Wednesday, March 11, 2009
When Irish Spots Are Smiling: Basic Color Inheritance In The Newfoundland Dog
Basic Color Inheritance In the Newfoundland Dog
(Courtesy of Cindy Williams, Pipeline Newfoundlands)
In every dog, every characteristic, such as size, color, eye color, coat, shape of skull, etc., is controlled by a pair of determiners which we call genes. At conception, when a new individual is formed, one gene is inherited from each parent so that the new individual has two genes to control each physical trait. A great many characteristics are what is known as simple dominant, or simple recessive traits. This means that if a gene for a dominant trait is present, the dog will show that trait even if he also carries the gene for the recessive trait. On the contrary, if the trait is recessive, it will be hidden in any case where the dominant gene is present.
This pattern is easily illustrated by the inheritance of the Landseer color in the Newfoundland. The color Black is dominant in Newfoundlands. Let us name the dominant Black gene with a large B and the recessive Landseer recessive with a small l. If we have a Black dog that carries only the dominant Black genes, we will call his color inheritance BB. Mated to a bitch that is also carrying only the dominant Black, the puppies produced will have a color inheritance as follows - every time:
BB mated to BB
BB BB BB BB BB BB BB BB
All of the puppies will be Black, and will be purebred for Black. Under no circumstances, no matter what they are bred to, can they ever produce Landseer offspring.
Now, suppose that one of these BB pups is mated to a Landseer bitch, or one of the bitches is mated to a Landseer male. The resulting litter will inherit the following pattern:
BB mated to ll
Bl Bl Bl Bl Bl Bl Bl Bl
Every puppy in the above mating will be Black, but each of them will carry the recessive gene for Landseer. This is not optional - it will happen every time, because the Landseer parent has no Black gene to offer, and therefore gives every pup a Landseer color gene.
Now suppose that you took two of the Bl puppies and mated them together or bred one of them to another Bl dog. You would expect the following results:
Bl mated to Bl
BB BB Bl Bl Bl Bl ll ll
Two puppies would be Black, and be purebred for Black, having taken a Black gene from each parent. About four would be Black, but carry Landseer, having taken one recessive color gene from each parent. Two pups would be Landseer, having taken the recessive Landseer genes from both parents. Generally, the number of puppies used to demonstrate the likelihood of color inheritance is eight, because this is large enough for color variations to be averaged.
If you study the following chart, it is easy to see that two Landseers can produce nothing but Landseers when mated, but apparently Black dogs can produce Landseers, because they can carry the recessive. In other words, when a dominant characteristic is not visible (i.e., Black), it is not present at all. With a recessive, however, it can be masked for generations only to reappear when the conditions are right - when it is teamed with another recessive gene.
It must be remembered, however, that the following Mendelian Expectation chart applies over large numbers, with the expectation to be exact in only lines 1, 2, and 6 where no variation will occur.
Since the color Grey is recessive, like Landseer, it is obvious that a great many dogs can carry Grey, but the only time a Grey pup will show up in a litter is when both parents give their recessive Grey gene.
Irish Spotting, on the other hand, is recessive to Black, but dominant over Landseer. If you bred a Black female that carried Irish (I) to a Landseer, you might expect the following:
IL IL IL IL BL BL BL BL
About half of the pups will be Irish Spotted, and half will be Black. All of the puppies will be Landseer recessive, because with one Landseer parent they have no alternative but to carry Landseer - the Landseer parent has to give a Landseer gene.
If you took the same bitch and bred her to a dog that was purebred for Black (BB), the resulting litter would be as follows:
BB BB BB BB BI BI BI BI
All of the pups would be Black, but about half of them would carry Irish Spotting, having taken the dominant Black gene from their father and the recessive Irish from their mom.
Take the same Black Irish Recessive female and breed her to a dog that is Black Landseer recessive and you could expect the following results:
BB BB Bl Bl BIrish BIrish IrishLandseer IrishLandseer
Most pups will be Black, carrying either Irish or Landseer. You will likely get some Irish Spotted pups, in the case where they took their father's Landseer gene and their mother's Irish Spotted gene. The Irish Spotted gene will dominate the Landseer gene, as will the Black. You cannot get Landseers out of the breeding unless both parents carry a Landseer gene. It must be noted that when dark Landseers are bred into a line that is known to carry Irish Spotting, it can be difficult to determine if you have a very dark Landseer or a very light Irish Spotted.
If you took two dogs that were Black/Irish Spotted and bred them together, you could expect the following genetic breakdown on color:
BB BB BB BI BI BI II II
Some of the pups will be Black, and purebred for Black. Some will be Black carrying Irish, and a couple will be Irish, carrying only Irish.
MENDELIAN EXPECTATION CHART
SIRE AND DAM PROGENY
LL LL LL LL LL LL
BB LL BL BL BL BL
BL BL BB BL LL BL
BB BL BB BB BL BL
LL BL LL BL LL BL
BB BB BB BB BB BB
(Editor's Note: How do you determine the sex of a chromosome? Pull down its genes.)