The Maternal Grandsire
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Breeders’ Briefcase by Amy & Bonnie THE MATERNAL GRANDSIRE by Cindy Vogels Genetics work in horses and mice has However, if one chromosome contains produced revolutionary and exciting new the gene for brown eyes but another insights that may influence your canine one contains the genes for green eyes, breeding decisions. For years, horsemen long-accepted Mendelian theory states have acknowledged a phenomenon called that only the genetically dominant the maternal-grandsire effect, when chromosome will be expressed. The outstanding males do not immediately theory also states that genetic dominance reproduce their greatness in the next is unrelated to the sex of the gene donor. generation. Instead, they produce When both genes are expressed, they are daughters who are outstanding dams. considered to be co-dominant. Coat color, An oft-cited example is Secretariat, for example, is an area in which both perhaps the greatest thoroughbred of genes can sometimes exert influence. all time. Secretariat’s achievement was Other times, both genes are recessive, not matched by his direct get, who by but one is nonetheless more dominant and large were unremarkable, but rather than the other, thus allowing a recessive was passed on through his daughters, gene to be expressed. Recessive genes many of whom went on to produce great may also be expressed when both contain performers. Dog breeders, too, have the same protein code for a trait. noted that an extraordinary male, while A Startling Study not producing extraordinary offspring, will In 1969, Dr. W.R. Allen startled the world often produce daughters who are prolific with a study that seemed to indicate and exceptional dams. For years, there certain genes might be gender-related in was absolutely no scientific explanation their expression. Allen bred horses and of this phenomenon in which traits skip a donkeys, and during pregnancy measured generation and are passed along only by levels of the pregnancy hormone called female offspring. Recently, however, an equine chorionic gonadotrophin (ECG). article documenting scientific evidence of Normally this level is high in horse- the maternal-grandsire effect appeared in horse crosses and low in donkey-donkey issue number 242 of Equus, an outstanding crosses. According to Mendel, it should horse publication. I acknowledge that not have made any difference which article for providing me with much of the species served as sire or dam. The levels information in this column. should reflect a combination of the two Some Genetics Background species, and would either be a moderate In each cell of a dog’s body there are level (indicating co-dominance), or if one 39 pairs of chromosomes, one set from species dominated, the level would be each parent. Each chromosome pairs off either high or low. Surprisingly, the mares with a corresponding chromosome of the (horse females) bred to donkeys exhibited other parent, and in each chromosome low levels of ECG, much like a donkey- there are thousands of genes, which donkey cross, and the contain the protein codes that determine jennies (female donkeys) bred to horses every physical trait. Within a pair of registered high levels of ECG, as in a chromosomes will be pairs of genes from horse-horse cross. Although no definitive each parent that determine various conclusions were reached, it appeared traits. When the genes are not in conflict that the sires’ genes were the only factor with each other - both expressing brown affecting the ECG levels in the females. eyes, for example - there is no problem. The females’ genes were silent. PAGE 10 * THE WEIMARANER MAGAZINE * JANUARY 2010 It was not until 1986 that the topic get, while females, for their own well- reappeared in the literature. A research being, control the size of their offspring. team headed by Dr. Azim Surani used mice Imprinted genes are quite possibly to create embryos in which all the genetic involved in traits inherited polygenically. material was received entirely from If only some of the genes are switched either one parent or the other. Since the on, the work of the geneticist tracking material was transmitted in appropriately inheritance becomes more complicated. matched pairs, Mendelian theory would The implications of this finding go far have predicted that the embryos would beyond the world of Thoroughbred racers. develop normally, since it was only the Already, a number of imprinted human presence of two genes for each trait, genes have been pinpointed. Ongoing and not the sex of the gene donors, that mapping of the canine genome should was considered relevant. Again, however, increase the likelihood of detecting Mendelian expectations were confounded, imprinted genes in dogs. The most as the all-female gene pairings resulted important contribution would probably in large placentas with little embryonic be in the realm of canine health, but material. The all-male gene pairings eventually we might have the tools to produced the opposite result: small track the inheritance of many canine placentas with large embryos. Surani’s characteristics that seem capricious in team concluded that some genes do not their skipping of generations. follow Mendel’s laws. Some are “switched on” before fertilization and are always Dog breeders should be aware of this expressed, while others are “switched possible maternal-grandsire effect. Keep off” and never expressed. The sex of the in mind, however, that outstanding males gene donor is the factor that determines tend to be bred to outstanding females, which mode a gene will fall into. A so even if some of the male’s desirable theory called “genome imprinting” was genes are paternally imprinted, the created to account for this previously offspring of such matings will probably unformulated phenomenon. inherit some excellent traits from their exceptional dams. For example, this For example, say there is a canine gene year’s Kentucky Derby and Preakness that is paternally imprinted and, when winner, Charismatic, was sired by 1990 expressed, produces three-eared dogs. Preakness winner Summer Squall, who When the gene is not expressed, the dog is out of a Secretariat daughter. While has two ears. A three-eared male inherits Summer Squall’s prowess on the track the gene from his mother, but because could be traced to the maternal-grandsire a gene that is paternally imprinted is effect, he seemed to pass his greatness switched off when passed on by a male to along directly to Charismatic. However, its offspring, he will have all two-eared Secretariat’s mother appears another offspring. His male two-eared offspring time in Charismatic’s pedigree and will not produce three-eared dogs, but Secretariat’s sire Bold Ruler appears his daughters will, because a gene that is twice. So, the talented colt’s lineage paternally imprinted will be switched on points back to many outstanding in females. individuals. A pedigree, whether for Questions and Implications dogs or horses, always contains many Many questions still remain, and influences and variables. We dog breeders the literature is vague on why the tend to be impatient and are disappointed phenomenon might occur. Researchers when an outstanding male does not point to the significance of gender-related immediately reproduce his excellence. functions. For example, it appears that Remember the maternal-grandsire effect, males strive to produce virulent, robust and wait a generation. JANUARY 2010 * THE WEIMARANER MAGAZINE * PAGE 11.