Feeds:
Posts
Comments

Posts Tagged ‘Genetics’

Pro-Life Academy every Tuesday and Thursday.

Fertilization of an egg cell by sperm, as shown above, is an example of sexual reproduction. This method, as we shall examine today, affords the greatest degree of genetic variation among the members of a species, including our own. Following our last lesson where we discussed the ultimate identity of an individual as residing in its genetic composition, we turn our attention today to exactly how it is that such unique individuality arises.

As we’ve discussed, all somatic cells, which are all body cells except the sperm and egg– called gametes, have 23 pairs of chromosomes. These chromosomes, recall, are long strands of DNA containing segments whose nucleotide sequences are instructions for building structural and functional proteins. We call these segments genes.

A somatic cell in humans has 23 distinctly different chromosomes. We get a set of 23 from our mother via the egg, and a set from our father via the sperm. This creates the 23 pairs found in each somatic cell. A word about what makes each of the 23 distinct chromosomes so distinct.

Each of those 23 unique chromosomes is unique because it contains a set of genes that cannot be found on other chromosomes. Further, the genes of a given chromosome reside at certain locations, or loci, such that when a chromosome from the mother finds its homologous partner chromosome from the father, they have the same genes at the same loci from top to bottom. Such a pair are called homologous pairs or homologous chromosomes (from homo meaning ‘same’ and logos, meaning ‘structure or form’).

In our last lesson, we considered how a somatic cell goes about dividing to make two genetically identical cells in a process called mitosis. Now we consider how a diploid cell (one with 23 pairs of chromosomes, 46 total) goes about making gametes, which are haploid (just 23 chromosomes) in a process called meiosis.

It’s really quite simple.

First a diploid stem cell for either egg or sperm will double its number of chromosomes. When it does this, each chromosome is stuck to its carbon copy at a point in the middle. Then, as the cell undergoes the first of two cell divisions, rather than the two new cels receiving a copy of each chromosome in a pair, the pair itself is separated during the division.

In this illustration to the right, we see an example involving a single homologous pair of chromosomes. At the top, the stem cell contains a single pair (we can imagine the yellow chromosome as coming from my mother and the blue one coming from my father).

Then the cells undergo DNA synthesis, making a carbon copy of each chromosome, joined at the center.

Next, we see that the pair is separated into separate cells, then, each of these cells divides to create four gametes.

Along the way, and not illustrated here, some genes were swapped between members of the pair-more on that next time in a lesson on genetic diversity.

Now, about a woman’s biological clock. Is that just a nasty manipulation by women to rope guys into marriage at a relatively early age, or is there merit to it?

Both!

Okay, just kidding. I wanted to see if you were still with me.

There is great scientific merit to the clock.

Go back to the illustration of meiosis above. Start at the top. We’ll use my wife as an example for show and tell today.

When Regina was a mere fetus in her fourth month of development, all of her organs were maturing in their development, including her ovaries and all of the eggs that she would ever carry. By her fifth month of fetal development, her eggs performed their DNA synthesis, as seen in the second illustration. The eggs remained that way, and still do today. The DNA carbon copies remain joined in every egg until a given egg is selected for a given menstrual cycle. Only then does the DNA separate. The longer the egg remains in the ovary, the greater the probability that the chromosomes will not separate.

So, when we were married, Regina was 24 years old, with an excellent chance that the chromosomes would separate, or disjoin, as we say.

Now at age XX (come on, you didn’t think I was that insane as to divulge her age, did you?) a great many of her chromosomes will not disjoin during the second round of cell division. That event is referred to as nondisjunction.

Okay, Regina’s turning 42 this year, I am that reckless and insane. And she’s more beautiful now than ever (which still won’t save me from the dog house 😉 ). But it’s an important milestone. By age 42, 90% of a woman’s eggs are chromosomally abnormal. Thus, at age eighteen, 1:2,000 live births results in Down Syndrome. By age 42 that rises to 1:25.

So, as a woman gets older, the greater the probability of nondisjunction occurring.

That’s a mouthful for one day.

Class dismissed. See you on Tuesday.
.
.
.
Top photo: Quarandscience.com

Middle: bio.georgiasouthern.edu

Bottom photo:Growbrain.typepad.com

Advertisements

Read Full Post »

In light of our ongoing treatment of Sanger and the Eugenics Movement, it’s fair to ask if the eugenists have any merit to their argument.

No, they don’t.

From a Christian anthropological perspective, the least among us is made in the image and likeness of God. Jesus tells us in Matthew 25 that He will judge us by our treatment of them, as He identifies with, “the least of these my brothers”.

As for the genetic basis of their argument: genetics or environment?, the safe answer is probably both. We can train a chimp to play golf and even fly a spacecraft, but that doesn’t make it human. Aping (pardon the pun) human behavior does not change genetic and simian reality for the chimp. For humans whose genetic defects render their function as less than optimal, sub-par performance does not make them less human, or less worthy of human dignity. An individual need not exhibit or realize all of their potential functions at all times in order to be a member of the human family.

We know that certain traits are hereditary, having identified what genes on what chromosomes are responsible. Down Syndrome is the most famous and easily recognizable based on physical (phenotypic) characteristics. Certain psychiatric conditions such as the Schizophrenias appear to have a genetic etiology. Autism may well prove to be genetic as well. I’m currently involved in a research project that points in that direction.

To make matters murkier, to what extent do environmental (physical or psychosocial) factors influence, or exacerbate underlying genetic predispositions? Then there is the issue of the extent to which environmental factors influence and ameliorate the physiological and psychological effects of a genetic disorder.

Take autism as an example. Children with horrific deficits in communication, with a broad spectrum of associated developmental delays, would easily fit in to the eugenist’s list of targeted individuals. It’s my considered opinion that there is indeed a genetic, developmental defect at the root. With a prevalence in the population that is increasing, a moral and ethical decision needs to be made. What do we do with these children?

Having one myself, the answer is simple. Treat them.

The last decade has witnessed a revolution in the treatment in children with autism. Better speech therapeutic regimens, as well as social skills, special education, physical and occupational therapy, play groups, have all shown dramatic effects in children whose function was less than half their chronological age.

Unlike our chimpanzee friends here, these children are humans, being taught human skills. The environmental stimuli effect neural development to bring the child’s behavior and cognitions more in line with optimal human function. That’s environment being used to overcome genetic defect.

We’ve had great success after six years of daily work, several hours per day. We’ve also learned more about love in the process than we ever dreamed imaginable.

That doesn’t happen with sterilization and abortion. Eugenics proclaims that life has a monolithic standard of acceptability, that individuals not meeting its arbitrary and capricious standard ought never to have existed. Unable to murder the adult, eugenists will prevent the child. Such a standard says nothing about the targeted individual and everything about the sickness and evil of the ones who crafted it.

Genetics doesn’t describe our difficulties so much as it invites us to engage in growth as individuals, as civilizations.

That requires courage, imagination, and an appetite for innovation.

Most of all, it requires love.

Read Full Post »

%d bloggers like this: