I'm posting here response from Dr. Matt Hegarty of IBERS, University of Aberystwyth, to my post of 4th Dec.2021on dam of lines:-
"I've attached a couple of PowerPoint figures showing how matrilines could be disproportionately represented over time within populations - especially with the heavy use of bull lines and line breeding: -
"Dam lines are of importance in animal breeding as much of the selection is performed on the sires, which only carry a single copy of the X chromosome inherited from their mother. A bull's sons will not inherit his X chromosome, so any contribution it makes to the bull's EBV will not be passed to his male offspring (whereas his daughters will have 50% of his EBV and 50% of the dam's, as for all the other chromosomes). Over time, this can lead to predominance of certain matrilines if a limited number of sires is used within a population (or line breeding occurs).
"Many important traits to do with sexual development are carried by genes present on the X chromosome, particularly to do with fertility in both males (sperm motility and scrotal circumference, for example) and females (eg: age at first calving, ovulation rate, incidence of endometriosis).
"These factors also apply to the mitochondrial DNA, which is inherited solely from the dam in both male and female offspring (whereas daughters will inherit their sire's X-chromosome). There is evidence that production traits (i.e. weight, marbling) in cattle are affected by variation in mtDNA sequence, which is expected given the role of the mitochondria in energy metabolism.
"Monitoring of dam lines via pedigree evaluation and genetic testing is valuable to breeders, therefore. SNP chip testing incorporates both X-chromosome and mtDNA markers which can be employed for this purpose."