Horned, scurred, and polled inheritance is not as straight forward as we once believed it was. More on that later.
In this situation it is completely possible that the Angus sire was homozygous polled. If we call big P the polled allele and little p the horned allele, then this cow could be Pp and scurred. The Angus bull could be PP. Thus, half of the offspring would be Pp with scurs or polled, and the other half would be PP and polled. So, if the cow is scurred there is no reason to assume the Angus bull is not homozygous PP polled. The polled gene has been identified. It is located on chromosome 1.
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0039477 DNA testing companies have tests to see how many copies of the polled variant an animal has. Since horned is recessive (i.e. masked by the polled variant) it is very difficult to completely remove it from a population without a DNA test, which was not available till last year.
There are other genes which contain variants that led to the scurred trait. There is a gene,
TWIST1, on chromosome 4 that if the animal inherits one of the broken gene variants, the animal develops scurs. But, if an embryo inherits two copies of the broken gene variant, the embryo fails to develop and the pregnancy is lost. See
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0022242 The inheritance of "type 2 scurs" in French Charolais due to the
TWIST1 gene is independent of the chromosome 1 variant.
I'm not aware of any scurred variants that have been identified on the X and Y sex chromosomes. So, scurs is not sex-linked, but rather sex-influenced. Sex-linked means the gene is on the X or Y chromosome. It appears that genes involved in horn formation produce different levels of their products in males and females.
I have analyzed the genomes of 3,664 Angus animals, and in that sample we don't see any evidence of crossbred animals. If there are crossbred animals in that data set, they have a
very high percentage of Angus ancestry. If you have a straw of semen on a registered bull you
know is crossbred, I would be happy to pay to have it genotyped.
It is possible for a population or breed to change very rapidly due to selection. There are lots of examples of this in flies, mice, corn, bacteria, yeast, etc. Here is a blog post I wrote about the effects of selection on the genomes of Angus cattle.
http://steakgenomics.blogspot.com/2012/11/birth-date-selection-mapping.html