Understanding the role of genetics in coat colour

Key Principles:

• Homozygous Black individuals (male or female) will produce black progeny 100% of the time, regardless of their mating partner

• Heterozygous Black individuals (male or female) will produce black progeny at least 50% of the time, possibly more often, depending upon their mating partner

Ten Basic Rules:

1. All individuals have two sets of genes, one from each parent.
2. For any one characteristic (such as coat colour) only one of the genes is expressed.
3. In any individual, therefore, there are two genes for coat colour, but only one that is expressed.
4. Though only one gene is expressed, either gene may be passed on to progeny.
5. Each individual is given two letters, signifying the two genes (i.e. 'BB' or 'Br' or 'rr').
6. The letter 'B' signifys the gene for a black coat, and as an upper case letter it signifys a dominant gene.
7. The letter 'r' signifys the gene for a red coat, and as a lower case letter, it signifys a recessive gene.
8. A dominant gene will always be expressed over a recessive gene.
9. Animals with at least one copy of the dominant 'B' gene ('BB' or 'Br') will be black.
10. Animals with two copies of the recessive 'r' gene ('rr') will be red.

1. Dominance of the black gene. Any animal with the black gene in its genotype must be black.
2. Homozygosity or heterozygosity. An animal with the homozygous genotype 'BB' can only pass on the B gene, whereas an animal with the heterozygous genotype 'Br' can pass on either a 'B' or an 'r' gene.

Buying bulls:

How do I decide which bull to buy?

If colour is an important issue for your program, then there are a number of steps you can take to control the colour of your herds progeny. The first step is understanding what will happen when you mate a particular bull to your herd. To understand this, you need to know the genotype of the bull, and ideally the genotype of your females. Lancaster Simmental will provide you with the genotype for all our sale bulls, and classify them to help you choose the bull that will suit your program. Lets go through the possibilities of different genotypes using the diagram below:

How to read this diagram:

This diagram shows all possible progeny resulting from matings between males and females with the dominant black gene (B) and the recessive red gene (r).

Each 'punnet square' (or figure) below represents one mating.

The green squares are the two genes of the male.

The grey squares are the two genes of the female.

Each possible progeny is represented by a black or red square, the colour of the square indicating which colour their coat will be.

You can use this diagram to match the genotype of a bull you are considering buying, with the genotypes of your cows (or their possible genotypes) to see the resulting progeny:

1. If the bull is homozygous black, look at figures 1, 4 and 7 - you will note that all possible progeny are black.
2. If your bull is heterozygous look at figures 2, 5 and 8 - you will note that the colour of progeny from a heterozygous bull depends on the cows genotype. If the cow is homozygous black then all progeny are black (fig 2), if she is heterozygous then one quarter of the calves will be red (fig 5), if she is homozygous red then one half of the calves will be red (fig 8).
3. If the bull you are considering is homozygous red, then figures 3, 6 and 9 are the ones you want and you will see the progeny once again depend on the genotype of the females you mate the bull to. The mating still results in all black progeny if the female is homozygous black (fig 3), but results in one half of the progeny being red if mated to a heterozygous cow (fig 6), and all the progeny being red if mated to a homozygous red cow (fig 9).

If you can't see the diagram, click here...