"Without going too deeply into the genetics, some basic
genetic background is needed to explain how an OFA
certified animal can produce off-spring that develop
CHD. The following is a gross over simplification of the
genetics of the CHD; but does provide a framework for
understanding a problem with multiple gene control over
a single trait. Dogs have two copies of each gene that
controls CHD (the total number of genes that control or
contribute to the development of CHD is unknown; but it
is believed that many are involved). They get one gene
from their mother and the other from their father. Some
of the "good genes" can mask the presence of a "bad
gene". In other words some of the "good genes" are
dominant over the recessive "bad genes". Thus, the
physical expression of the "good genes" will be result
in good hips, even though the animal is carrying a
number of hidden (recessive) "bad genes". When the sperm
or egg is formed the two copies of each gene are
separated so that sperm and the egg end up with only one
copy of each gene. In a case where an animal had a copy
of one good gene and one copy of the bad gene, 50% of
the eggs or sperm would get the "good gene" and 50% will
get the "bad gene". Now recall that many genes are
involved in the development of CHD, so this separation
occurs for each of the genes that control hip joint
development. Thus, a very few of the eggs and sperm will
get all of the "good genes" and a very few will get all
of the "bad genes" but most will get a mixture of both
"good" and "bad" genes that control CHD. The outcome of
a particular mating will be determined by the match up
between the genetic make -up of the parents. If the bad
recessive genes line up incorrectly then more of the
puppies could have hip dysplasia. However, if the good
dominate genes line up well then many of the pups will
have good hips, but may still carry the hidden bad
genes. Complicating the situation even further is that
some genes may be good when combined with one set of
genes and bad when combined with different set of genes.
It is also possible for some genes to have a greater or
lesser influence on CHD than others, adding to the
complexity of the problem. Thus, one can see that the
genetics of CHD is verycomplicated."