Transferring biological information
Back to the genome. The genome contains biological information. How to transfer this information from one generation to the next? Once again, intuitively we know: this time it’s all about sex. Now think of the genome as a deck of cards (actually a double deck of cards: everything comes in twos, remember?). Let’s say that Dad has a genome consisting of only black cards and that Mom has a genome consisting of only red cards. Each parent now shuffles their cards (their genome), and then cuts their deck in two so that only half of their cards will get passed to the next generation. Sex happens. The resulting offspring (puppies in this example) will now have a new genome consisting of a full (double) deck of cards, half of which are black (from Dad), the other half of which are red (from Mom).
The fact that genes come in pairs (one copy from each parent) is a good thing as it allows biology to experiment with genetics and thus allows animals to evolve and become (just for an example) dogs. But it can also be not so good in that bad copies of genes can stay hidden. A healthy dog can be a carrier for disease genes. If two healthy (carrier) dogs are mated they can have puppies that are diseased, and other puppies that are healthy. And that’s at the heart of heredity.
An example of heredity
Let’s look at an example of heredity. Consider a black dog and a brown dog. Let’s say that brown is the recessive trait, i.e. it can be hidden, and that black is the dominant trait, i.e. it always reveals itself. Let’s call the gene responsible for the presence (or absence) of black or brown the gene ‘B’. Furthermore, let’s call the black version of the gene capital ‘B’, and the brown version of the gene small ‘b’. Now go back to the idea that you have two parents and that everything in genetics comes in pairs. To have a brown dog, we need two brown copies of the gene (bb). This dog is said to be “affected”. To have a black dog, we only need to have one black copy of the gene, while the other copy can be for black or for brown (BB or Bb). BB will give us a black dog that is ‘clear’ (not a carrier) for the brown copy of the gene. Bb will give us a black dog that is a carrier for the brown copy of the gene.
We now look at three possibilities for mating our black and brown dogs:
Black X Brown (‘Clear’ X Affected)
This will give all black puppies. All the puppies will be carriers for brown.
Black X Brown (Carrier X Affected)
This will give black puppies and brown puppies, in about equal numbers. The black puppies will be carriers for brown.
Black X Black (Carrier X Carrier)
This will give black puppies and brown puppies, but more black than brown. Some of the black puppies will be ‘clear’, while some of them will be carriers for brown.
Once again, genetics doesn’t get any simpler than this. Once again, it can get more complicated.