Firstly I'd like to apologise on the delayed post this one has taken a lot of time!
Shorthorn cattle are known for their versatility, adaptability, and productivity. They can thrive in different climates and environments. Shorthorn cattle are also famous for their variety of colours, which range from red, white, roan, or red white markings. But how do these colours come about? What determines the colour of a Shorthorn calf? In this blog post, we will explore the basics of genetics and how they apply to Shorthorn colours.
What is genetics?
Genetics is the study of how traits are passed from parents to offspring. Traits are characteristics that can be observed or measured, such as the colour of the coat, the shape of the horns, or the size of the body. Traits are determined by genes, which are segments of DNA that carry instructions for making proteins. Proteins are molecules that perform various functions in the body, such as building structures, regulating processes, or fighting diseases.
Each gene has two or more versions, called alleles. Alleles are variations of the same gene that may have different effects on the trait. For example, there is a gene that controls the presence or absence of red pigment in Shorthorn cattle. This gene has two alleles: one that produces red pigment, and one that does not produce red pigment. Each individual inherits two copies of each gene, one from each parent. The combination of alleles that an individual has for a gene is called the genotype. The genotype determines the potential of the trait, but it may not always be visible. The actual appearance or expression of the trait is called the phenotype. The phenotype is influenced by the genotype, as well as by the environment and other factors.
How are genes inherited?
The inheritance of genes follows some basic rules, which can be illustrated by using a tool called a Punnett square. A Punnett square is a diagram that shows the possible outcomes of a genetic cross, or the mating of two individuals. The Punnett square has four boxes, each representing a possible offspring. The alleles of the parents are written on the top and the left side of the square, and the alleles of the offspring are written inside the boxes.
To use a Punnett square, we need to know the genotypes of the parents. For simplicity, we will use letters to represent the alleles. For example, we will use R to represent the allele for red pigment, and r to represent the allele for no red pigment. We will also use capital letters to represent dominant alleles, and lowercase letters to represent recessive alleles.
Dominant alleles are those that can mask or hide the effect of another allele, while recessive alleles are those that can be masked or hidden by another allele. For example, R is dominant over r, which means that an individual with one R and one r allele will have red pigment, because the R allele will cover up the r allele. An individual will only have no red pigment if they have two r alleles, because there is no R allele to mask them.
Let’s say we have two parents who have red pigment, but they both carry one r allele. Their genotypes are Rr and Rr. We can use a Punnett square to predict the possible genotypes and phenotypes of their offspring. Below I've attached an simplified Punnett's Square.
We can see that there are four possible genotypes for the offspring: RR, Rr, Rr, and rr. However, there are only two possible phenotypes: red pigment and limited red pigment. The probability of each phenotype is calculated by dividing the number of boxes with that phenotype by the total number of boxes. In this case, the probability of red pigment is 3/4, and the probability of limited red pigment is 1/4 (Shorthorn whites often have red over their polls, in their ears and around their coronet bands). This means that out of four offspring, on average, three will have red pigment and one will have limited red pigment (white).
How does this apply to Shorthorn colours?
The colour of Shorthorn cattle is determined by two genes: one that controls the presence or absence of red pigment, and one that controls the distribution of red pigment. The first gene has two alleles: R, which produces red pigment, and r, which does not produce red pigment. The second gene has two alleles: S, which distributes red pigment evenly, and s, which distributes red pigment unevenly. The combination of these two genes results in four possible phenotypes: red, white, roan, red with white marks.
Red Shorthorn cattle have the genotype RRSS, which means that they have two copies of the R allele and two copies of the S allele. This means that they produce red pigment and distribute it evenly, resulting in a solid red coat.
White Shorthorn cattle have the genotype rrss, which means that they have two copies of the r allele and two copies of the s allele. This means that they do not produce red pigment and distribute it unevenly, resulting in a solid white coat.
Roan Shorthorn cattle have the genotype RrSs, which means that they have one copy of the R allele and one copy of the r allele, and one copy of the S allele and one copy of the s allele. This means that they produce red pigment and distribute it unevenly, resulting in a mixture of red and white hairs, giving a roan. White limiting can leave an RrSs animal appearing to be solid red but genetic testing and potentially offspring would disprove RRSS. I have posted an example of a heifer I had mistakenly registered as red (reference red roan Diamond Eva Maid 40E). She when clipped as a yearling showed limited roaning under her tail. In production she is the dam of a white calf which would have disproved my observed colour if clipping hadn't before.
Red White marks Shorthorn cattle have the genotype Rrss or rrSs, which means that they have one copy of the R allele and one copy of the r allele, and two copies of the s allele, or two copies of the r allele and one copy of the S allele. This means that they produce red pigment and distribute it unevenly, resulting in patches of red and white on their coat.
To understand how these colours are inherited, we can use the Punnett square again. For example, let’s say we have two roan parents, with the genotype RrSs and RrSs. We can use a Punnett square to predict the possible genotypes and phenotypes of their offspring.
We can see that there are 16 possible genotypes for the offspring, but only four possible phenotypes: red, white, roan, or red white marks. The probability of each phenotype is calculated by dividing the number of boxes with that phenotype by the total number of boxes. In this case, the probability of red is 1/16, the probability of white is 4/16, the probability of roan is 8/16, and the probability of red white marks is 3/16. This means that out of 16 offspring, on average, one will be red, four will be white, eight will be roan, and three will be red white marks.
In conclusion, Shorthorn colours are a fascinating example of how of codominance. Shorthorn colours are determined by two genes that control the presence and distribution of red pigment in the coat. These genes have different versions, called alleles, that can interact in different ways to produce different colours. Some alleles are dominant, which means that they can hide the effect of another allele. Some alleles are recessive, which means that they can be hidden by another allele. An example would be white limiting factor. And some alleles are codominant, which means that they can both show their effect at the same time. Codominance is what causes the roan and bicolor phenotypes in Shorthorn cattle, where both red and white colours are visible in the coat. Shorthorn colours are a simple way to understand the basics of genetics, and a wonderful way to appreciate the diversity and beauty of Shorthorn cattle.
Below are photos from a previous sale catalog there are thousands of variations to roan and red with white marks and red and white colour patterns and are not limited to my examples.
Red White Marks
Red and White
red roan (Diamond Eva Maid 40E)