Do these genes make my backside look big?

An intriguing example of epigenetics is evident in the size of the backside of ‘callipyge’ sheep – leading to the genuine question: do these genes make my backside look big?

Inheritance of traits is not just about which genes parents pass on to their offspring. It is also about which parent passes on which gene, and whether a gene is switched on or off in the process.

As mule breeders have known for thousands of years, the offspring you get from crossing a horse and a donkey depends on which animal is the sire (father) or dam (mother) – you get a mule when the sire is a donkey and the dam a horse, and a hinny when the sire is a horse and the dam a donkey.

Scientists believe the differences between the mule and the hinny are caused by a phenomenon known as epigenetics, which relates to certain molecular tags attached to genes. These tags can switch genes on or off and affect the traits of a person, animal or plant.

Offspring inherit one gene copy from each parent, and for most genes both copies are switched either on or off.

However, for a few genes, epigenetic tags switch off the gene copy from only one parent.

According to CSIRO Research Scientist Dr Ross Tellam, an intriguing example of epigenetics is evident in so-called callipyge sheep.

Meaning ‘beautiful buttocks’ in Greek, the callipyge gene - when mutated - endows sheep with big, muscular backsides.

Callipyge sheep show muscle enlargement (left) only when the mutated gene is inherited from the father. Credit: Agricultural Research Service, United States Department of Agriculture (image permission granted).

“This is a fascinating mutation that has large effects on muscle, but it does not obey the traditional laws of genetics,” Dr Tellam says.

“Strangely, the callipyge trait is only seen in offspring with one mutated gene copy, inherited from the sire. If the mutation is inherited from the dam or from both parents, there are no visible effects. And even more strangely, this has nothing to do with the sex chromosomes.”

Another epigenetic phenomenon is the large offspring syndrome in cattle and sheep – a result of embryos exposed to unusual environments.

It can be caused by assisted reproduction techniques, but also by high urea diets in ewes.

“Epigenetic switches can be affected by environmental signals – nutrients, toxins, climate and social factors such as stress and parental care. This can happen during early development but also later in life, and the effects can in some cases last for several generations,” Dr Tellam says.

“Epigenetic tagging of genes also happens throughout life and in some instances may be reversed. Epigenetic inheritance therefore allows more flexibility in adapting to changing environments and this is a major evolutionary advantage to a species.”

Much of Dr Tellam’s research over the past decade has focused on improving our understanding of epigenetics, particularly its role in livestock production traits including muscling in sheep.

This may help livestock producers optimise breeding and management practices, and develop new technologies to increase efficiency and profitability of their business enterprises.

This article is reprinted with permission from the CSIRO Livestock Industries E-newsletter. CSIRO Livestock Industries can be found at http://www.csiro.au/org/CLI.html.