Color Genetics Primer

A bit outdated, this is the classic tome on this subject: The Inheritance of Coat Color in Dogs

Okay, first you’ll need a primer on our friend, Gregor Mendel. Gregor was a Roman Catholic monk who loved to garden. He liked to experiment with peas and the like. He found out that certain traits in the peas were dominant and others, recessive. For instance, green coloring in peas was dominant over white coloring. So, whenever he bred white peas and green peas, he always got green peas!

This was working out great, until he bred those green peas back to their white parents (how do peas breed, anyway?). Gregor couldn’t believe it when some of the peas were white. How did green peas bred with white peas make white peas? It astounded him even more when he bred the green peas in the “litter” back to their green parents, and some of the peas came out white.

Obviously, genetics of peas wasn’t so simple! So why did the green peas make white peas? To show you why, lets use some Punett Squares. Mr. Punett developed this handy method for graphically explaining why the genetic mixes came out as they did. When looking at a Punett square, it is arranged like this:

Example Punett

Father Trait A

Father Trait B

Mother Trait C

Offspring: AC

Offspring: BC

Mother Trait D

Offspring: AD

Offspring: BD

From this, we see that 25% of the offspring had traits AC, 25% were BC, 25% were AD, and 25% were BD.

Let’s assign green peas big G’s and white peas small w’s for the sake of argument. This is technically incorrect, but I’ll show you why later.

Brother Mendel thought that his peas were simply G if they were green and w if they were white. Punett would express this like this:

Gregor’s Peas (Wrong)

Trait G

Trait w

G

From this, it would seem as though 100% of the offspring had the G trait.

What Gregor didn’t realize at first was that, like sex chromosomes (female are XX, male are XY), there are two components for any trait, including color. What Gregor was seeing was phenotype (physical trait) and missing the genotype (genetic trait) So, again using Mr. Punett’s invention, we see that:

Gregor’s Peas (correct)

Trait G

Trait G

Trait w

Gw

Gw

Trait w

Gw

Gw

100 % of the offspring had both the G trait, and the w trait. Their genotype was Gw, but because the white color was recessive, it didn’t show up, so the pea had a green phenotype. In later breedings of the sibling original white peas back to the new generation of plants, we see that:

Trait G

Trait w

Trait w

Gw

ww

Trait w

Gw

ww

The odds are now that only 50% of the peas are Gw (phenotype, however, is green), with 50% of the peas being ww (white).

Now, before I told you that this Gw business was technically incorrect, and that’s true. When dealing with traits, you need to keep the same letters to show blending of the same traits. Usually, it is named after the dominant trait, so in this case, green would still be G, but white would be g. Got it? Not so bad, huh?

Okay, do you think you’ve got it? I hope so, because this is the end of your beginning genetics course!

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