Lesley Partridge
At the Central Region Autumn Open Day I took a selection of printed photo panels which I had made to illustrate different aspects of the Jacob breed. The two topics discussed on the day were Jacob markings and ear set and temperament. The following article expands on this discussion a little more.
Although they don’t look it, Jacobs are essentially black sheep carrying two dominant black genes, Ed Ed BB. In the first three months of life in the embryo it is an important time for the distribution of pigment. Embryonic stem cells are called the Neural Crest where they generate a wide variety of cell and tissue types during embryonic and adult development. The cranial Neural Crest gives rise to the majority of the bone and cartilage of the head and face, nerves, muscle tissues and pigment cells.
There are three important sites where the pigment or Melanoblasts first start to develop, on the top of the head, near the eye, near the ear, and several other places on the body. Once the pigment cells have finished migrating they then take up positions at the base of hair follicles which are then referred to as Melanocytes. A few pigment cells then migrate outwards joining up to form larger patches on the top of the head. According to Cattanach 1999: “they then spread out until they meet under the chin, then spread down the sides of the body.”
There is a recognised importance for the black Melanocytes to be present on the brain area to regulate mood and temperament, but a failure of them not reaching there will cause seizures and diverse behaviour.
RECESSIVE WHITE SPOTTING
The Jacob carries a recessive white spotting ss gene which affects not only the change of black to white, but also the structure of the wool making the white areas much finer. This can be seen in lambs’ fleeces of the ‘patchwork/Intermediate’ type where the white wool appears short and fine, and the black is coarse and long. Experience in breeding Jacobs suggests that the recessive white ss gene in some sheep can produce different strengths of pink skin.
STRONG ss WHITE
In its strongest form it produces very blood red pink skin. Pedigree breeders try to select for ‘clean’ skin in their show and breeding stock. Some Jacobs will stay 90% clean pink skinned for most of their lives.
MEDIUM ss WHITE
Some sheep are born pink skinned but develop pale spots with increasing age. Skin blemishes can fade during the winter months only to appear again in the presence of the summer ultraviolet light. In others, the black base pigment can gradually show through as freckling allows fibres of black wool to grow from the spot of pigment.
WEAK ss WHITE
Although born pale, the whiter woolled skin areas can develop into a blue/grey colour, darkening with age .This skin colour was quite common in the early days before breed improvement had started.
BLACK PATTERN MARKINGS
It seems there is very little scientific information about ‘Black Patterns Over White Spotting’ in livestock. My own field observations suggest there seems to be a big variation of intensity of the black marking gene over a recessive white gene background. The chart below shows the range of black marking variation I believe to exist:
- Large body spots Plate 2 A, D and Plate 3 A,D,F
- Small body spots Plate 2 B,C Plate 3 B,C,E
- Head mask Plate 2 A,B,C,D Plate 3 A,B,D,F
- Spotty feet Plate 2 B Plate 3 A,B,D,E
- Black on leg – late 3 D
- Dirty nose – Plate 3 B
- White head Plate 2 C Plate 3 C, E
- White on ear Plate 2 C Plate 3 C
- Panda eye – Plate 3 E
In the seventeenth century Jacobs were known as ‘Spanish sheep,’ kept for their attractiveness in the parks of large English country estates by the landed gentry. Since those days the body conformation is vastly improved but the genetic marking structure is still present. Jacob Sheep Society members try to breed for the Plate 2 type which makes up the standard for the show ring.