Genetics - Basic Facts
First, let's very briefly review what you probably learned in high school biology . Chromosomes are found in the nucleus of every cell. The normal human being has 46 chromosomes (23 pairs). The first twenty-two of these pairs of chromosomes are called autosomes, and, as you can see on page A-2 of your textbook, each of the autosomes is matched with a similar chromosome. The twenty-third pair are referred to as the sex chromosomes, and determine gender, among other things. An individual may have either two X chromosomes (which would be a female) or an X and a Y (a male). Each chromosome carries thousands of smaller units of genetic material, referred to as genes. An allele is one of a pair of genes. You only have two alleles for each gene (since you get one from each parent).
For example, there is a gene for eye color. Let's say that you receive an allele from your mother for brown eyes and an allele from your father for brown eyes. Obviously, you are going to have brown eyes. What if you received an allele for blue eyes and an allele for brown eyes? Then you would have brown eyes because brown is dominant. If an allele is dominant, it dominates the other allele. The dominant characteristic will be present in the individual, and the recessive (non-dominant) one won't.
Yeah,
yeah, yeah, and it can tell you what color your flowers will be, too, right?
How nice.
Actually, although many textbooks use an example of the colors of flowers and Mendel's research was with peas, genetics has a lot more immediate practical applications than building a better house plant.
My husband
broke his back and was in the hospital for a week while the physicians
all waited for him to come out of shock so they could operate. They assumed
he was in shock because the time it took his blood to clot was several
times what is normal. Since a typical operation of the type he was facing
would take a pint or two of blood, and, at this rate, he would require
around 16 pints of blood, they were understandably somewhat concerned.
Finally, a specialist in blood disorders was called in and diagnosed my
husband's condition as Bernard-Soulier syndrome, a rare, autosomal recessive
condition characterized by excessively large platelets (a type of blood
cell). His blood never would clot in normal time. He had the surgery, along
with literally gallons of blood, and then another surgery. This story does
not have a happy ending. He died.
So, what
does this have to do with genetics? Well, it was a genetic disorder, and
we had two children, both girls. What is the probability that they inherited
the same blood disorder? That seems a pretty important thing to know, don't
you think? The first thing that was important to know was that it was recessive.
That means that the children would have had to have received an allele
from both parents to be affected. It also means that we knew that their
father had two alleles for this disorder, since it was recessive, and he
had it. Therefore, it was certain that each of the girls had received one
allele for Bernard-Soulier syndrome. Each of the girls also received one
normal allele from me (their mother), and, because the normal gene is dominant,
they are both fine. However, both are carriers, and it is possible that
they could pass this on to any future children, IF their spouses are also
carriers. The fact that it is autosomal means that boys and girls have
an equal chance of being affected. It would have been very comforting to
have read that this disorder was carried on the Y chromosome (which would
mean only males would have it, and therefore, my girls were fine). That
wasn't the case. Below are some Punnett squares, which are a simple way
of figuring out what the probability is of having a particular trait. Obviously,
these work the same whether the gene in question determines Huntington's
chorea or eye color, but you most often hear this type of example related
to birth defects, because that concerns people a whole lot more than other
inherited traits like whether you can roll your tongue or have hair growing
on your ear lobes.
As you can see from the diagram above (which is called a Punnett square, incidentally), if a defect is dominant and a parent is homozygous for that trait, meaning both alleles are the same, then all of the children will have that defect. This is true even if the other parent has NO alleles for that trait. Your textbook gives a few examples of traits which are dominant and recessive.
The one fortunate fact in this whole
story is that Bernard-Soulier syndrome is recessive, as are most birth
defects which have been discovered to date. The Punnett square below shows
the outcome if a trait is recessive.
So, as you can see, since the child will inherit only a normal gene from the mother, it is guaranteed that he or she will NOT have this particular disorder. (Not surprisingly, their blood tests indicate that they are fine.) We also know that their father's genotype (the alleles he had for a specific gene) must have been dd, that is two defective genes, otherwise he would not have had this disorder, because, by definition, to have a recessive trait, you must have two alleles for that trait. We also know that each child must have inherited a defective gene (because they could only get a defective gene from their father). We know that each child must have a normal gene which dominants the defective gene, because they do not have the disorder. We also know that the children will all be carriers for this disorder, that is, they do not have it themselves, but carry the gene that causes it, which could be passed on to the next generation.
As you can, hopefully, see, this information can be very useful, especially if you are discussing a disorder which is very painful or even fatal.
Genetics: A few complicating factors
The above is only a very brief introduction to the basics of genetics. If you read more about genetics, even basic textbooks will mention a few of the complicating factors, such as sex-linked traits and co-dominance. A few other complicating factors (of many) for you to think about are:
- Many human characteristics, such as intelligence, are assumed to be polygenic, that is, determined by more than one gene.
- Evidence is mounting that certain behavioral characteristics, such as shyness, are genetically determined (if you are interested in this topic, you might want to find a basic review of research on behavioral genetics).
- The fact that a characteristic is genetically determined does not mean it cannot be affected by environment. Eye color and hair color are genetically determined, but you can always change these by using colored contact lenses or dying your hair. You might argue with that, saying that your actual eye color is the same. However, height, which has a strong genetic component, is also determined by environment. You may have noticed that tall people tend to have tall children and short people have shorter children than their tall friends do. Certain races tend to be shorter, such as the Japanese and Latinos. Environment still can have a significant effect. Consider the fact that the current generation of Japanese young adults are inches taller (on the average) than the pre-World War II generation. No, it is not some subtle result of bombing, it is a result of better nutrition.
- There is an important difference between genetic determination and genetic predisposition. If a trait is genetically determined, whether, or how much you develop it is dependent on your genes. If you have genes for blue eyes, your eyes are going to be blue. Period. It doesn't matter if everyone else around you has brown eyes or if your mother really wanted a brown-eyed baby. The same is true with many birth defects. If your child is born with Down syndrome it is because he or she has three (instead of two) of the 21st chromosome. So, yes, your child actually has 47 chromosomes in this case, instead of the usual 46. You can do a lot to help your child get an appropriate education, but he or she will always have Down syndrome. You didn't do anything to cause that and you can't make it go away.
- A genetic predisposition means that a person is more likely to develop a disorder or characteristic, under certain environmental conditions (which we usually cannot identify, unfortunately). Two common examples are alcoholism and schizophrenia (a type of mental illness). Children who have a parent who is alcoholic, or schizophrenic, are more likely to develop these problems than the general population. You might think that this is due to growing up with a parent who is alcoholic or mentally ill, which cannot possibly be the ideal environment for a child. However, even children who are adopted very young show a higher probability of developing the same disorder as their biological parents, even if they have never even MET that parent. So, these children are more at risk, but most of them develop normally (a larger percentage than you would expect if genes were the sole cause). Identical twin studies also show that, while if one twin is alcoholic (or schizophrenic) there is a high probability that the other twin will be, too. there are also plenty of cases where the other twin has no observable problem. Obviously, if genes were the only determinant, when one identical twin was an alcoholic, the other would be, too (since identical twins have the same genotype). I think this is a very important point. I have heard people make such statements as "Well, of course he is going to have a problem with alcohol, both of his parents were alcoholic, you know." And it seems to me so ignorant to just write off a child as 'doomed' that I just want to smack people who do it. Being a professional, I don't go around smacking people, of course, but it is definitely a temptation at times.