Theory of colour inheritance in alpacas

By Elizabeth Paul

B.App.Sci., (App.Biology) R.M.I.T.,
Cert. Animal Technology, F.I.T.

Reprinted by permission of Elizabeth Paul and the Australian Alpaca Association (AAA)

The author wishes to thank Dr David Propert, formerly Associate Professor of Human genetics, Department of Applied Biology and Biotechnology, R.M.I.T., for his constructive comments and advice.

Introduction

Coat colour in mammals is almost entirely dependent on the presence or absence of the pigment, melanin, in the skin and hair. There are two distinct forms of this pigment: eumelanin (brown/black) and phaeomelanin (red/yellow).

Melanin is produced in granules by cells called melanocytes, which are concentrated in the hair follicles, the skin epidermis and the retina of the eye. Colour in these areas is determined by the size and shape, as well as by the type, number and distribution of the granules. The nature of the granules is affected by many factors, both internal and external. Colour inheritance patterns for small laboratory animals, particularly mice, and some larger domestic animals such as dogs, cats, horses and cattle have been intensively studied. Many of their genotypes and phenotypes have been described (Searle, A.G., 1968).

If a living cell is removed from an animal and examined under a microscope, the large, clearly defined nucleus can easily be seen. The nucleus is the control centre which directs all the activities of the cell.

The nucleus consists mostly of DNA (deoxyribonucleic acid), the hereditary material of life. DNA is formed into long threads, called chromosomes, which occur in pairs within the nucleus.

A gene is a very small segment of a chromosome which determines a particular characteristic of the cell. Since chromosomes occur in pairs, there are two sites where the gene may be found. These sites are called loci (singular, locus) and the two forms of the gene are called alleles. If the alleles are the same as each other, they are termed homozygous; if they are different, they are termed heterozygous. There may also be more than two alleles of a given gene and these are termed multiple allelic series.

Every cell of the animal (except for the sperm and egg cells) contains exactly the same amount of DNA and, therefore, the same chromosomes and genes as every other cell.

Each species of animal has a specific number of chromosomes called the 2n number: in humans, 2n = 46, or 23 pairs of chromosomes. Of these, 22 pairs are called somatic or ‘body’ chromosomes and one pair is the sex chromosomes where an XX pair gives a female and an XY pair gives a male.

To illustrate the nature of inheritance, the process of sperm production in the sperm-forming cell of a sexually mature male will be described briefly.

At the start of the process, matching pairs of chromosomes come together within the nucleus of the cell. Each chromosome duplicates itself and divides into two identical halves along its length. This creates a group of four chromosomes for each pair. When all the chromosomes have paired up and divided, the nucleus itself divides into four new nuclei.

Each new nucleus takes only one of the four chromosomes from each chromosome pair. In this way, the 2n number of chromosomes of the male animal is exactly halved. The four new nuclei now become mature sperm, each containing only one chromosome from any given pair and each having only n number of chromosomes.

If the alleles of a particular gene on a given pair of chromosomes are identical (homozygous) then all four sperm will have identical alleles derived from the pair. If the alleles are different (heterozygous) then two of the sperm will have one allele and two will have the other allele. Eggs in the female are produced in a (more or less) similar way.

At fertilization, the 2n number of chromosomes will be restored. However, the new baby may have a different combination of alleles (its genotype) which will give it different characteristics (its phenotype) from one or both of its parents. This is the basis of genetic variation within a species (apart from mutation).

The genotype is the genetic make-up of the animal which is fixed at the moment of conception. The phenotype may be described as the physical expression of the genotype. It depends on the interaction of the animal’s genotype with the changing environment in which the animal finds itself during its lifetime (Evans, B., Ladiges, P.Y. and McKenzie, J., 1995).

To create a theoretical genotype, a physical characteristic (the phenotype) which can be observed in most of the animals of a species may be regarded as the dominant allele of the gene controlling that characteristic.

Assume that, in the species above, the 2n number is 4: that is, they have two pairs of chromosomes, one somatic and one sex pair. A particular characteristic is observed and designated as P. The recessive allele is then p. (Any letter can be chosen but, once it is, all the alleles of that gene must have the same letter.)

Assume also, that the alleles are located on the somatic chromosome (i.e. not sex-linked) and that P is completely dominant to p. Animals displaying the characteristic must, therefore, have at least one P allele in their genotypes. They may be PP (homozygous for the dominant allele) or Pp (heterozygous). Animals which do not display the characteristic must be pp, or homozygous for the recessive allele.

A heterozygous male animal will have a genotype of Pp and will produce sperm with chromosomes carrying either P or p. If he is mated to a female also heterozygous for P, the cross will appear as per the following example. (Note that the P alleles have been given superscripts ‘s’ for sperm and ‘e’ for egg to make it easier to see what happens to them; the sex chromosomes have not been included here.)

Progeny genotypes:

• 1, 2 and 3 will show the characteristic, since they all have at least one P allele in their genotypes.
• However, No 1 is homozygous for P and can pass on only P to its own progeny.
• Nos 2 and 3 are heterozygous and can pass on both P and p alleles.
• No 4 is the homozygous recessive animal which will look different from both its parents and its siblings. It can pass on only the p allele to its progeny.

Colour inheritance analysis

The mode of fleece colour inheritance in alpacas raises many interesting questions. Two of the most intriguing are the nature of the base colours and the production of coloured progeny from white parents.

The two wild species of South American camelids, vicunas and guanacos, are both reddish-fawn in colour over most of their bodies. If alpacas are the descendants of these two species (Hoffman, E., and Fowler, M.E., 1995), it would be reasonable to assume that the base colour of alpacas is also reddish-fawn. Guanacos also have grey or black colour around their heads and this may be the source of the black gene in alpacas.

The proposed model is based on the following assumptions:

1. fleece colour inheritance in alpacas follows the Mendelian principles of segregation and independent assortment;
   
2. there are two base colours: red and black. The genes controlling these colours are designated as R, the gene for red colour, and r, its recessive allele; and B, the gene for black colour, and b, its recessive allele;
   
3. the red and black genes interact in an additive manner to produce brown colour; and
   
4. there is another gene which controls the production of pigment and, therefore, the expression of colour. This gene is designated as M, and its recessive allele, m.

Thus, for an alpaca to have a coloured fleece, it must have at least one colour gene and one M gene in its genotype.

There are likely to be other genes which control distribution and intensity of colour, but to keep calculations fairly simple, a three-gene model only will be considered.

In order to test the correlation between theoretical and actual mating results, the Australian Alpaca Association Herd Books were surveyed.

Table 1 shows possible genotypes for this model together with their proposed phenotype groups.