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Mendelian Inheritance Explained

July 10, 2020

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. Traits aren’t copied with 100% accuracy from parent to offspring, though. Since traits are inherited from both parents, all sorts of 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. One litter of puppies can see a wide range of different traits between coat color, pattern, puppy size, and etc. Some puppies can even end up looking completely different from their parents. This can be explained in part by Mendelian inheritance.

Illustrated puppies with different color coats

No matter how much a puppy may appear to rebel against their parents’ genetics, all of their genes came from Mom and Dad. The reason some genes might express the traits they code for differently is because of alleles

An allele is a variant form of a gene. Let’s use the 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. Humans and dogs are diploid organisms, meaning they inherit two alleles for each gene (one from each parent). The way these alleles combine affects how a trait is expressed. 

 

Dominant and recessive alleles 

Some alleles are dominant, while others are recessive, but most are somewhere in between a scale of 0 (recessive) to 1 (complete dominance). The allele for a 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 two alleles from both parents combine to make the offspring’s genotype. A genotype is the allele combination of a gene. Since alleles come together in pairs, the possible genotypes for coat color, in this case, are BB, Bb, and bb.

Black dog with uppercase B lowercase b genotype

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. 

Most genetic loci actually have incomplete dominance. This can lead to all sorts of variations in how phenotypes are expressed. Instead of the dominant allele expressing fully, the alleles could blend together in different ways. A dog’s height and body size are traits affected by many alleles with varying levels of incomplete dominance. 

 

Genotype and phenotype

Genotypes are unseen (unless you test your dog’s DNA), while phenotypes are what you can see on the outside. Phenotypes are the physical expression of the trait the gene codes for. 

Computer coding works in a similar way. Think of everything you’re seeing onscreen now as the phenotype. What you can’t see is an entire programming code (the genotype) telling the page what to display.

Four black puppies with uppercase B genotypes

The puppies above all have a dominant B allele that overpowers the recessive b allele, so all of their genotypes translate to 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 parents’ alleles to show the possible genotypes that their offspring could inherit as well as the probability of inheritance.

Punnett square with two black dogs uppercase B uppercase B on top and uppercase B lowercase b to the left. From top left to bottom right: uppercase B uppercase B, uppercase B uppercase B, uppercase B lowercase b, uppercase B lowercase b

Since the father has two dominant B alleles, the puppies have a 100% chance of showing a black coat phenotype. 

Punnett square with two black dogs uppercase B lowercase b on top and uppercase B lowercase b to the left. From top left to bottom right: uppercase B uppercase B, uppercase B lowercase b, uppercase B lowercase b, lowercase b lowercase b

If the father has a dominant B allele and a recessive b allele, the chances of a puppy showing a brown coat phenotype are 25%

Puppy with brown coat and lowercase b lowercase b genotype

Because the puppy didn’t inherit the dominant B allele, which leads to the production of black pigment in her coat, her pair of recessive alleles is expressed as a brown coat. She’s the brown sheep of the litter.

Empty Punnett square with two black dogs lowercase b lowercase b on top and uppercase B lowercase b to the left.

Pop quiz time: Try filling out a Punnett square assuming the father had two recessive b alleles, then figure out the chance of puppies showing a brown coat phenotype.

(The answer will be shown at the bottom of the page)

 

Inheritance across generations

Sometimes traits will skip a generation or two (or even ten!) A trait that’s attached to a recessive allele will only be expressed when it is transmitted by 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.

Still interested in how inheritance works? Learn about how recombination can cause genetic variation between puppies and their parents or take a closer look at coat color genetics.

 

 

Punnett square with two black dogs lowercase b lowercase b on top and uppercase B lowercase b to the left. From top left to bottom right: uppercase B lowercase, uppercase B lowercase b, lowercase b lowercase b, lowercase b lowercase b.

Answer: The puppies have a 50% chance of showing a brown coat phenotype.