All living things inherit their traits from their parents, whether it’s a person’s height, a dog’s coat type, or a flower’s petal color. Since traits are inherited from both parents, considerable mixing and matching can occur on the genetic level. The environment can play a big role as well. There are many complex factors that determine the variation between parents and offspring, but we’ll take a look at more simple traits that are affected by Mendelian inheritance.
In the nineteenth century, Gregor Mendel conducted a wide array of experiments using pea plants in the garden of his monastery. Over the course of eight years, he looked at how the plants developed as he cross-fertilized them with each other. His observations and theories, though not appreciated during his lifetime, were rediscovered in the early 20th century and were critical in the development of modern evolutionary biology
Mendelian inheritance explained with dogs
Instead of pea plants, let’s look at simple traits in dogs. Within one litter of puppies, a breeder can observe a wide range of traits including coat color, pattern, puppy size, etc. Some puppies can even end up looking completely different from their parents. This can be explained in part by Mendelian inheritance.
No matter how much a puppy may appear to rebel against their parents’ genetics, all of their genes are inherited from their sire and dam. For each locus, a puppy inherits one allele from the sire and one from the dam. An allele is a variant form of a gene, and it may be dominant or recessive, which affects how it is expressed. To learn more about different modes of inheritance (MOI), please visit our blog.
Let’s use a simplified case of the B-locus in dogs as an example. The B-locus is a gene (TYRP1) that can lead to otherwise black coats being lightened to brown, depending on which alleles are inherited. This impacts solid black dogs, dogs with areas of black on their coats, and even the black pigment in dogs’ noses.
Dominant and recessive alleles
As mentioned previously, some alleles are dominant, while others are recessive. To learn more about genetic terminology and the unique MOIs of the coat color loci, you can click here. The allele for black coat color is dominant, so it’s denoted as an uppercase B. The allele for brown is recessive, so it’s a lowercase b.
The alleles inherited from each parent combine to make the offspring’s genotype. Since alleles come together in pairs, the possible genotypes for coat color, in this case, are BB, Bb, and bb.
When a dominant allele pairs with a recessive allele, the dominant allele is expressed. A recessive allele is expressed when paired with another recessive allele.
Genotype and phenotype
Genotypes are unseen (however, you can test your dog’s DNA), while phenotypes are the physical expression of the trait the gene codes for.
The puppies above all have a dominant B allele that overpowers the recessive b allele, so all of their genotypes translate to the black coat phenotypes.
Mendelian inheritance visualized with Punnett squares
We can predict the probability of how genotypes and simple phenotypes are passed on using a Punnett square. This table combines the parents’ alleles to show the possible genotypes that their offspring could inherit as well as the probability of inheritance.
Even though the dam has a Bb genotype, since the sire has two dominant B alleles, the puppies have a 100% chance of showing a black coat phenotype.
However, with the dam’s genotype remaining Bb, if the sire has a dominant B allele and a recessive b allele, the chances of a puppy showing a brown coat phenotype are 25%
Because the puppy didn’t inherit the dominant B allele and she has two recessive b alleles, the default black pigment will change in structure, which is expressed as a brown coat phenotype. She’s the brown sheep of the litter.
Please note, the illustrations are intended to easily distinguish black from brown and are not meant to incorporate the variants at other loci (such as a facial mask or tan points). Additionally, savvy breeders know that there are subloci at the B locus which can interact to impact phenotype. Embark provides these subloci results in your dog’s profile.
Inheritance across generations
Sometimes traits will skip a generation or two (or even ten). A trait that’s due to a recessive allele will only be expressed when it is inherited from both parents. Since there’s a chance that a recessive allele may not combine with another recessive allele for several generations, it can take time to see it fully expressed as a phenotype.