The following was written in response to Heidi Schlenker’s original article here: https://www.facebook.com/photo/?fbid=25639616755625262&set=a.219157078097913
Mothers are the powerhouses of the pedigree. This is true for all mammals and most other species that rely on the maternal inheritance of mitochondrial DNA. But it carries a lot of implications in horse breeding, especially for rare breed perpetuation and preservation.
In horses, across history, it has been the tendency to give more attention to recording sires–the stallions. This is often because a stallion can produce a large number of offspring from which his value and inheritance of traits can be evaluated for consistency, whereas a dam’s production is limited and it is harder to assess for consistent quality. However, many historically wise breeders have and more breeders today are recognizing the importance of tracing dam lines in breeding selections. The comprehensive write up below from Heidi Schlenker does a great job covering the many points between the roles and stallions and mares in offspring traits with emphasis on the mares.
During the domestication of horses the Y chromosome in stallions was drastically reduced to only one or two haplotypes while the X chromosomes maintained around 100 haplotypes (Kavar and Dovc, 2008). After domestication, interbreeding from different regions, and gene mutations or conversion, modern horse breeds carry an estimated six haplotypes of the Y chromosome with still many more for the X chromosome (Kentucky Equine Research, 2014).
Why is this important? Haloptypes mark and carry ancestral links associated with gene function, genetic diversity, and patterns of inheritance for a specific group. While stallions have one copy of X and one copy of Y with less haplotypes, mares pass two copies of X with more diversity of ancestral genetics in more haplotypes. During embryonic development, these two Xs undergo a process whereby one X is inactivated in each cell at random and then through cellular division groupings of cells (or cells sharing the same role) inherit the X of the originals. This means there is a 50/50 chance in mares of each X chromosome from passing and expressing its genetic type and function. But the exception is in the mitochondria, the part of the cell that fuels all cellular function. Mitochondrial DNA is inherited only from the dam. It can mutate over time but because the mutation rate is low, its DNA allows us to “trace maternal lineage through hundreds of thousands of years” and help us decode breeds’ genetic narratives (Bigler, 2020). And it all goes through the mares.
This mitochondrial DNA dictates the cellular performance of a horse from embryo through its entire life. It controls the oxygenation and oxidative capacity of the body, the ATP output of cells for performance and stamina, and the nutritional conversion of metabolic energy for tissue growth and repair. This means that the mare’s mitochondrial DNA impacts the ability for their offspring to have more balanced development, stronger performance potentials, and lower rates of cellular degeneration.
Of course good stallions are important, especially for maintaining or passing on desired conformation and type. Good stallions need to be evaluated by their parentage and production to assess true phenotypical qualities and influence of his genetics in his offspring. Any functional strengths or weakness faults of a stallion can be inheritable, just as from the mare. But, as Heidi Schlenker so succinctly put it, “𝐂𝐨𝐧𝐟𝐨𝐫𝐦𝐚𝐭𝐢𝐨𝐧 𝐝𝐞𝐟𝐢𝐧𝐞𝐬 𝐡𝐨𝐰 𝐚 𝐡𝐨𝐫𝐬𝐞 𝐦𝐨𝐯𝐞𝐬. 𝘔𝘪𝘵𝘰𝘤𝘩𝘰𝘯𝘥𝘳𝘪𝘢 𝘥𝘦𝘧𝘪𝘯𝘦 𝘩𝘰𝘸 𝘸𝘦𝘭𝘭 𝘵𝘩𝘢𝘵 𝘮𝘰𝘷𝘦𝘮𝘦𝘯𝘵 𝘪𝘴 𝘱𝘰𝘸𝘦𝘳𝘦𝘥. It takes both.”
So as horse breeders and as rare breed stewards we really need to give more attention to both sires and dams in selection. And this means better acknowledgement of the mothers who are often overlooked. To maintain good quality of form and type, mares should be chosen to compliment the stallion but for performance goals they should also be selected to supply the right power. After all, the mothers are reservoirs of each breed’s ancestral power.
Additional references:
https://nigms.nih.gov/biobeat/2020/05/the-maternal-magic-of-mitochondria
https://ker.com/equinews/tracing-genetic-material-mares-stallions/
https://www.sciencedirect.com/science/article/abs/pii/S1871141308000747
