GENETICS

Dr. C. George Boeree
Shippensburg University


Chromosomes, DNA, and Genes

We inherit most of our physical traits and some of our psychological traits from our parents via genes. Half of our genes come from mom, half come from dad. So we inherit some of our traits from each.

Each person is “built” on the basis of a “blueprint” consisting of two sets of 23 chromosomes. (Mosquitos have 3 pairs of chromosomes; Onions have 8 pairs; Carp have 52 pairs!)

Chromosomes are packages of DNA, which is the ultimate genetic material.  DNA stands for deoxyribonucleic acid. It looks like a twisted ladder (or double helix).

The “rungs” of the ladder are made up of pairs of four bases (the opposite of acids) we refer to as A, T, C, and G. These stand for adenine, thymine, cytosine, guanine. A always pairs with T; C always pairs with G. so there are four possible combinations: AT, TA, CG, and GC.

To switch metaphors from ladders to words, the four possible combinations of bases function as four "letters," and three letters at a time function as "words" called codons. This means we can have up to 64 different words.

Sequences of words, i.e. “sentences,” are what make up genes. The sum total of all our genes, all our genetic information, we call our genome. There are about a billion base pairs and 25,000 genes in the human genome.  I should mention that long stretches of DNA are devoted to regulating other genes, and even longer stretches seem to do nothing at all.

By means of partial half-strings of DNA-like material called RNA, the genes communicate the instructions that other parts of the cell need to make specific proteins. Proteins are like tiny machines that guide chemical reactions, and each one is specialized. One protein, for example, is hemoglobin, and its specialty is carrying oxygen.

Proteins are made up of amino acids. There are 20 amino acids, and they are what the codons actually refer to. Since there are 64 codons and only 20 amino acids, there is in fact a lot of redundancy built in to our genes.  That's a good thing when you consider that mistakes do happen.


The Sex Part

The human body has about 100 trillion (one with 14 zeros.) cells, all of which come from a single cell. Each cell has identical chromosomes in their nuclei - except for two specialized types of cell. Sperm cells and egg cells only have a single set of chromosomes - half of what other cells have. When the sperm cell enters the egg cell, the two sets combine to form a complete set, and life begins as a complete cell called a zygote.

In this way, we get one set of chromosomes from mom, one set from dad, in different combinations - hence the fact that brothers and sisters look similar to each other (and to mom and dad), but not the same. This is called sexual recombination.

For example, if mom has a set we will call A and a set we will call B, and dad has a set we will call C and a set we will call D, then there are four obvious combinations: AC, AD, BC, and BD -- representing four “flavors” of children. But this is far from all the possibilities.

Actually, each parental contribution to their child is a mixture of chromosomes. Mom has 23 in group A and 23 in group B, but when it comes time to contribute some genes to junior, chromosome 1 may be from A, chromosome 2 from B, chromosomes 3 through 8 from A, chromosomes 9, 10, and 11 from B, and so on. Over 8 million different possible combinations. And the same goes for dad.

So each brother or sister gets a different combination from each parent. Identical twins, though, come from a single egg-sperm combo (the zygote splits once, then each half goes on to be a different baby), so they have the same combination of mom’s and dad’s chromosomes - and so they look the same.  We can sometimes still manage to tell them apart - one may have a slightly flatter face, or a narrower head, for example - but these traits come from their positions in the womb, i.e. these are actually environmental effects.

When the gene that comes from mom and the corresponding gene that comes from dad are both the same, you obviously inherit whatever trait that gene leads to. But just as likely, the two versions differ, and then something has to give. Often, one version is more powerful - the dominant gene. It then “over-rides” the information from the other version - the recessive gene.

Eye color works a bit like this, at least when we simplify it a bit: If you have blue eyes, then you have a matching pair of genes - b-b. If you have brown eyes, then you might have a matching pair - B-B - or one of each - B-b or b-B. Brown is dominant and overwhelms the blue, which is recessive. So, if both mom and dad have blue eyes, then all their children will have blue eyes, because neither of them has the gene for brown. But if either mom or dad has brown eyes, all bets are off. It is likely that somewhere between half and all the kids will have brown eyes.

This actually explains what is going on when people say some characteristic "skips a generation". Sometimes, two people with brown eyes or brown hair will have a baby with blue or green eyes or red or blond hair, like one of the baby's grandparents. Really, both parents were "carriers" of the recessive gene, and both happened to contribute those genes to their baby. This, of courses, is a serious issue when it comes to illnesses that might be hiding in your genes. This is why doctors will ask you about your parents' and grandparents' health when trying to diagnose you.

Most traits, however, are tied to many genes, and predicting what children will be like is nearly impossible.

I should mention that the 23rd pair of chromosomes serve a special function in this regard: They determine the sex of the zygote (and so you). The X chromosome is pretty normal looking (X shaped), but another is odd - the Y chromosome. It is missing a section. Females have a matching pair of X’s. Males have one X and one Y. The genes on that missing section would have otherwise contributed to the creation of femaleness and inhibited other genes that contribute to maleness.  Notice that it is the father's chromosome that determines the sex of the baby:  If it is an X, the child will be female; if it is a Y, the child will be male.


