human and horse genes are inherited in a Mendelian manner. We are usually
unaware of their existence unless a variant form is present which causes an
abnormal or different phenotype. We can follow the inheritance of the
abnormal phenotype and deduce whether an allele is dominant or recessive.
dominant condition is transmitted in unbroken descent from each generation
to the next. Most matings will be heterozygote (Aa) to homozygous (aa)
recessive. We would therefore expect every child of such a mating to have a
50% chance of receiving the mutant gene and thus of being affected.
Huntington’s disease is an example, progressive mental deterioration
beginning in the 40’s. The age at which this appears is imprinted and
depends on whether it is the maternal or paternal copy of the allele that is
dominant. Using Mendel's peas Yellow is dominant to green. If a Y allele is
inherited the offspring will always be yellow.
recessive trait will only manifest itself when homozygous. If it is a severe
condition it will be unlikely that homozygotes will live to reproduce and
thus most occurrences of the condition will be in matings between two
heterozygotes (or carriers). An autosomal recessive condition may be
transmitted through a long line of carriers before by ill chance two
carriers mate. Then there will be a ¼ chance that any child will be
affected. Cystic fibrosis is an example. A green pea allele is recessive. It
will only be expressed if the offspring inherits an allele from both parents
so the offspring is homozygous for this factor.
can also show incomplete dominance. When a true breeding black animal (AA)
is crossed to a true breeding white (aa) and the first-generation offspring
are grey-blue (Aa). Then when the offspring are mated to each other the
second generation’s offspring consist of 1 black, 1 white and 2 grey-blue.
Since the black and white reappear undiluted in the second generation the
genes have not merged but have remained independent, despite the fact that
in a heterozygote (Aa) they affect each other.
alleles are manifested in a heterozygote (Aa). An example of this is found
in some human blood groups where both forms of the allele are found in the
red blood cells of MN individuals. Both forms of the genes are expressed.
Neither gene is dominant or recessive they exist side by side.
autosomal traits generally it makes no difference how these will be
expressed in offspring regardless of the sex of the parent and the sex of
the offspring. This will be mentioned again under sex balancing. Only if the
trait is imprinted will sex balancing make sense for an autosomal factor. To
date there are no know horse traits known to be imprinted.
things to keep in mind when looking at pedigrees:
a condition is common, this does not mean the allele is dominant. Extra
digits (polydactyl, 6 fingers or toes) are rare but caused by a dominant
allele. Just one copy inherited by an individual will mean the individual
will display the trait.
offspring differs from both parents the offspring is displaying a recessive
condition since both the parents must have been carriers but it was not
expressed. This is where for example where both parents are green but the
3. If the parents are related to
each other, perhaps by being cousins, there is an increased chance that any
gene present in a child may have two alleles identical by descent. The
degree of chance that both alleles of a pair in a person are descended from
the same recent common ancestor is the degree of inbreeding of the person.
The above is not a complete list
of the various forms of dominance and recessivity but they are the main ones
that you need to understand before looking at the other forms.
It is important to understand that
a gene is not termed “dominant” because it somehow subdues a recessive
allele. When a dominant allele (A) coexists with a recessive allele (a) in a
heterozygous genotype (Aa), they do not interact at all. It simply reflects
the mechanisms by which specific alleles are expressed in the phenotype. It
has nothing whatsoever to do with superiority and the frequency with which
they may appear in a population.
As breeders you will often see
comments like “he stamps his stock” as if this is an expression of
superiority of a stallion’s genes. All it reflects is that the stallion may
be homozygous (AA) or (aa) for that trait not that he has superior genes
that somehow subdues less worthy genes. Whether this “stamping” is good or
bad will also depend on the mare and whether the trait is important for the
sort of horse being bred. The expression stamping of stock as such gives the
wrong impression as it only reflects that an individual maybe homozygous for
a particular trait. It is important only for giving you an increased
probability of the trait being inherited by the offspring. It does not mean
the stallion’s genes subdue some other less desirable gene carried by the
The same idea applies if you want
to avoid an undesirable characteristic in a mating.