January 9, 2009

Sheep Farm Husbandry - Breeding coloured sheep

Sheep, husbandry, breeding, coloured sheep, genetics of colour in sheep, chromosomes, alleles, dominant and recessive genes, mutations

By Dr Clive Dalton

Who wants coloured wool?
Breeding coloured sheep to produce wool for home spinning and weaving has been very popular in New Zealand, where enthusiastic breeders formed the Black and Coloured Sheep Breeders’ Association (BCSBA) in 1976, to foster interest and help breeders sort out the genetics of pigmented wool. Many breeders started off keeping a black or coloured lamb for a pet which then became the founder of a coloured flock.

Knowledge of basic genetics needed
Before starting, it’s important to understand some basic Medelian genetics. The Austrian monk Gregor Mendel worked out the laws of inheritance of single genes for traits like colour and shape in sweet peas, and these apply to pigmentation in sheep.

Note that when we come to breeding for traits that are controlled by many genes like growth, fertility and fleece weight, then “population genetics” based on statistics are used, using the normal distribution as discussed in an earlier article.

These basic genetic principles have bored many generations of students, and are quite complicated when you get deeply into pigmentation. Finding out what alleles you have in your sheep is rather like a detective story, so you would be well advised to join the BCSBA to get help from breeders who have done so much work on sorting things out. Here are some of the basic points on Medelian genetics for reference.

Chromosomes, genes and alleles
  • Inside the nucleus of all body cells are string-like structures called chromosomes.
  • They are present in pairs and the sheep has 27 pairs, two of which are sex chromosomes that control the sex of the individual.
  • On each chromosome are genes which are the basic units of inheritance.
  • When the body cells divide (called meiosis) the daughter cells are identical in all respects to the parent cell with the same number of chromosomes.
  • When the sperm and eggs divide (called meiosis) they end up with half the number of chromosomes - so after fertilization thing are back to normal.
  • The place on the chromosome where the gene is situated is called a locus (plural is loci).
  • Each gene is made up of separate parts called “alleles” and it’s the alleles that separate when the chromosomes divide. This is often misunderstood.
  • Each offspring therefore ends up with a sample half of the genes of each parent – controlled by the laws of chance.
  • Dolly the sheep was a “clone” made by taking the cells from her mother’s udder and multiplying them, missing out meiosis. So Dolly had identical genes to her mother and not a sample half.
  • Some genes are “lethal” and cause the death of the animal either early in life or later. Others are “semi-lethal” and disrupt normal health but the animal lives.
  • Some alleles appear to be transmitted as a group in cell division rather than separately and this is called “linkage”.
  • Some alleles are on the sex chromosomes so only the males may inherit a trait or only the females. This is called “sex linkage”.
Dominant and recessive
  • Some alleles of a gene are “dominant” and mask the effects of a “recessive” allele (see Table below).
  • A capital letter is used for a dominant allele (e.g. A) and a lower case letter for a recessive (e.g. a).
  • So the animal having alleles from both parents could be AA or Aa or aa.
  • Knowing the relative dominance of each allele to any other means they can be ranked in an “allelic series” which helps in predicting what the animal looks like.

Phenotype and genotype
  • The phenotype is what the animal looks like.
  • The genotype is what is in its genes – it’s genetic makeup.
  • Sometimes the phenotype accurately reflects the genotype, but other times it does not.
Homozygote and heterozygote
  • A homozygote is an individual that has identical alleles at a specified locus (e.g. AA or aa).
  • A heterozygote has non-identical alleles at a specified locus (e.g. Aa).
  • A mutation is a change in a gene and gives rise to new alleles at a particular locus.
  • Mutations are the main way new heritable material occurs.
  • Genes mutate spontaneously but it doesn’t happen very often. Lethal or semi-lethal genes have been the result of mutations in animals.
  • The short legged Ancon sheep was a mutation.
Loci in coloured sheep
There are currently seven known loci controlling pigmentation in sheep:
  • The A (Agouti) locus with 18 alleles controlling pigmentation over the body.
  • The B (Brown) locus with 2 alleles controlling the presence of black or brown pigmentation.
  • The C locus with 2 alleles controlling the presence of albinism (no pigment).
  • The E (Extension) locus with 2 alleles controlling the presence of dominant pigmentation.
  • The P (Pigmented head) locus has (it is thought) 4 alleles (3 semi dominant) controlling pigmented heads. The black head of the Dorper has some of these alleles.
  • The S (Spotting) locus with 2 alleles controlling the presence of white markings on pigmented areas.
  • The T (Ticking) locus with 2 alleles controlling “ticking” or spotting or flecking.
  • The Rn (lethal roan) locus with 2 alleles found in Asiatic breeds, some of which are now here in New Zealand, e.g. Karakul.
General points
  • The A locus with all the 18 known alleles is the most important one controlling colour patterns and you need to be aware of these (see Table below). If the sheep has either one or two of the (dominant) white/tan alleles, then it will be a white sheep.
  • The E locus is also important as it works with the A locus and allows the full expression of the Agouti locus. There is the wild E+ allele and the ED dominant black.
  • The C locus is not very important.
  • The B and S loci are very important.
  • At the B locus, the dominant B+ allele causes black pigment, and the recessive Bb produces a chocolate brown also called “moorit”.
  • Few brown sheep (BbBb) are seen, as B+B+ and B+Bb are black (because they both have the dominant B+).
  • In brown sheep (BbBb), a good guide to identifying them is that they go grey on the nose and ears with age.
  • At the S or spotting locus, the wild dominant allele S+ gives unbroken pigmentation and has no white markings, and the spotted allele Ss is a recessive with white markings on the pigmented areas.

Can you see why it can be difficult to sort out the genotype of a coloured sheep? Breeders are continually crossing sheep in test matings to see what colours are produced and hence what their likely genetic makeup is likely to be. It’s a great challenge and one you’ll face when you start a flock.

How to start a flock of coloured sheep
  • Start off with white sheep and buy a coloured ram and see what happens. You will most likely get some coloured and some white lambs. Carry on putting black rams (of unknown genotype) over the white and coloured sheep in the flock and be content with the outcome.
  • If you want to get into the genetics of your flock, then this could take time and you should seek help from BCSBA members.
  • Buy coloured ewes and mate them to a coloured ram (of unknown genotype) and see what happens. Here at least you would get mostly coloured offspring – of unknown genotype but an experienced breeder could probably predict with some accuracy what they were – at least the main genotypes.
  • Buy rams of known genotype from a breeder such as (aaBBSS) for a black self-colour ram without spotting, or AwAwbbSS for a brown mouflon without spotting.
  • If the ram’s genotype is known then the colours and patterns of his progeny can be predicted with more certainty.
  • Starting with a white flock means that you can at least select the wool type you want to breed for before adding the colour. But it will take more time.
  • All rams used must be sound and free from structural faults. Pigmentation should be secondary to health and structural soundness.
This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.

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