Mutation

Mutation is the “misspelling” of genetic words. For example, the instructions for building CF proteins normally looks like this:

ATT-ATC-ATC-TTT-GGT-GTT-TCC

Some people are missing the second ATC sequence. So their RNA “neglects” to put a specific amino acid (phenylalanine) in the CF protein. The CF protein has the job of “gatekeeper” in the cell membrane, maintaining the levels of water and salt. Without that missing phenylalanine amino acid, it can’t do its job, and certain tissues, such as the lining of the lungs, dry out. It may not seem like such a big deal, but this tiny mutation is the one that causes cystic fibrosis.

Most diseases, I should note, involve many genes, just as most traits do.

Evolution is based on whether or not some particular creature does well (meaning survives and reproduces) in a specific ecosystem - or not.   This is called natural selection: Nature “selects” who will survive and reproduce and who will not, or, more precisely, what genetic material will get passed on and what will not.

Most of the variation in animals is due to sexual recombination. But the variations that are most important to evolution over the millenia are the mutations - specifically, the mutations of the DNA in the sperm and egg cells and in the zygote.

Most mutations have no effect, because there is so much genetic material not directly tied to protein production and because there is so much redundancy in the coding.  Some mutations are disasterous and lead to spontaneous abortion or early death. Others are less dramatically debilitating, and simply reduce the chances of survival and reproduction. Very rarely, a mutation actually enhances an organism’s chances within a certain environment, and is passed on to its offspring.

There are three main causes of mutations. First there is spontaneous mutation - just glitches in the natural process of DNA copying.

Next, we have physical mutagens or radiation. Any kind of electromagnetic wave that is high energy - most commonly, x-rays, uv or ultraviolet, and gamma radiation. These kinds of radiation are found in soil chemicals, in cosmic radiation from space, and in the air as radon (the most common form). It is also found in medical testing, nuclear power plants, television sets, and so on.

Third, there are chemical mutagens (both natural and man-made). One example is nitrous acid, which comes from digestion of nitrites, which are used as food preservatives.  One of our biggest concerns are the various chemicals that have made their way into our environment.  The effects of pesticides such as DDT were first brought to the public's awareness back in the 1960's when Rachel Carson published Silent Spring (1962).  As we have come to expect, the giant chemical companies (with the help of the agriculture department and the media) tried desperately to silence her, calling her "an hysterical woman."  Of course, many years later, DDT was outlawed and she even got the Presidential Medal of Honor.

A more recent concern are xenoestrogens - chemicals that mimic estrogen - such as bisphenol A, found in the plastics used for water bottles, baby bottles, and the linings of tin cans.  They are suspected of having a role in a large number of reproductive problems such as low sperm count, spontaneous miscarriages, low birth weight, reduced brain growth, a decrease in the numbers of boy babies, and the feminization of boys.  It is also suspected as having a role in breast cancer, prostate cancer, asthma, insulin resistance, hyperactivity, autism, and various immune disorders. 

Other potential culprits include PCBs, which never biodegrade.  They are absorbed by simple ocean life, which are eaten by larger ocean life, which are eaten eventually by polar bears, birds, seals, and, of course, us.  It is found in breast milk and is related to liver disease and the birth of babies with both male and female genitalia.  Also suspected are phthalates, used in PVC, nail polish, and even sex toys.  These are associated with problems like liver damage and the birth of boys with small genitalia.  If you are interested, you can find all about these chemicals in a book called Our Stolen Future (Dumanoski et al., 1996).  The authors of this book are, of course, now being labelled as "hysterical" by the giant corporations.

(Remember how global warming was ridiculed by the oil industry and the Bush administration? Or how President Reagan said that most pollution is caused by trees?)

Unfortunately, if a mutation doesn’t kill you, you may pass it on to your children. Inheritance is actually the most common way of getting a mutated gene! When we talk about people having a genetic tendency to get certain cancers, for example, we are talking about these inherited mutations. On the plus side, most mutations are recessive. It should also be noted that cells are capable of some DNA repair, and there is quite a bit of redundancy built in.

But again, if the mutation is genuinely helpful, natural selection may lead to it becoming the norm in a population, perhaps contributing to the creation of a new species in the process.  We are the result of millions of mistakes that worked out well.


The human genome project is an effort to map as completely and accurately as possible all the genes in human DNA. It started in 1990, when we knew the location of a handful of genes. It was 90% complete in 2000, and was 99.99% done in 2003.  Scientists have in addition discovered the function of nearly 50% of our genes.

There were a few surprises:  Although the human genome is comprised of more than three billion bases, this is only a third as large as scientists had predicted.  And it is only twice as large as that of the roundworm.   We share about 98.8% of the same DNA with our closest relatives, the chimps. That's ten times more similar than mice are to rats!  It was also discovered that 99.9% of the sequences are exactly the same for all human beings.  We are not as special as we sometimes like to think.

Knowing the human genome has awesome potential: It will help researchers to develop new drugs, tailor drugs for specific problems and specific patients, detect and predict illnesses earlier, even in newborns, and lead to gene therapy for various illness, even cancer. Gene therapy is the term for the actually repairing of genetic misspellings.  This is without a doubt the greatest discovery of the 20th century.


© Copyright C. George Boeree 2002, 2007