Sanskrit for Computers?

 Genes And Memes

All about  Genes

 [This is the first of a two part blog series which covers the genes. I will take up memes in the next part]

 How come Giraffes came to have such long necks?

It would seem that the ancient ancestors of the Giraffe were more like other herbivores. They did not have necks that were particularly long, as these are in modern day Giraffes.

How did the long necks evolve in nature? Many people believe that  early animals with normal necks had to repeatedly stretch these out to eat leaves that were higher up from the ground, and that this “effort” slowly made their necks grow longer. And that the offspring of these animals were then born with longer necks.

This explanation is totally wrong! This is not the way biological evolution proceeds in nature. The observed fact is that traits that are acquired within the life time of an animal (through effort or accident) cannot be inherited by the next generation.

What gets inherited?

It is widely known that we do inherit characteristics (or traits) from our parents. These inherited traits include hair colour, skin colour, dimple, hairline, height etc. etc. The important point is all the characteristics that we inherit from our parents were in turn inherited by them from their parents. No characteristic that we acquire in our life time can be passed on to the next generation as an inborn trait.

How do traits pass on to the kid from the parents?

The answer is -- through the genes. Genes store the information (as recipes or blueprints) needed for the cell to assemble proteins, which eventually yield specific physical traits. I can learn a new trick or language in my life time. I may acquire a scar as result of injury. But there is no way that “information” regarding these acquired skills or traits can pass on to my genes that would form the blueprint for my offspring.

Let us understand a bit more about genes. Many genes group together to form chromosomes, and these reside within the nucleus of each cell that constitute our body. Thus, a chromosome may be understood as a gene cluster.

Normal human cells have 46 chromosomes (as 23 pairs), while a special type of cell, the germ cell (egg and sperm) has only 23 unpaired chromosomes. When a sperm from the male parent fertilises the egg of the female parent, the 23 chromosomes from the sperm pair-up with the 23 chromosomes in the egg. Thus, a fertilised egg (also called a zygote) comes to have 46 chromosomes – 23 each contributed by the father and the mother.

Genes and Evolution

All of us know that the offspring are not exactly like the parents; nor exactly like each other. Several factors can contribute to variations among the offspring.

It is easy to understand that the traits of kids (caused by chromosomes from both the parents) are not the exact replica of either the mother or the father.

There are some other causes too that contribute to natural variation among the offspring. Let us consider these --

We had earlier talked about the germ cells (eggs and sperms with only 23 unpaired chromosomes). These germ cells are generated by a special process of cell replication called "meiosis". The chromosomes (of the male or female) undergo genetic recombination during meiosis. Thus the genes within the chromosomes constituting the egg  cells of the female (or the sperm cells of the male) are not identical for the same person.  Each egg (or each sperm) within the same individual,  is genetically distinct from all other eggs (or sperms) of the same individual.

Thus, meiosis and sexual reproduction are sources of genetic variation among siblings born to the same parents.

A more rare cause for variation among offspring is genetic mutation -- that is, the errors that occur while genes are replicated during meiosis -- causing the egg or the sperm cells to contain genes that are not found in either the egg or the sperm cell. These accidental errors are called mutations. What is significant is that these mutations are cumulative – in the sense that in future these changes become part of the inherited (mutated) gene recipes. To an offspring, a mutated gene is not different from a non-mutated gene. Both these are liable to replicate during meiosis, and get passed on to future generations.

These natural variations result in differences of "success rates" among the offspring. That is, some individuals fare better in the tasks of growing up, finding food, avoiding early death, finding mates, and generating healthy offspring of their own. To a great extent, this success depends on the environment. Thus, it is natural that genes that lead to greater “success” have a greater chance of moving on to future generations.

Back to the Giraffe

Okay, now that we know about genes, let us see how the neck of an animal can evolve and become longer – in response to an environmental circumstance where long necks are an advantage for survival.

As always, Giraffe calves are born with variations. These variations are not a result of environmental pressures or of effort by the parents. These are accidental consequences of the process of genetic recombination during  meiosis, sexual reproduction and random mutations. Irrespective of the environment, some calves are born with slightly longer necks and others with shorter necks. The environment becomes significant only when a particular type of variation increases (or decreases) the “success rate” of the offspring.

In case of the Giraffe, it would seem that at a particular period in history, there was a significant advantage in possessing long necks. Offspring with longer necks survived in larger numbers to adulthood, could find mates, and could reproduce. This meant that genetic mutations that provided a bias towards longer necks cumulated across generations. And giraffe necks started getting longer across generations.

And why did the growth of the necks stop at a particular length? Because, beyond this critical length, the “costs” of having a long neck balance the “benefits”. This provided a counter-pressure against further growth of the neck. This is the point of “stability”, as determined by natural conditions.

How is the gender of the baby determined?

Out of the 23 chromosome pairs, the last is called the sex chromosome pair. This pair can be either XX or XY. Females always have XX, while the males always have XY.

In case of an unfertilised egg cell, the 23rd (unpaired) chromosome can only be X. On the other hand, the 23rd (unpaired) chromosome of a sperm cell can either be X or Y.

Thus, when the 23 chromosomes of a sperm pair-up with each of these of the egg, the 23rd pair of the zygote (the fertilised egg) can be either XX or XY. This is the "switch" that determines whether the egg must "assemble" into a male or a female body. If XY, the result is a male offspring; while if XX, the result is female.

Everything else being equal, the probability that 23rd chromosome of a sperm cell is Y is about 50%. This means that the probability of a male offspring is around 50%. And also that only the sperm cell from the father contributes to determining the gender of the offspring -- the egg of the mother has no role to play in this.

Not that it matters much – but the fact is that the gender of the child is determined by the sperm of the male. The egg of the female would seem to have no contribution towards this!

Original link: http://creative.sulekha.com/genes-and-memes_307731_blog