by Phil Albro
Like almost everything else alive in this world, human beings can mate with other members of their own species to produce new living beings. They can not produce live offspring from matings with other species. Higher forms of life (higher than slime mold, for example) reproduce sexually, that is, there are male and female subtypes in the species. Only matings between a male and a female can produce live offspring. So far, so good.
Individual human beings (“people”) have unique combinations of characteristics that make them at least slightly (more often a lot) different from every other individual human being. This is because the “instruction manual” for producing the particular combination of characteristics that make up an individual is written when a particular man mates with a particular woman on a particular day in history. It's all tied up in chromosomes, genes, DNA.
This isn't really high school Health class, although it was probably there. If you took high school biology it was there too, but the teacher was being very careful not to have you learn more about S – E – X than you needed to know! So not surprisingly, genetic transmission of characteristics, the “genetic code” and all that stuff, the whole reason geneticists bother to study family trees, is a little vague to a large number of people. I hope the following simplified review will help eliminate some of the folklore and misunderstanding in this area. Please remember that it is simplified! I will skip the subject of mitochondrial DNA, for example. In what follows, words that show up in blue and underlined are defined. If you don't know what one of those words means, click on it and a new window will pop up containing the definition. Just close that window to return to this article.
Starting with DNA:
DNA is an acronym standing for deoxyribonucleic acid, the chemical substance that contains the genetic information (blueprint, design specs) in all living things (except a few viruses.) This chemical is a “polymer”, like plastic only different, in the form of a long chain. Four simple chemicals (bases), many copies of each, are hooked together in what looks like a random order but in fact is anything but random, to make a chain millions of bases long. How long is that? Well, your body is made of cells too small to be seen without a microscope (except for a few giant ones. The world is full of exceptions.) The DNA in one of your tiny cells, if uncoiled and stretched out, would reach over six feet! Anyway, the genetic code is based on the sequence of the four bases. A given sequence codes for a given part of your body (one of its enzymes, for example.) Change the sequence by changing one base out of, say, 200,000, and you would get a different enzyme that might work, might not work, or might even work better!
Genes and Chromosomes
The DNA chains are organized into two major regions, the genes, and something we are only beginning to understand. That latter is guessed at, and could be spare parts, all sorts of unproven ideas. We'll stick to the regions that are divided into genes. Each “gene” is a section of the DNA chain that has the code for one particular chemical (protein) your body uses. Human beings have a few tens of thousands of genes. The genes are strung together like beads on a necklace to make “chromosomes”. Bacteria have only one chromosome, while humans have 46. Other creatures have more, or less than 46 chromosomes. A mating between members of two different species that don't have the same number of chromosomes can not work. The chromosomes have to be able to match up in corresponding pairs.
A human's 46 chromosomes are in the form of 23 pairs. That is, there are two “copies” of each unique chromosome, one that came from the father and one from the mother. All the genetic information that tells how to make a human being is coded in the sequences of bases in the genes that are strung together in these chromosomes. Each and every cell in your body except your red blood cells (always an exception!) has a duplicate set of the 46 particular chromosomes that define “you.” (Trillions of backups! That should make you feel secure!) [An aside: They don't totally define you, since the environment and your life experiences play a role too! They do, however, make sure you are human and not a kangaroo rat. That just makes genealogy different from genetics!]
Male and Female
O.K., so far so good. Now to make life complicated. There is one exception to the bit about there being 23 matched pairs. Males have one so-called “X” chromosome (from their mother) and one so-called “Y” chromosome (you guessed it – from their father) as the 23rd “pair”. These contain the coded information for “sex-linked” characteristics, like whether or not you'll be able to grow a beard. They are obviously not matched, and don't even look alike under the microscope. Females have two Xs, one from each parent. So if you have a Y-chromosome you're male. If not, you're female.
When a male and a female mate, each provides a single copy of each gene, that is, 23 chromosomes, to the newly conceived child. This is because the germ cells, sperm and ova, are produced by a unique system so they don't get two of each chromosome. Since the woman has no Y chromosomes, while the male has an equal chance of providing an X or a Y, obviously, the father's contribution determines the sex of the child. It was Henry VIII's fault he wasn't getting an heir, not his wives' fault!
Anyway, one thing should be obvious. Only the biological parents can contribute any genetic information to the child. Marriage is irrelevant. Step parents contribute none; guardians contribute none; adopted parents contribute none. They will contribute intangibles – values, habits, psychological traits, even physical results depending on how they fed the child! But not any genetically inherited traits.
There is an interesting trivia item associated with this - since a girl has two X chromosomes, she received the same amount of genetic informtion from her father as from her mother. But a boy receives somewhat more genetic information from his mother, since his X chromosome (from her) provides hundreds of genes but the Y chromosome (from his father) contains only 27 genes!
