[Originally published in the Summer 2011 ASSET Magazine]
Just Talking Donkey…Genetics
By Susan White – Halfpint Hollow Miniature Donkeys
Two of my loves are miniature donkeys and genetics. Of course, being in the breeding business, I am most interested in the genetics of donkey conformation, but it is the genetics of color that is presently being unraveled. The pictures and pedigrees that you and I send in to the American Donkey and Mule Society for registration as well as the DNA of the donkeys that have been tested are being documented and analyzed by ADMS and by Dr. D. Phillip Sponenberg. Originally we had only observations available, but now DNA work is a contributing factor to greater understanding.
Donkey genetics is very complex, but going back to the basics helps as it has been a very long time since many of us studied biology. Donkeys carry 31 pairs of chromosomes in each cell, one of each pair donated by the dam and the other by the sire. The exception is in the sex cells (egg and sperm cells), as they must carry half each, so when the egg and sperm unite, the developing cells will then again carry the full number necessary for an individual. These chromosomes are made up of strings of genes at specific locus points. Some genes are dominant so that the individual needs to only receive one copy of that gene for it to be expressed (show) as its phenotype and others are recessive, meaning they will only be expressed if there are two of them with no dominant gene to “hide” them. We represent genes with letters (and superscripts.) As an example, in the most simplistic form, we could represent the dominant gene for Dun with D and the recessive non-dun with d. The genotypes DD (homozygous for D) and Dd (heterozygous) would produce a dun colored donkey. To have a non-dun donkey produced, it would need a double dose of the recessive gene, dd, receiving one from each parent. Unless our donkey shows (has the phenotype of) a recessive trait, we don’t always know if it carries a recessive gene for that trait as it could be heterozygous and have the recessive trait “hidden” by the dominant trait.
In reality, geneticists represent the dominant Dun gene with D+ (“+” is for “wild”) and the non-dun gene with Dnd, so it is the superscript which will tell us whether it is the dominant or recessive gene. At the Dun locus point on a specific chromosome, we could have the wild type dominant allele or the recessive non-dun allele from one parent and one of those two alleles on the specific chromosome from the other parent. If one parent is heterozygous D+Dnd, then there is a 50/50 chance the offspring will receive one or the other gene from that parent.
There are many locus points on the chromosomes that determine color and the outcome is the result of how these affect the expression of each other. Two of the donkey’s main color loci, other than Dun, are the Extension (E) locus and the Agouti (A) locus. Black is associated with the Extension locus (E+_.) Chestnut red is recessive and if homozygous recessive at the E locus (EeEe), it is believed to “hide” the expression of black. Bay red is associated with the Agouti locus (AA_.) The alleles E+ and AA are dominant whereas Ee and A+ are recessive. Sorry for the confusion…Sometimes the wild form of the gene is dominant and sometimes it is recessive. Since not all the DNA work has been completed, many of the combinations are theoretical, but it is believed that one would have a black donkey with E+_ DndDndA+A+. (“_” means the second symbol could be either dominant or recessive.) That would read dominant at the Extension locus, recessive non-dun at the Dun locus, and recessive at the Agouti locus. Even though only three loci are represented in the above example, it is important to know that theoretically about thirteen loci determine color.
Before the advent of DNA work, it was believed that roan horses were the result of codominance where a dominant chestnut and a dominant white produced a strawberry roan and a dominant black and a dominant white produced a blue roan. Since then a roaning gene has been discovered, but there may be other factors as well that determine roaning. Donkey and horse genetics are not identical, but many similarities do exist.
Most miniature donkeys have a white muzzle, eye rings, belly and inner legs, but we all know of a few donkeys with no light points (nlp) at all…This trait is recessive, so if two true NLP’s are bred to each other, one can be guaranteed an NLP offspring.
Spots have been discovered to be dominant, but there is no guarantee of having spotted offspring from two spotted donkeys. This is because no donkeys homozygous for the spotted gene have been found. It is believed that it is lethal for an embryo to be homozygous for Spot, therefore, if all spotted donkeys are heterozygous (SpaSSpa+) for Spot, there is a 25% chance they won’t throw a spotted offspring.
There are a number of “dilution” genes, the most understood of which is the dominant Dun gene. An example is a donkey that codes for dark red will show a lighter red if it also carries the Dun gene.
It is indeed exciting to think that our miniature donkeys are an integral part of this scientific investigation into miniature donkey color genetics! Science is about questioning, testing, and analyzing. Sometimes we must throw out old concepts when new facts arise. As for the genetics of conformation, at this point we need to continue being good observers and breed wisely.
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