Genetics

Genetics


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GENETICS SOME THOUGHTS
By Jim E. Quarles
16-02-2000

Gregor Mendel: Geneticist.

Johann Gregor Mendel was born in what is now called the Czech Republic on July 22, 1822.

In 1843 just eleven years before Doctor Heckle discovered and described the discus fish that now honors his name Gregor Mendel was laying the groundwork of modern genetics.

Let me state before going forward with this article I am not a geneticist, nor do I lay claim to any extended knowledge on the subject. But I find the subject very interesting when considering its application with regard to discus fish we see and keep in our tanks today.

Mapping of the genetic patterns in discus will have to be done sooner or later since the hobby keeps expanding at a fantastic rate. New strains or color types are appearing almost by the hour. And little if any development is being done on the sound principles developed according to Mendel's laws of genetics. It all appears to be pure random selection.

MENDELS LAWS OF GENETICS

  • Each parent contributes one factor of each trait shown in the offspring.
  • The two members of each pair of factors segregate from each other during gamete.
  • The blending theory of inheritance was discounted.
  • Males and females contribute equally to the traits in their offspring.
  • Acquired traits are not inherited.

Now some interesting facts about Gregor Mendel. At the age of 21 Mendel became a friar at the Augustinian monastery in Brno, Czechoslovakia, which was a center of learning for those who studied theology, philosophy and what then were the natural sciences.

From 1868 Gregor Mendel was the abbot of the monastery. In January 6, 1884 he passed away. The study and development of how organisms pass genetic variations was first explained by friar Mendel and history owes that honor to the insight of Gregor Mendel.

ONE MAJOR PROBLEM WITH DISCUS GENETICS.

The principle known as genetic drift.

Charles Darwin's theory of evolution explains how the natural order of all life forms evolved and provides the general ground work for further studies that have been refined into what is commonly known as Darwin's Theory of Evolution. While not totally accepted by all, it is so far the best scientific method devised to explain the natural order of life, as we know it.

Natural selection is not the only mechanism of evolution. Genetic drift is considered just as important as natural selection. Random genetic drift is a stochastic process. One aspect of genetic drift is the random nature of

Transmitting alleles from one generation to the next given that only a fraction of all possible zygotes become mature adults.

The easiest case to visualize is the one, which involves binomial sampling error. If a pair of diploid sexually reproducing parents (such as discus) have only a small number of offspring (as in captive breeding) then not all the parents alleles will be passed on to their progeny due to changing assortment of chromosomes at meiosis. In a large populations this will not have much effect in each generation because the random nature of the process will tend to average out. But in small populations the effect can be rapid and significant. This is a real problem with captive breeding programs using a limited number of individuals with a small number of serving off spring. Such as in the culturing and breeding of discus fish.

Why?

In all populations, there are specific allelic frequencies. These frequencies are not fixed, however. They fluctuate with every breeding season. The only way these allelic frequencies might remain static is if the equilibrium was met. This principle is called as the Hardy-Weinberg equilibrium statement.

It states that in a sexually reproducing diploid species, allelic frequencies will remain the same if five conditions are met.

Conditions for the Hardy-Weinberg Equilibrium.

  • Large populations size
  • Random mating (and random matching of alleles)
  • Isolations (no migration)
  • No mutation
  • No natural selections (all individuals contribute equally to the next generation.

When we consider the captive breeding of discus it becomes very clear that few of these conditions can be met.

Even in nature and the natural habitat where discus are found these conditions are not fixed at all times. And it certainly becomes next to impossible to create them in a captive-breeding program on such a limited scale currently in use in the hobby today.

Random genetic drift is more than this. For example imagine a population of 100 discus fish where 50 of them has the genotype AA for (perhaps shape,) say 7 are Aa and only 1 is aa. Now imagine that the heater fails and 10 of the fish are killed. Regardless of which discus died, there will be significant affect on the allelic frequencies.

The point here being that with limited stocks, "genetic" disasters are anything that results in lost individuals in small populations unselectively. The result is that the small surviving populations are unlikely to be representatives of the original populations in its genetic makeup. - This creates what is known as the bottleneck effect.

Let's consider another example of genetic drift known as the founder effect. In this case a small group of individuals breaks off from larger populations and forms new populations. (Asian Discus Selected populations) This type of effect is well known in human populations. It explains why the blood group B is almost totally lacking in the American Indian population. Their ancestors arrived in very small numbers across the Bering Strait during the end of the last ice age.

BOTTLENECKING HAS BECOME A MAJOR
PROBLEM IN CULTURING DISCUS AND OTHER
TROPICAL FISH.

When you hear people talking about captive breeding to protect the wild populations you can be sure they do not understand the principles of bottlenecking and limited genetic pooling of captive breeding.

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