Introduction to Color Genetics

In color genetics, a trait represents the visible, measurable, or quantifiable expression of one or more genes. The set of chromosomes carrying the genes forms the individual’s genotype, while the expression of those genes influenced by the environment constitutes the phenotype. To understand color inheritance, it is essential to define several terms. First, we must know that there is a relationship of dominance and recessiveness between the genes that determine color. Dominant genes will be represented by capital letters, and recessive ones by lowercase letters. Whenever a recessive gene is paired with a dominant one, the latter will prevent the expression of the recessive gene in the phenotype, although this gene will appear in the individual’s genotype. Genes are made up of pairs of alleles that are inherited independently from each parent, and that are located on the chromosome at a specific locus. Among these alleles, epistatic or hypostatic relationships will be established depending on whether they allow the expression of or mask the genes they refer to.

The genes that transmit color are two pairs, and their dominance/recessiveness and epi/hypostasis relationships are as follows:

• Gene B: transmits black color, is dominant over chocolate and recessive regarding yellow.
• Gene b: hypostatic allele that allows the transmission of chocolate color and is recessive regarding black and yellow.
• Gene E: This gene does not transmit a specific color; it is the epistatic allele that acts by nullifying the dominance of yellow color, and therefore allows the expression of chocolate or black.
• Gene e: hypostatic allele regarding E, which allows the expression of yellow color dominant over black and chocolate.

Genetic Maps by Color

The combination of these genes in terms of color (Bb) and transmission (Ee) will produce 9 different specimens with the following genetic maps:

BLACK

• EE BB: Pure dominant black
• EE Bb: Black carrying chocolate
• Ee BB: Black carrying yellow
• Ee Bb: Black carrying yellow and chocolate

YELLOW

• ee BB: Pure yellow, with normal pigmentation
• ee bb: Yellow carrying chocolate, with lack of black pigmentation in mucous membranes
• ee Bb: Yellow with normal pigmentation in mucous membranes, carrying chocolate

CHOCOLATE

• EE bb: Pure chocolate, can produce black and chocolate puppies depending on the cross
• Ee bb: Chocolate that can produce puppies of all three colors depending on the cross

Explanation and Examples of Color Genetics

According to Mendel’s theory of inheritance, there are two types of genes. Those named with capital letters are dominant genes and those named with lowercase letters are recessive genes. Simplifying quite a bit, for each trait, we have a pair of genes, one from the male and one from the female. For example, if blue eye color is gene a and brown eye color is gene A… which is the dominant one? Brown, so a child who inherits gene a from their mother and gene A from their father will have a genotype Aa for eye color, so they will have brown eyes, but they carry the blue color, so they could have children with blue eyes if their future partner also carries gene a.

In summary, dominant traits appear whenever the gene that defines them is present, while for the recessive trait to appear in the phenotype (external appearance), the individual must have two recessive genes:

• AA: brown eyes, pure, does not carry any other color.
• Aa: brown eyes, carrier of blue color.
• aa: blue eyes.

If crossed with another aa individual, there will never be offspring with brown eyes. That is what defines the dominant gene: two recessive specimens cannot produce a dominant one, therefore, two yellow Labradors will never have black puppies, and two chocolate Labradors neither. Because a cross between a pure chocolate Labrador and a pure yellow Labrador gives 100% black puppies, why? In the Labrador, color is not defined by a single pair of genes, but by four genes (two from the male and two from the female).

The E and B Genes Explained

Gene E: Black gene associated with yellow. This gene defines whether the dog is black or yellow. In capital letters it is black, so a dog that carries this gene will not be yellow. It will be black or chocolate, depending on the genotype of the other genes.

Gene e: Yellow gene. A Labrador that carries one e gene will be a carrier of yellow, it can be chocolate or black. A Labrador that carries two ee genes will always be yellow.

Gene B: Black gene associated with chocolate. A Labrador that has a B gene will not be chocolate. It will be black or yellow, depending on the rest of the genes.

Gene b: Chocolate gene. A Labrador that carries one b gene will be a carrier of chocolate. A Labrador that carries two bb genes will be chocolate or yellow (depending on the rest of the genes).

Genes E and e define the color: black or yellow. However, genes B and b define the oxidation of black.

Chocolate color is an oxidation of black, so for a puppy to be chocolate, it not only has to have a bb genotype, but it must also have at least one E gene, since if the puppy does not have the black gene, it cannot oxidize that color. In cases where the puppy only has yellow and chocolate genes, it will be born depigmented (nbp), because the chocolate gene oxidizes the color and eliminates pigmentation. In the case of black, nothing happens, but in yellow, the Labrador has light eyes and lack of pigmentation in mucous membranes, which is highly undesirable and not admitted by the standard. And very likely with future skin problems.

Color Genetics Review

EEBB: A pure black Labrador, meaning one that cannot produce offspring of other colors, has a genotype EEBB.

This means it will give each puppy one gene from each group; since it only has E and B, it will only transmit black color, so its puppies will be E_B_ (from here on, the dashes are unknowns, since we do not know the genotype of the other breeder). When gene E and gene B are together, the Labrador is always black.

EeBB: Black Labrador carrying yellow. It will transmit gene B to all its offspring, so it will not have any chocolate puppies (they will all be B_, then black dependent on chocolate). And it will transmit gene E to 50% (black puppies) and gene e to the other 50% (which could be yellow if they receive another e gene from the other breeder, or black carriers of yellow if they also receive an E gene).

EEBb: Black carrying chocolate. It has two E genes, so it is black and does not carry yellow. It has one B gene, which dominates gene b, which is why it is not chocolate. This Labrador will not have yellow puppies, because it will always transmit gene E, but it will transmit the black gene B to 50% of its offspring (which will not be chocolates) and gene b to the other 50%, which may or may not be chocolate depending on whether they receive another b gene or not.

eeBB: Yellow Labrador. The desired yellow, it has two e genes, so the color is yellow, and it also has two B genes, so there is no oxidation. Pigmentation is good. It will only have black and yellow puppies (depending on the cross).

EEbb: Chocolate Labrador. Two black genes oxidized by two chocolate genes. It will only have black and chocolate puppies (depending on the cross).

Why do a pure yellow and a pure chocolate produce 100% black?

eeBB x EEbb ….. we know that each breeder will transmit one gene from each group.

Breeder 1 (eeBB) will always transmit eB (it has no others), so its puppies will be: e_B_

Breeder 2 (EEbb) only transmits Eb, for the same reason, it cannot transmit any other, so its puppies will be: E_B_

All puppies from this cross are EeBb. What color is this? They have one black gene and one yellow gene, black would dominate, so they are not yellow. And they also have another black gene and one chocolate gene… according to what we have seen, black would dominate and they would not be chocolate. We are looking at a black puppy carrying both colors.

And to finish, there are also:

• Eebb: Chocolate Labrador carrying yellow. Can have puppies of all three colors.
• eeBb: Yellow Labrador carrying chocolate. Can have puppies of all three colors.
• eebb: Yellow Labrador carrying chocolate. Undesirable cross, the two chocolate genes oxidize the yellow and the dog is depigmented (nbp). It is not admitted by the standard.

Of all the possible combinations of these nine genotypes, 44 different crosses are obtained, which we specify in the tabs below with their types and percentages.