Miniature Horses and Tunis Sheep

Basic Color Genetics

Color genetics of the horseThe bodies of all animals are made up of microscopic building blocks called somatic cells. A cell consists of a cell wall and a nucleus, which is more or less in the center of the cell.

Between the nucleus and the cell wall is the cytoplasm. The genetic material is carried on chromosomes in the nucleus of the cell. Each chromosome is actually a pair of rod shaped structures. For each pair, half came from the father and half from the mother.

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.

punnet square An easy way to figure your chances is to use what geneticist call a Punnet square. This allows you to see that three possibilities from this breeding result in pinto horses, with a capital T. Only one, the tt, will be normal colored.

That in a nutshell, are the basics of genetics: the results can be predictable when you are mixing and matching genes. All it takes is a bit of mathematics. Here are some of the genes and their results.

Now back to color, the presence of color in a mammal is due to the presence of granules of pigment in pigment bearing cells called melanocytes. Where melanocytes are present the animal has the potential to produce color. Melanocytes serve an important function in shielding deeper structures from harmful ultraviolet radiation.

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