For my project, I researched information on genetics and cloning. I concentrated on dogs because I have an interest in animals and I love dogs. This applies to the information I have learned in my Biology course.
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Genetics:
Just like in any organism that reproduces sexually, there are parents cells that combine (eggs and sperm) to make offspring. In the cells are genes and in the genes are alleles, and on the alleles are where the DNA is carried that passes on the traits of the dog. Some traits are fur color, eye color, size, or nose color. Now, when the cells reproduce, there can be a lot of random matching up of the parent traits that get passed on to the offspring. The traits carried on the sex cells (eggs and sperm) have many possibilities. Only one egg and one sperm meet.
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For example, if the male parent has black fur and the female parent has brown fur there is a 25 percent chance that the puppies will get brown or black fur and no other color. Let's take a look at how genetics works:
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It turns out that whether a dog is black or brown depends on the alleles it has on a particular locus - the B locus. There are two possible alleles - B is black, and it's dominant over b, which is brown. Here's a list of the possible genotypes and what dogs with those genotypes would look like:
BB - two copies of black, so puppy will be black.
Bb or bB - one copy of black, one of brown. Black is dominant, so puppy will be black.
bb - two copies of brown. Puppy doesn't carry any black, so it must be brown.
Punnet Squares
The mystery of the chocolate puppy can be solved by working out the genotypes of the parents. If we calculate some breeding results from all the possible crossings of BB/Bb dogs, we can work out which breeding/s will produce chocolate puppies, and so from that we can have a fairly good guess at the genotypes of the parents. The quickest and easiest way to calculate breeding results is to use a Punnet square. Firstly we'll see what happens when we breed a Bb dog with a BB dog.
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Put the genotype of each parent in the boxes at the top and the side (the title cells). We put just one gene letter in each cell, and it doesn't matter which parent goes across the top and which down the side (it makes no difference to the results).
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Each of the four cells represents one possible puppy genotype. To work them out, we trace up from the cell to the column title and across to the row title. It doesn't matter which order we do this in (whether we look at the row or the column first).
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Review the finished Punnet square. We can see that all the puppies in this litter would be black. There are no bb puppies, and they're the only dogs that can be brown. So we haven't found Jet and Ed's genotypes yet. We need to keep looking!
Crossing a BB dog with another BB dog will obviously only produce BB puppies, so Ed and Jet cannot both be BB. From the example above we now also know that it can't be right that one of them is BB and the other Bb. So the only other option we can try out is Bb x Bb:
So that's three black puppies (BB, Bb and bB), and one brown. It looks like we've found what we were looking for. Because the brown gene (b) is recessive, it can be hidden, which means a dog can carry one copy of it without displaying it. That means that two dogs carrying brown can be black themselves, but produce brown puppies. It can be a bit of a problem when it comes to breeding, because recessive traits, such as brown, can remain hidden in lines for many generations, then suddenly appear when a dog carrying the trait happens to be bred to another with it. This is why breedings sometimes throw complete surprises, like silver (blue) Labrador puppies in a breed which, to all intents and purposes, contains no silver at all. That one lone recessive silver gene (d, on the D locus) has been passed down from generation to generation, completely unknown to the breeders, until finally it's met another one. It might have come from a cross breeding with another breed many years ago, which doesn't show up on the pedigrees and no longer has any effect on the look of the dog. Such rare recessive traits can be impossible to eliminate from a breed, simply because you can't tell which dogs carry them. In recent years, however, genetic testing has helped to identify the carriers.
Anyway, so now we know that Jet and Ed both have the genotype Bb. However, if we wanted to get brown puppies in the future by just breeding black dogs, how would we know which black dogs are Bb and which are BB, without doing any genetic testing? Luckily for us brown is quite common in Labs, so it's not like trying to find out which Labs carry silver, where you could do hundreds of test breedings and still not get silver pups because the chances of finding another dog with the gene are so slim. In the absence of genetic testing, the way to tell if a black dog is Bb or BB is to breed it to a brown.
We can be certain that the brown parent will be bb, because it can't be anything else. If the black parent is BB, we'll end up with all black pups, because each pup can only possibly inherit B from the black parent. All of the puppies will have the genotype Bb. But if the black parent is Bb, half of the puppies will be brown. How do we know this? By doing a quick Punnet square!
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