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:

fleece colour inheritance in alpacas follows the Mendelian principles of segregation and independent assortment;
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;
the red and black genes interact in an additive manner to produce brown colour; and
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.

Genotypes with at least one of each dominant gene, R, B and M, were assigned to Brown. Similarly, those with at least one R and one M, but no B were assigned to Red; and those with at least one B and one M, but no R were assigned to Black. Of the eleven genotypes assigned to White, eight have colour genes but no M gene and two have M but no colour genes. The last White genotype is recessive for all three genes and may represent an albino.

Theoretical results

The following theoretical results were obtained.

All 27 genotypes from Table 1 were crossed in all possible combinations, which gave over 2,000 progeny results. Each progeny genotype was assigned to its phenotype group, and the progeny phenotypes were estimated as a percentage of the total progeny from each type of cross.

Crossing all White genotypes with all White genotypes gave 200 progeny genotypes, of which 176 (or 88%) were Whites.

There were 24 colour genotypes, 8 each of Reds, Browns and Blacks (or 4% of each).

Crossing all Red genotypes with all Red genotypes gave 85.6% Red genotypes and 14.4% White genotypes.

Similarly, crossing all Black genotypes with all Black genotypes gave 85.6% Black genotypes and 14.4% White genotypes.

Crossing all Brown genotypes with all Brown genotypes produced 83% Brown genotypes, 4.9% each of Red and Black genotypes, and 6.8% of White genotypes.

Crossing all genotypes with all genotypes gave progeny of 40% Brown genotypes, 30% White genotypes and 15% each of Red and Black genotypes.

An example of the method used is shown opposite (Example 1).

Actual results

The actual results were obtained from the Australian Alpaca Association Herd Books Volumes 2, 3, 4, 5 and 6. Only matings where both the parents’ and the progeny’s colours were recorded have been used. For this reason, AAA Herd Book Volume 1 was excluded from the survey, as were all imported alpacas.

Phenotypes of alpacas were assessed as follows: fawn and roan alpacas were assigned to Red; silver greys and blacks were assigned to Black; and browns and rose-greys were assigned to Brown.

White alpacas were assigned to White.

Multi-coloured alpacas were assigned according to the mix of colours listed. For example, a Dark Fawn/Light Fawn/White alpaca was assigned to Red, while a Dark Fawn/Medium Grey alpaca was assigned to Brown. A total of 14,535 actual matings producing progeny were assessed in this way. All theoretical and actual results are summarised and compared in Table 2.

Table 2
Comparison of Theoretical
and Actual Results

In order to form a base level of parent phenotypes, all imported alpacas listed in the AAA Herd Books Volumes 1-6 inclusive were assigned to the same phenotype groups as above, and the results summarised in Table 3.

Whites x Whites
Approximately 36% of actual matings produced coloured progeny, compared with a predicted result of 12%. There were more than three times as many Brown and Red progeny phenotypes as Blacks in the actual results. Approximately 62% of all White x White matings produced White progeny phenotypes compared with a predicted result of 88%.

Whites x Reds
The actual results agree with the predicted results.

Whites x Browns
A much higher proportion of Red phenotypes and a lower proportion of Browns than the expected results, appeared.

Whites x Blacks
The actual results produced five times as many Red phenotypes as expected, with a lower than expected proportion of Black phenotypes.

Reds x Reds
Almost 20% of the progeny born were recorded as Brown phenotypes where the model predicted none at all.

Reds x Browns
Twice as many Red phenotypes were recorded for the actual results as were predicted by the model.

Browns x Browns
There were approximately three times as many Red and Black progeny phenotypes in the actual results as expected.

Blacks x Blacks
A large proportion of Brown phenotypes were produced in the actual results where none were predicted.

All x All
Actual results gave over 80% coloured progeny and 17% white progeny, compared to predicted results of 70% Coloured phenotypes and 30% White phenotypes.

However, taking all these factors into account, it would seem reasonable to draw the following conclusions:

More research into actual matings and pedigrees would enable genotypes to be more accurately assessed. Colour Herd Books would make this faster and easier. Of particular value would be records of multiple progeny to the same pair of parents. Research into the nature of the pigments would also be of great interest.

In the above model, White genotypes carrying colour genes will produce some Colour phenotype progeny when crossed with White genotypes carrying M genes. These White genotypes may be termed ‘carrier’ Whites. A cross between a carrier White genotype and an albino White genotype will only produce White phenotype progeny.

There is anecdotal evidence in the industry that white-fleeced alpacas with pigmented skin, when mated together will produce the occasional coloured progeny – or even twins (Hand, H.)! White alpacas with permanent pink skin, when mated together, produce pink-skinned, white progeny. This may be explained in terms of the model by assuming that carrier Whites with colour genes represent white alpacas with pigmented skin, and that carrier Whites with M genes represent pink-skinned white alpacas. (Some pink-skinned cria have pigmented spots on their lips or eye-rims which grow and spread as the animal matures.) An albino genotype would represent a true albino alpaca with pure white fleece, pink skin and pink eyes. Albinos occur in low frequencies in many species of animals, although rarely, if at all, in some.

It is interesting to note that dark-eyed and blue-eyed animals with white coats also occur in many different species. In dogs, dark or ‘ruby’ eyed whites are thought to be produced by the presence of a ‘chinchilla’ gene. This gene has a paling effect on red/yellow pigment, causing red dogs to have biscuit, fawn or cream to near-white coats. Dark-eyed white Pekingese dogs, when crossed, have apparently produced coloured litters (Little, C.L., 1973). A similar effect is found in horses, where a ‘cremello’ gene acts on red/yellow pigment to produce cream or near-white, dark- or blue-eyed horses, called ‘cremellos’ or ‘perlinos’ (Sponenberg, Dr P., and Beaver, B.W., 1992).

Searle, A.G., Comparative Genetics of Coat Colour in Mammals, Logos Press Limited, London. 1968.Evans, Barbara K., Ladiges, Pauline Y., and McKenzie, John, Biology Two: Survival Mechanisms, Continuity and Change. Second Edn., Rigby Heinemann, Melbourne. 1995.Hoffman, Eric and Fowler, Murray E., DVM. The Alpaca Book: Management, Medicine, Biology, and Fiber. First Edn. 1995, second printing 1997. Clay Press, Inc. Herald, California.
Australian Alpaca Association, Herd Books, Volumes 1-6.
Hand, Heather, ‘East Gippsland scores a Double’. P. 38, Alpacas Australia, Issue No. 25, 1998.
Little, C.L., The Inheritance of Coat Colour in Dogs. Fifth Edn., Howell Book House, New York. 1973.
Sponenberg, Dr Phillip and Beaver, Bonnie W., A Complete Guide to Horse Coat Colours. Breakthrough Publications Inc. 1983, reprinted 1992, Carvajal, Columbia, South America.