Genetics and Inheritance
Most breeders understand terms like 'cells' and 'genes', and know them to be the building blocks of life. However, they themselves are built from many parts. In this section, we will drill down beyond the 'cell', right to the very core of life - genetic code.
There are trillions of cells that make up each dog. Within the majority of those cells is a nucleus.
The nucleus contains 39 pairs of chromosomes, totalling 78 chromosomes. One of these pairs are the sex chromosomes, X and Y, which determine the dog's sex. If a puppy inherits two X chromosomes, it will be a female. If it inherits an X and a Y, it will be a male. The other 38 pairs are called autosomes, and these determine all of the other traits the dog will develop. Chromosomes are essentially large lengths of DNA wound tightly, in order to fit them within the nucleus. Of interest, the dog has a large number of chromosomes for a mammal, which leads to a higher tendency to 'shuffle'
DNA is otherwise known as deoxyribonucleic acid. Within the well known helix shape of DNA, there are nucleotides. There are four types: adenine (A), thymine (T), guanine (G), and cytosine (C). The combination of nucleotides can be thought of like a code, and this code can produce different traits in the dog, by producing proteins that make up bones, hair, muscle, organs, and more.
Along the strands of DNA, are sections called 'genes'. Each gene has a variety of 'alleles' - which can be thought of as the 'options' that gene has. For example, the gene for MDR1 multidrug resistance has two options: mutant (causes MDR1), and wild type (does not cause MDR1). Each puppy will inherit two alleles, one from each parent.
So, if we have a sire who has two MDR1 mutant alleles, and a dam who has two MDR1 wild type alleles, the puppies will always inherit a mutant allele from the sire, and a wild type allele from the dam. The puppies will therefore be mutant / wild type for MDR1.
In the chart below, we can see a punnet square which shows the male and the female genotype. When mated, we can see the combinations of puppy genotypes we can get. We can receive from this combination puppies that are heterozygous (WT/M) and puppies that are homozygous (M/M and WT/WT). Heterozygous means the puppy has inherited two differing alleles, and homozygous means the puppy has inherited two of the same allele.