Genes involved in footrot: Patterns of inheritance
Footrot is a disease that costs New Zealand sheep farmers over $80 million a year to treat. It can cause lameness, weight loss and even death in sheep. Scientists at Lincoln University have developed a test to see what kind of genetic resistance a sheep has to getting footrot.
1 = very high resistance
2 = resistance
3 = no resistance
4 = susceptible
5 = very susceptible
Lets call the footrot gene F and label these genes F1, F2, F3, F4 and F5 (with the number indicating the score of that allele).
Each adult sheep will contain 2 copies of each of their genes, including the footrot gene. This is because all animals and most plants have pairs of chromosomes. These chromosome pairs (called homologous pairs) contain the same sequence of genes, so each of the two chromosomes will have an allele for the same genes.
A sheep could have any combination of footrot alleles. For example this ram has one F1 allele and one F5 allele. We can write this most quickly as F1/F5 or 1/5 if someone knows what we are talking about. As a result this sheep is very resistant to footrot.
When this ram produces its sperm, only one of each homologous pair of chromosomes goes in to each sperm. This is so that when the sperm fertilises the egg, the offspring created gets a full set of chromosomes with half from each parent. In this case, half of the ram’s sperm will get the chromosome with the F1 allele. The other half will get the chromosome with the F5 allele. A similar process happens in the mother sheep to produce eggs or ova. It is called meiosis or gametogenesis.
Either of these types of sperm is equally likely to fertilise an egg.
We can summarise this whole process in a Punnett square.
If the ram from this example, F1/F5, is crossed with a F1/F5 sheep then:
In this case, there is a 1 in 4 chance that the offspring of these sheep will by F1/F1 and very resistant to footrot. There is a 2 in 4 chance that the offspring will be F1/F5 and very resistant to footrot and a 1 in 4 chance that they will be F5/F5 and be very susceptible to footrot.
It is assumed that the footrot alleles are codominant, which means that both are expressed. In a F1/F5 sheep, because both resistance and susceptibility are present and expressed, the sheep will be resistant to footrot.
Which of these offspring are likely to be culled?
Are there any other offspring that might be culled if a farmer knew the FGMT scores of the sheep?
Do you think the FGMT is a good thing for sheep?
FGMT has the potential to save farmers lots of money, by choosing to breed sheep with high resistance to footrot.
Get information sheet: Impacts of the footrot gene-marker test on New Zealand sheep farming
- 20 November 2007