and only one dominant gene, the calf will always be black.

What has been a little surprising to us this year is the genetic throwbacks we are getting with our heifers. These heifers were born before we moved to the ranch so we have no records that show their maternal lineage but I know for certain the sire is black Angus (BB). Since our herd is so heavily influenced by black Angus, we would anticipate similar results to above.


So when we end up with a red calf, what has changed? We are breeding a black animal to a red animal in both scenarios so why is the outcome different? The answer has to do with unknown genotypes in the cows.

With a purebred black bull (BB), the sire will throw a dominant gene (B) 100% of the time. It doesn't really matter what the dam throws because it will be always be overshadowed or matched by the sire. The resulting phenotype is 100% black calves.

With the purebred red bull, he will always throw recessive genes so the situation is reversed and it is the dam that provides the genetic information that will ultimately determine phenotype in the calves. Though her phenotype is manifested as a black cow, she could have a genotype of BB or Br.

So in the case of the red calf, the Punnett square looks like this:

B r
r Br rr
r Br rr

In this mating there was a 50/50 chance for the phenotype of the calf to be black or red. In this case the calf turned out red (rr). We know that the dam is all black, but does this mean that the grand-dam was red? Not necessarily, she could have been black (Br) too and thrown the r gene to her daughter where it went unexpressed until passed down to her granddaughter where the recessive gene paired with another recessive gene to finally express the red coloring. The red cow could have even been further back in the lineage since with each generation there was a 50% chance to pass on that recessive genetic information.

So far we have had two calves from our first-calf heifers that have been red and since the rr/Br pairing only results in red calves 50% of the time, there could likely be more cows that hold the recessive gene. I honestly hadn't put much thought into genetic expression in the herd and was just planning for all black calves but now that we have seen this type of genetic throwback I am thinking that we may have a lot more cows that carry that recessive genotype and could continue to express that phenotype over time if we keep using red bulls. Very interesting...

Well that concludes today's science lesson. Cheers!

1 comment:

csw said...

I didn't know those were called "PUnnett" squares. I knew the idea, but not the name. Very cool. Glad you guys had a good branding weekend!

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