The number of chromosomal pairs varies by species, humans have 23 pairs, horses have 32
pairs or 64 chromosomes. The sex cells or "gametes" (egg and sperm) are not
somatic cells. They carry only one chromosome from each pair. When the stallion's sperm
carrying 32 chromosomes unites with the mare's egg carrying 32 chromosomes, the resulting
foal will have the required 64 chromosomes. That's how the genetic make-up or genotype is
passed on.
The sex of the offspring is determined by the sex chromosomes (X or Y) found in the
sperm. A normal male offspring will have both an X and a Y chromosome, while the normal
female will have two X chromosomes. Thus the female may only pass on an X chromosome. The
sex of the offspring is determined by the male. ALWAYS. He alone has an option of giving
an X and producing a female or giving a Y and producing a male.
A gene is a segment of a DNA molecule which instructs a cell to produce a certain
protein resulting in a reproducible trait. Each gene is located in a fixed location
(allele) on the chromosome, and is the basic unit of heredity. Each chromosome carries
many different genes upon it, they may be compared to beads on a string. The possible
combinations of these are in the millions.
Phenotype is the visible, external appearance of an individual. It is the physical
manifestation of the genotype. Your miniature's size and color are both phenotypes,
anything that you can see is the phenotype.
So the genotype is the information which gets transmitted to the offspring, while the
phenotype is the physical results from that information. Environmental conditions will not
affect the genotype, but they can alter the phenotype. If two horses receive the genes to
be black in color, that is the genetic potential for both. Depending on diet and exposure
to the sun, though, one's coat may be black while the other may be reddish.
Now that you understand the various terms, let's look at all that can happen with just
one single pair of genes. A popular gene in horses is the Tobiano gene that produces the
typical pinto. For means of identification geneticists and horse breeders name genes with
letters of the alphabet. We call the Tobiano gene "T" and the gene for
non-Tobiano (in this case normal, solid color) "t" . For a horse to be pinto it
needs just one T. For the horse to be normal color it needs two t's. So we have TT and Tt
horses which are pinto, and tt horses which are normal colored.
What happens when you breed a TT with a tt? Each parent is genetically pure for this
trait, one producing only T genes, and the other producing only t genes. As we discussed
previously, each half of the gene pair is called an allele. When an individual has two of
the same alleles for the same trait, it is called homozygous. In this example each parent
is homozygous. (If the individual has two different alleles for a given trait, it is said
to be heterozygous.)
Okay, in our example, we are breeding a TT horse and a tt horse. What happens? The TT
parent contributes a T allele in its gamete, and the tt parent contributes a t allele in
its gamete. The first generation (noted as F1), with a Tt genotype, turns out pinto, since
all it takes is one T to make a pinto. The T, because it "outvotes" the t, is
called dominant. The t, which doesn't show up in the phenotype, when the dominant T is
around, is called recessive.
Now the plot thickens when you breed various members of the F1 generation. That's
because the laws of probability kick in. Because all of the F1 horses carry Tt genes,
there are more possibilities -- four possible outcomes, to be exact.
