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Size Genetics...Article: The Genetics of SizeAuthor: MindiBown Kennels I'm sure at least some of you have wondered at the amazing variation in size when looking at Chihuahuas. Of even greater mystery may be watching 2 normal or even quite large parents produce a tiny, tiny offspring. How can that happen? It's pure genetics. The gene that controls size is different from most in so much as it has 6 parts, or alleles, and that each parent passes on 3 of theirs. Which 3 is pure Tattslotto stuff. Spin the wheel and see what numbers drop. The size alleles can be best described as either + or -. We then add the 6 together and get a total. With six present in any living mammal we have an infinite variety of variations. For example: +,+,+,-,-,-, = 3 of each. Think of the +'s as "up 1", and the -'s as "down 1" The first three cancels out the next three so we end up with 0, or your proverbial "Average". How about +,-,+,-,-,-. Add them up and we get 2 up and four down. end result = 2 down or below average size. One more. +,+,+,+,+,-. 5 up and 1 down = 4 up. or in other words, a BIG boy. Are you starting to get the idea? Ok, let's start passing things on to the kids. Let's take 2 average parents. +,+,+,-,-,-. = Dad and +,+,+,-,-,-. = Mum. Let's give them a litter of 3. Pup 1. We'll take (at random) -,-,+. 2 minuses and 1 plus from Dad and let's say -,+,+. 1 minus and 2 pluses from Mum. Pup one therefore is -,-,+,-,+,+. up and down we end up back at 0, or average size. An average size pup from 2 average sized parents. What else would you expect? Well how about Pup 2? We'll have -,-,-. 3 minuses from Mum AND -,-,-. three minuses from Dad. What size pup do we get? -,-,-,-,-,-. down, down, and down again 6 times means one very tiny pup. For Pup3 We'll go the opposite. Let's have all the pluses that both Mum and Dad can give. That's a total of +,+,+,+,+,+. 6 ups or a Chihuahua masquerading as an elephant. AND ALL THREE FROM AVERAGE PARENTS That's the best part. But when you see how size is inherited, it all starts to make sense doesn't it? But genetics is only half the story with size. It's been fairly well documented that the human race is getting bigger and bigger with each generation. When you look through museum reproductions of early settler's cottages etc. the height of doorways and the length of beds stand out as being so small by today's standards. This growth in the human population has been put down to an improvement in diet and better health care. Science has pretty well proven that diet has played a major role in this phenomena. Diet is just one factor in what is generally referred to as "Environment". And environment plays a major role in the size of Chihuahuas as well. There is more to size in Chihuahuas than pure inherited genetics. Environment plays a definite role, as does an as yet unknown genetic link to problem areas in the Chihuahua physique that inhibits growth. Some years ago when we had not long been breeding Chihuahuas, we had occasion to have a litter arrive in the depth of winter. As luck would have it, that particular time coincided with a breakdown in the family chain saw. Hence, our supply of firewood was being rationed. In short, the house was b.......y cold. The litter arrived and they were all fit, healthy and eating well. It wasn't long however before it became obvious that all was not well. All pups cry, but this litter cried non-stop. In particular, at night when the fire had died down. We realised the pups were cold and we went out and invested in our first heat pad. A good thermostatically controlled unit. As soon as the pups were put on the heat pad, the incessant crying stopped. The interesting part however was observing the pups weight. It has always been our practice to weigh each pup daily. This litter had shown a small but steady gain each day. A bit down on normal but regular and consistent. As soon as the heat pad was installed the pup's weight gain increased DRAMATICALLY. Not just a little bit but significant and noticeable. On a graph, the weights all took a sudden surge upward. So what does this mean? Quite simply, that environment also plays a part in puppy growth. Environment is meant in this case to entail both diet and the environs of birth. Quite obviously diet and such has a bearing on growth. The only question is whether the effect is temporary or permanent. The last factor is a congenital factor affecting growth. Research from the Genome Project has thus far shown a link between size and certain organ development such as the Thyroid. What is not certain as yet is whether size is affecting the development process or the development of certain organs and glands is affecting size. It seems pretty obvious however that the very tiny tiny examples of our breed have more health problems than their normal brothers and sisters. At this stage it's still a "Chicken & Egg" discussion, but the connection is obviously there. The one over riding conclusion that should be emphasised is that the deliberate down sizing of our breed below a (say) 3 lb limit is downright foolish. I would go so far as to suggest a change in the standard to give a preference for a size range of 4 1/2 to 5 1/2 lb.. Not the current, smaller is better with no lower limit. (C)Copyright 2007, this article is copyright protected Special thanks to MindiBown Kennels for permission to display this article on RoyaltyChi Chihuahuas website |