The study of heredity is perhaps the most esoteric of all the sciences. By its very nature, it often gives rise to more questions than answers. While such sciences as physics and chemistry are descriptive (an engineer can calculate the maximum load that a bridge can handle, and a chemist knows what the end product of a chemical reaction will be); heredity can only provide probabilities and predictions, unless of course one is talking after the fact.

The History

Most of what we know of heredity can be traced back to an Austrian Monk by the name of Gregor Mendel. Mendel was fascinated with pea plants, and the factors the drove the colors and shapes of the peas from parent to offspring. Amazingly, while modern microbiological methods have antiquated much of what Mendel hypothesized, like Newtonian physics, his theories are really "close enough" for most hereditary discussions.

The Chromosome

The basics of heredity are really quite comprehensible and can be described by rather simple mathematics. The underlying processes of heredity, are of course, ultimately driven by cellular division, and an understanding of the division of a cell is necessary to fully understand the mechanics behind it all.

Every living organism is made up of cells. Some animals, like bacteria, are very simple and may be made up of one cell. Other organisms, like plants or animals may be made up of billions or trillions of cells all working in unison. Regardless, the cell is the brick of life. Inside each of these cells is deoxyribonucleic acid or DNA, and this DNA contains all of the information about an organism.

This DNA is generally clumped together into propeller-like structures known as chromosomes. While the actual number of chromosomes an organism may contain differs among species, chromosomes are almost always paired.

The Genes

Now everyone always hears about the genes. Almost without fail, the term is misused. Many people talk about genes being inherited, and this is incorrect. Genes are NOT inherited. A gene is a site or a location on a chromosome that controls a specific trait. A gene can control eye color, hair color, and even dimples. However, the actual gene is, generally speaking, the same for all organisms within a species. For instance, the gene for hair color is on the same chromosome in the same location for you, for me, and for every other human being that is walking the earth right now.

Of course the question then arises, if genes are all the same for a species, how does diversity or differences in a species occur? The answer is the alleles. It is the allele that is the inherited component of heredity, and it is the combination of alleles or the possible combinations that cause genetic diversity.

A simple analogy might be something like a string of Christmas lights. The entire string of lights could be though of as a chromosome. Each individual socket could be thought of as a gene. To put it in genetic terms, every string of lights out there is essentially the same, and each socket is the same for every string of lights. The only room for difference then could be in the light bulbs themselves. We can think of the light bulbs themselves as the alleles. If we just considered the eight primary colors, it is very easy to see that huge differences could be observed between one string of lights to the next. It is the different colors that occur in the sockets that cause the differences in the string of lights. On a large string of lights, say 100 sockets or more, the number of differences that could be created between one string of lights to the next, is tremendous. If you then realize that higher organisms are going to have about twenty or so chromosomes, or strings of lights for our scenario, it is easy to see that the differences are almost infinite. This is how genetic diversity arises.

Dominance, Recessiveness

With only a few exceptions, chromosomes are always paired. Humans have 23 pairs of chromosomes for a total of 46. If we assume that each gene controls a specific trait, then there are two alleles that control each and every trait we have. This is not concrete, and often times some traits are in fact, controlled by multiple genes, but for our purposes its close enough.

While there are two alleles for every trait, many times, only one gene will actually physically express itself in an organism. This is where the terms dominant and recessive come into play. Some alleles are known as dominant. When a dominant allele is present at a gene, it will always express itself physically. Other alleles are recessive. It takes two recessive alleles for a trait for an organism to express that trait physically. Again, let's go back to the

Christmas light scenario to help visualize these concepts. We can imagine two strings of lights, laid out on the ground side by side next to each other, we can think of the whole string of lights as a chromosome. If we take a look at the first socket on both strings of lights, we can think of this socket as our gene. These two sockets, in combination, are going to control a certain trait. Now, there are lots of possibilities as to what color lights bulbs go in these sockets.

We could for example, have a red bulb in each socket. In this case, we will have red light. We could also have a red light in one socket and a white bulb in another. If you took a look at the light coming from the combination of these two lights, it is going to be red. So we could say that the red bulb is dominant to the white bulb. The white bulb is still there, but the red bulb is too strong for the observable light given off by these bulbs to be white. It's going to be red. This is how dominance works. In this example, we would say that a red bulb is dominant and the white bulb is recessive.

However, let's say that in another scenario we have two white bulbs. In this case, the light being given off is going to be white. It cannot be any other color. The two recessive white bulbs when paired give off white light. This is how recessive alleles work.

Genotype and Phenotype

Now scientists use a type of shorthand to express these characteristics. You can pick an arbitrary letter; let's use the letter "b" in this case. A dominant allele (the red) is written as a capital B, a recessive allele is written as a lower-case letter b. When you see a capital B, you know that allele is the dominant color red, and when you see a lower-case b you know that the allele is for the recessive color white.

So to make some sense out of these letters, here is how it works.

B = red
b = white

Of course, we have to remember that alleles are paired, one allele from each chromosome to make up our gene. That being the case, at gene 1, we can have some very different combinations. For example:

BB = two dominant red bulbs, we will have red light.
Bb = one dominant red bulb and one recessive white bulb, we still have red light
bb = two recessive white bulbs, we will have white light

The actual makeup of the alleles is known as the genotype. BB, Bb, or bb, this represents the actual alleles that an organism has at that site. You can see, there are three possible genetic combinations.

The phenotypes of an organism are those physical traits that are actually expressed. So in this case, even though we have three possible genotypes at socket 1, (BB, Bb, or bb), we can only have two phenotypes. Red, which would have genotypes of either BB, or Bb, or white, which would have a genotype of bb. No other phenotypes are possible in this scenario.

