Myth that genetic material in an older stallion alters with
age and their progeny are not as good as when they were in their prime.
Genetic material does in fact alter with age. You yourself are genetically
different from how you were yesterday. It is a fact that by the time you
finish reading this you will be a different person. What will happen is in
the next half hour your genes will be changed by mutation
. This is a very basic genetic
concept. Mutation is happening all the time, over generations and within a
particular animal. This is why growing and aging occurs. Mutations are also
the reason evolution occurs and they may have both beneficial as well as
1980s, the change in DNA was unknown. However, it is something that is
constantly occurring. For example, a gene that causes one form of cancer has
a relatively slow rate of mutation and occurs
later in life. All genes have their own rates of mutation and can be
influenced by outside factors such as the dropping of the atomic bomb on
Hiroshima (an extreme example).
Within any human (this applies to all species) hundreds of millions of cells
are involved and during a lifetime most cells continue to divide and
thousands of miles of new DNA is produced each day. All sorts of errors
occur when this material is copied. Each chromosome has its own specialized
bit of DNA attached and this gets shorter with age. Steve Jones
(Galton Laboratory) compares this to photocopying, a
little is lost each time until the original message disappears. This process
also occurs in mitochondria (the cells powerhouse), which become shot full
of errors as aging occurs. Death occurs when the genetic message no longer
Most mutations described above are known as somatic mutations and their
ultimate fate is to disappear when the cell in which it occurred, or its
owner, dies. Mutations are only of genetic consequence to the next
generation if it occurs in a germ line cell so that it stands a chance of
being inherited. It is known that a complex series of
repair mechanisms in spermatogonia and
other cells that can reverse damage to the DNA. These repair mechanisms are
extremely effective and repair more than 95% of all mutations before they
are passed on.
above is correct it gives the impression this, could be a very common
occurrence and be of importance to breeders. This is not correct. From a
breeder's point of view as a general rule mutation
can best be ignored and there is no discernible disadvantage in using an
can use this to advantage as many older stallions stand at reduced service
fees and there is no known reason why the progeny will be of a lesser
quality than when these horses were in their prime. Mares can have more wear
and tear from their mothering efforts and there are some disadvantages with
breeding older mares. They may not be so good at mothering the foal both
before and after birth.
I was fortunate
to have received a very detailed email from Ernest Bailey
, Ph.D. Department of Veterinary Science M.H. Gluck
Equine Research Center on this topic. Be aware this was just a private email
and not intended as a discussion paper but it contains sufficient
information to demonstrate that older stallions may represent great value
--- Yes, DNA changes. Even
identical twins are not truly identical
because mutations will independently and spontaneously occur which make
them different at the level of DNA sequence. But they will be exceedingly
difficult to find among the 3 billion base pairs, which make up the genome.
-- If DNA did not change then we would not have the genetic variation upon
which to select. Obviously change occurs. I can not account for neglecting
that in my previous email to you.---The questions are, what is the mutation
rate and what impact do mutations have?
---Radiation and chemical mutagens can increase the rate dramatically. But
this is not normal.
---There are DNA repair enzymes that constantly analyze the DNA sequences
and repair defects when they occur. The difference in mutation rate for
mitochondria that you refer to is related to the fidelity of the ploymerase
, which replicates the DNA. It just does not do as good a job as the
polymerases. Consequently the mutation rate for mitochondria is higher
than for genomic DNA. Fortunately, almost all the genes, which are
responsible for the function and structure of mitochondria, are the product
of nuclear genes!
--Some of the cancers, which you described, are the product of a defect in
DNA repair enzymes.
---There are approximately 3 billion bases in the genome. Most of them do
not code for genes and the mutation of those bases is inconsequential.
Also the DNA code is such that most base changes will not result in a
change of amino acid. Furthermore, many changes in amino acid have no
impact on the function of the gene.
So... what is the rate of mutation?
Your argument suggests that mutations are very, very common. Why then,
doesn't the entire species degenerate over time? Each year, each
generation, there would be an accumulation of gene defects that would cause
a collapse of the species.
That this does not happen suggests an interaction between mutation rate and
selection rate such that cells with defective genes are not viable.
Furthermore, the genes, which make the cell viable, also make the tissue and
the animal viable. Selection may occur at all levels of the organism.
We do know that older women are more likely to give birth to children with
chromosome defects, such as Down's Syndrome. That is thought to be a
product of the nature of oogenesis. The eggs develop during puberty then
sit until maturation. During the intervening time random mutational events
can occur which do not affect the ability of the oocyte to develop but
which affect the viability of the resulting offspring. --- The same effect
is not observed for men. This may be because the selective process on the
male reproductive tract is significantly greater. Sperm are generated
throughout reproductive life in males. Cells, which are genetically
deficient, will not survive.
You have constructed some logical arguments. But I do not believe that
they will survive the math.... mutation rate vs size of the genome vs
amount of coding genes in the genome vs selection for viable cells.
We have not observed a decline in the quality of offspring from a male over
There is no basis to suggest that the quality of an individual would be
less from an older male than from a younger male. If the DNA in a male
degenerated in such a fashion, then it should degenerate in the entire
species at the same time and at the same rate.----
The counterbalance must be selection.