Compliments of Tina Porter
Have you ever wondered what color you next litter of puppies will be? Or why your two sable dogs produced a litter of black and tans? It all has to do with genetics — specifically coat color genetics.
Just to make sure we are all starting out in the same spot, here is a quick refresher on basic Mendelian genetics. If you already have a good grasp of genetics you might want to skip this part.
Trait: a feature or characteristic that is inherited. It is usually represented by a letter. Example — coat length = L
Gene: the specific segment of DNA on a chromosome that codes for a particular trait. Example — gene for coat length is located on chromosome 12.
Allele: version of the trait/gene. Each allele is given a version of the letter assigned to a trait. Example — the gene for coat length has two alleles — one for the smooth coat (L), and one for plush coat (l).
Chromosomes: “packets” of genes. Dogs have 78 chromosomes, 38 pairs and two sex chromosomes (determine gender). The paired chromosomes (sometimes called autosomes or homologous) contain the same. Think of it in terms of a shoe store. They have 38 different pairs of shoes (chromosomes). They are all different styles (different chromosome pairs carry different genes), but each pair have the same style (homologous chromosomes have the same genes). This means you have two copies of a gene — one on the right shoe, and one on the left. As you look closely at your pair of shoes you’ll notice they are not exactly the same — one is a righty, the other a lefty (you have two copies of the gene, but maybe not the same alleles). One of the shoe/chromosomes you inherited from the sire, the other from the dam.
Genotype: what alleles you have for a particular gene. Remember you have two alleles, one from mom, one from dad. A genotype is written as two letters, each letter represents an allele.
Heterozygous: has two different alleles in the genotype. Example — Ll
Homozygous: has the same alleles in the genotype Example — LL or ll
Phenotype: what the dog looks like. Example — a dog with the genotype ll has the phenotype of a plush coat.
Dominant: an allele that is expressed in the phenotype no matter what other allele you have. Dominant alleles are written as capital letters. Back to our coat length example — if a dog has the genotype LL or Ll they have a short coat. L is dominant to l, short hair is dominant to long hair.
Recessive: An allele that is only expressed if a dominant allele is NOT present. Recessive alleles are written as lower case letters. The only genotype a long haired dog can have is ll. (* Yes, I know there are “smush” coats out there. There is another gene that modifies coat length aside from “L”).
We all know that you get half of your chromosomes from the sire, and half from the dam. But how can you tell which ones the pups will get? Simple — you can’t. But you can make predictions using a Punnett Square.
Let’s use this sample problem to give you an idea. You breed a heterozygous short haired bitch (let’s say her father was a plush coat so you know she has at least one recessive allele from him) to a plush coated dog. You want to know many plush coated puppies you will get.
Alleles L= smooth coat, l= plush coat
Sire’s genotype — by looking back we remember that he can only be plush coated if he has both recessive alleles, so hmust be ll.
Dam’s genotype — as discussed earlier she is heterozygous, so L l.