BRAHMAN NEWS SEPTEMBER 2011 Issue #172
by Sarah Streeter
The ability to confidently apply selection pressure and increase the frequency of the polled trait in tropical breeds has positive implications for animal welfare, productivity, labour requirements and safety of station staff. The frequency of the polled gene in Brahman cattle is generally low compared to some other breeds. In the past, breeding polled Bos. indicus cattle has been complicated, due a complex interaction of the polled, scurred and African horn gene. However the recent release of a gene marker test for the trait has allowed us to distinguish a homozygote ('true poll') polled animals with a high level of confidence in Brahman cattle. Homozygote polled sires provide a more rapid rate of infusion of the polled gene into a Brahman herd, with 99% polled/scurred progeny when mated with horned cows compared to just 45% polled/scurred progeny from the same matings using a sire with only one copy of the polled allele (heterozygote). This article aims to provide the background required to understand the mating outcomes from different polled/horn genotypes, which can now be determined using a gene marker test.
There are a few terms that it can be useful to become familiar with when understanding simple inheritance in relation to the poll/horn trait.
Genotype: The genetic makeup of an animal (ie. can't be seen, but can be determined with genomic technology such as gene marker tests.
Phenotype: The observed performance or trait, as a result of gene expression (eg. coat colour, horn status, 200d weight)
Chromosome: A long strand of DNA/genes
Gene: A DNA sequence which sits at a specific location on a chromosome, which controls or contributes to a phenotypic outcome
Gene marker: A detectable gene or fragment of DNA used to identify a linked gene that is otherwise unable for us to detect (hence the term 'marker' )
Homozygous: Gene is made up of two identical alleles
Heterozygous: Gene is made up of two different alleles (ie. a 'carrier')
This is easy to remember if you think of 'Homo' as meaning 'same, 'Hetero' as meaning different.
An animal has two copies of all genes. When the animal produces its own gametes (sperm or eggs) the gamete will have only one of these genes randomly selected, so it ultimately only has half the genes required for an animal. When an embryo is formed, it will again have a full set of genes, but one of each gene from each parent. So an animal's genes are a mix of 50% randomly selected from the female parent and 50% randomly selected from the male parent.
With simple inheritance, there are two forms of a gene (each called an 'allele') and one can be dominant over the other. For example, say we have the genotype BW in sheep (heterozygous), where B=black colour and W=white colour, but black is dominant over white. The phenotype of the sheep would be black. If it was BB (homozygous), it would still be black, but if it was WW (homozygous) it would be white (as there is no B gene to dominate). This works the same for Pompes disease in cattle. Say if the tt allele causes pompes, and TT doesn't, and TT is dominant. An animal that is tt tt would have pompes. A TT tt animal would not have pompes disease even though it is carrying the disease gene, as the TT is dominant and overriding the effect of tt. This animal can be called a 'carrier', as it is essentially carrying the gene and can pass it on to progeny. In some cases, one gene is not completely dominant over another, and the resulting phenotype can be a mix of both. For example, if B in sheep wasn't completely dominant over W, then the BW sheep may show black and white spots.
Lets go back to the TT tt (heterozygous) pompes animal or pompes 'carrier'. This animal's sperm or egg will have a copy of either TT or tt, which will be purely random. The TT TT homozygous animal can only produce a sperm/egg that has the gene TT, as there is no tt to pass on. This is where we come to predicting mating outcomes.
Say we mate a TT TT animal with a TT tt animal. The progeny has equal chance of having either one of each parent's genes. We can easily sketch this out with a punnet square (Table below), where we can see that this cross would result in 50% of progeny being TT TT and 50% being TT tt. If TT was dominant, then 50% of progeny would be carriers of tt, but no progeny would show signs of the tt trait (Pompes).
Some gene test, such as Pompes, can tell us the exact genotype of the animal as the exact location of the gene has been identified. In the case where a trait is controlled by a number of genes, or where the exact location of the gene is difficult to detect, a gene marker test may be developed. Pieces of DNA that lie close to each other on a chromosome tend to be inherited together. A gene marker is a piece of DNA which is easy to detect, and is known to lie close to a gene driving a trait of interest. The gene marker acts just as that, a 'marker' or 'flag' letting us know that the gene is likely to be close by. Because what is actually being measured is not the gene itself, a gene marker result is unlikely to be 100% accurate. However accuracy can be understood and improved with industry validation.
The polled gene in Bos indicus cattle is not entirely simple, as it is controlled by more than one gene. Also, the dominance is not simple, rather there is the intermediate phenotype 'scurrs'. That is, rather than either the poll or horn gene being dominant over each, the heterozygote animal is more likely to be scurred. But this isn't always the case, and that is why it is difficult to tell an animal's genotype on phenotype alone. The use of the gene marker test tells us, to a high level of confidence, whether an animal is a homozygous poll (PP). An animal that carries no copy of the poll gene is a homozygote HH, while an animal that carries one copy of the polled gene is a heterozygote PH. Validation of the test results have shown that in most cases (98%) PP animals are polled. PH animals are most often (50%) scurred or polled (40%) and in few cases horned (10%). HH animals are mostly horned (93%), although few can be scurred (6%). This is summarised in the Table 1.
The same process can be used for predicting mating outcomes where simple inheritance is at work.
Let's cross a homozygote polled animal (PP) with a homozygote horned animal (HH) (Table below). All progeny from this mating would have the genotype PH, which from the validation we know have a high possibility of being scurred or polled, and only a negligible chance of being horned.
This is where we can clearly see the rapid rate of infusion of the polled gene into a horned cow herd by using a PP sire, as all progeny will carry the poll gene, and we know from validation that 99% of these animals will be polled or scurred (Diagram 2).
Let's cross a homozygote PP animal with a heterozygote PH animal (Table below). 50% of progeny from this mating would be genotype PP and 50% PH. If you were to mate these animals only once, it would be random to which progeny outcome would result. But if you mate a PP bull to 100 PH cows, then you would expect to see the ratio of PP:PH calves approach 50:50 (Diagram 3).
Table 2 shows the progeny likelihood for all possible matings of poll/horn
This article has been written with the purpose of simplifying the poll gene marker test results and mating predictions. It is acknowledged that an ambiguous allele associated with the poll and horn gene appears to occur, which means that the phenotypic outcomes of the genotypes are not 100% probable. However, the validations so far indicate that the ambiguous outcomes are uncommon in Brahman cattle, therefore for the purpose of simplifying the concepts the ambiguous allele has been ignored. For updated results of industry validation, go to http://www.beefcrc.com.au/PolledGeneMarkerTest
Table 1: Poll/Horn genotype and resulting phenotype from validation animals (simplified)
Table 2: Progeny likelihood for all potential combinations of sire/dam genotypes.