Inheritance

That is the nuts and bolts of genes, chromosomes, alleles, dominance, and recessiveness. So, the question that everyone wants answered, is "How does it relate to inheritance?" Well, it's really pretty simple.

We know that we must have two alleles for each gene because chromosomes are always paired right? Speaking very simply, what that means is that we must inherit one allele from mom, and one from dad, 1 + 1 = 2.

Now remember, mom and dad cannot pass on anything to their offspring that they do not have in their own genetic makeup. When the cell divides into sperm cells and egg cells, the offspring will receive one allele that dad has, and one that mom has, however, this is where the probabilities come into play. There is really no way to predict with certainty what allele the offspring will inherit. All we can do it perform mathematical probabilities and predict what CAN occur.

Let's say we have two individuals with genotypes of Bb from mom and Bb from dad. Both individuals give off red light, but carry an allele for white light. If we breed these individuals we get something like this.

		MOM
		B	b
DAD	B	BB	Bb
	b	Bb	bb

From a mating such as this, one of the four offspring will have two alleles for red light (BB), two will have one allele for red light and one for white light (Bb), and finally one will have two alleles for white light (bb). However, remember, this is simply predictive; we have no guarantee of what pairs will actually join during the fertilization process. It is, a proverbial crap shoot.

However, the phenotypes for these offspring will be three with red light, and one with white light. As you can see, even though two parents both have red lights, an offspring with white light is very possible.

You can play around with some other combinations. If one parent is BB, and another is Bb, you will find that all offspring will have red light. Conversely, if both parents are bb, the only possible outcome will be white lights.

Breeding True Strains

As you can see, it gets a little tricky when a parent carries one dominant allele and one recessive allele. The diversity in the offspring can vary quite a lot. Of course, there is no way to know what the genotype of the parents' actually is, so often times, a trait must first be observed in an offspring to determine what the genotype the parents have.

It is this diversity that may breeders be it plants or animals try to avoid. They often breed individuals that have the phenotypes that they are looking for over many generations, and exclude those individuals that do not have the phenotype. Over time, a "true strain" is developed. In other words, all offspring are BB (or bb if the recessive characteristic is being selected for) and are thus incapable of producing offspring that have a genotype different than that of their parents. These "true breeding strains" are essentially what humans have developed into breeds, be it horses, cattle, corn, or dogs.

Some Common Myths

When it comes to heredity, it seems like fiction outweighs fact more often than not. One of the most misunderstood concepts is that of the recessive allele. For some reason, many people believe that if a trait is recessive, it is detrimental or perhaps even dangerous. This is in fact, only a half-truth.

Lots and lots of recessive traits are in fact, benign. For example, if you have straight hair, guess what? You have two recessive alleles. Also, if you have dimples, you have a recessive trait. Blue eyes and blond hair are also recessive traits that may people exhibit. As all of you blond haired blue-eyed people out there can probably attest, these traits are in no way harmful to an organism's health.

What is true about recessive traits is that almost always; extremely dangerous diseases are in fact, controlled by recessive alleles. In the grand scheme of things, this makes perfect sense. If a deadly genetic disease were controlled by dominant alleles, the potential to run rampant through a population is very real, if the disease did not affect the organism until later in life after sexual maturity had been reached. The result would be the extinction of the population in a very short time. Nature quite simply does not work this way.

Another misunderstood and sometimes even abused concept is the co-efficient of inbreeding. Software that claims to calculate how much inbreeding actually exists in an individual animal seem to be very popular. The mathematics behind the calculation are actually based upon the number of times common ancestors occurs in a pedigree, and are described in detail on other websites. When considering a COI, one should remember that while the number may represent the percentage of common ancestors, it does not and it cannot, describe the actual genetic diversity that any individual organism contains.

To expand on this topic, we can go back to our earlier example of the mating of two individuals with the genotypes of Bb x Bb. When we performed this mating, we produced four offspring with the genotypes of BB, Bb, and bb. If we compared the BB and the bb offspring from this mating, you can see, it is theoretically possible for two offspring with the same parents to have no common alleles.

Some people like to claim that the COI can act as a predictor of genetic disease. This is also a fallacy. The driving theory behind this hypothesis is that as genetic diversity goes down, the incidence of disease and mutation goes up. This is based upon the recessive allele principal that we have already discussed in some detail. If an organism does carry a recessive allele that is deleterious, but lying dormant due to the presence of a dominant allele, there is a possibility that it will pass that recessive trait to its offspring. As closer and closer breedings occur, the chances of those two recessive alleles matching up at a gene increase. This would result in an offspring that has two recessive alleles for the deleterious trait, and an offspring that manifests that trait. Since we have already shown earlier that most of the time there is no way to know what the actual genotypes of the parents are, any claim by the COI to accurately predict the incidence of disease is at best, stretching the truth.

Another common myth is that offspring inherit more genetic material from their mother than their father. Again, this is another half-truth. As stated earlier, offspring inherit ½ of their genetic makeup from their mother and ½ their genetic makeup from their father. The exception is mitochondrial DNA. The female egg cell contains mitochondria, the male sperm cell does not. A small amount of genetic material is contained in the mitochondria. This DNA is only inherited from the mother.

Yet another common myth is that a female's genes somehow become less potent with age. Well, kind of, but not really. All females are born with a finite number of eggs in their ovaries. As the individual ages, radiation, chemicals etc., can and do affect the eggs contained in the ovaries. This can lead to an increase in genetic mutations caused by environmental conditions, but the actual genetic material does not change, so to speak. It is the potential for mutations that increase, not the actual genetic makeup of the mother.

This is a very rudimentary look at genetics, and yes, there are always exceptions to almost every rule. However, it is the basics, and they occur probably 99% of the time.