We're All Mutants
Let's look at those 22 pairs of chromosomes not including X and Y. Even though in the simplest theory there should be matching genes in the same order on the two chromosomes of a given pair, that isn't quite how it works. Through the ages there have been mutations (changes) in the genes of any given individual, and some of those changes have been passed on to that person's offspring. A change that can be passed on will be one that modifies the characteristic in a way that doesn't prevent survival. Blue eyes, for example, probably came about when particular mutations in the genes that control eye color occurred. Then the descendants of two blue-eyed parents eventually made up a significant portion of the population in a particular area. Suppose a blue-eyed mother and a brown-eyed father have children. The pairs of chromosomes containing genes related to eye color in the children will not be exact duplicates of each other. (Incidentally, if you have genes for both blue and brown, you get brown. Brown is said to be dominant over blue.) We call two genes that code for the same product, but which are not identical, alleles. Since the same consideration applies to all of the thousands of human characteristics, the odds are that the two chromosomes making a pair are never identical in any person today. Each of a pair of identical twins has a set of 46 chromosomes identical to that of the other, but each twin has 23 pairs of chromosomes the members of which will be not quite identical.
Some of the modifications that will occur as mutations are not as harmless as blue versus brown eyes. Some will not allow that baby to live, while others cause significant health problems like sickle cell anemia. Other problems that can occur, not as mutations but as accidents during reproduction, can cause conditions like Down's Syndrome, which involves there being three copies of chromosome 21 instead of two (two stuck together when they should have separated during cell division.) But there is another aspect of there being 23 non-identical pairs of chromosomes in a person that we should think about. In most cases, only one of the two copies of a given gene in a given cell of a given person is “active”, or being used. We don't understand exactly how, but something determines, in each cell of your body, which of the two copies it will be. In some of your cells the copy you got from your mother will be “on” and in other cells it will be the copy you got from your father. The advantage to you is that if one of the copies is defective, the other may be good and keep you going.
Why We're Each Unique
Let's look at a couple of numbers. You have 23 chromosomes worth of different genes. You have a second copy of each of the genes on 22 of those chromosomes, and a similar but not identical collection of genes on the 23rd if you are male (X-Y), a matched set if you are female (X-X). A given germ cell (sperm or ovum as the case may be) will contain 23 chromosomes, one copy of each. Let's say that for the male, the particular chromosomes will be identified as numbers 1m – 23m. For a female they will be 1f – 23f. Let's suppose these happen to be the germ cells that merge in a pregnancy. The child now has all 46 of those identified chromosomes. It grows up and produces its own germ cells. One of them gets 1m, 2m, 3f, 4m, 5f, 6f, 7m, 8m, 9m, 10f, 11m, 12f, 13f, 14m, 15f, 16f, 17f, 18f, 19f, 20m, 21m, 22f, 23f (X). If there is something that determines which, f or m, of a given pair goes into a given germ cell, we don't know what that might be. But you can see, there are 23 x 23 or 529 unique combinations. When this person mates with someone, there are also going to be 529 unique combinations for the germ cell from the other person. Thus there are 529 x 529 or 279,841 different possible combinations of genes if these two people become parents. That is, their next child can be one of 279,841 possible different children. Suppose one of those potential parents might mate with any one of the 3,000,000,000 members of the opposite sex living at the present time (ignoring for simplicity that some are too young and some are too old). Then because each person could have unique alleles of a given gene, the next person to be conceived in this world could be any one of 529 x (529 x 3,000,000,000) = 839,523,000,000,000 unique people. To put that into some sort of perspective, the total number of people who have ever lived on the Earth back to the creation of human beings is estimated at roughly 11,500,000,000. That means at least 100,000 times as many unique people could exist as have existed so far. I find that comforting, myself.
Also keep in mind that if two people with all 46 chromosomes identical did manage to get born (as identical twins do), they still would not necessarily have the same members of each pair of genes activated. Their exact experiences during development in the womb and thereafter probably affects the selections. Furthermore, there is a phenominon called "recombination." Some of the genes on one of your chromosomes, for example one you got from your father, will exchange with the corresponding genes on the matching chromosome you got from your mother. This can only happen with paired chromosomes, so is not going to happen with the male Y-chromosome. This is a random process, and there is no way to predict which or how many genes or pieces of genes will trade places during your lifetime. If the trade involves the germ cells or their precursors, it ensures the offspring will never be quite identical to the parents. So it is safe to conclude, no two exactly identical people have ever existed, even if their genes gave them the potential to be identical. Moreover, there will continue to be new mutations, further increasing the number of new, unique persons that could come into existence. Life is really something, isn't it!
Taboos Aren't Only for Primitives
Let's look at the implications of that “advantage” that in many cases, if you have one “good” copy of a given gene you will be o.k. Suppose both copies are bad? You're in trouble. You will be missing something you need to live a completely normal life at best, and may lack something essential to life itself at worst. Suppose both of your parents share a mutation such that each has one of its pair of a specific gene defective. Each of their children has a 75% chance of getting at least one good copy of that gene. But suppose you are in the other 25% and get a “bad” copy from both of them? Tough luck for you! Who are most likely to share a “bad” mutation like that? Brothers mating with sisters, or parents with their children. Second most likely (in a given generation?) First cousins. This is the scientific reason why incest is taboo, and why most societies ban marriage between first cousins. The observation that the offspring of siblings and other very close relatives are more likely to be genetically defective than the offspring of matings between unrelated, or less closely related persons, was made long before the genetic reason for it was known. A few diseases you may have heard of that require two “bad copies” of a gene are cystic fibrosis, sickle cell anemia, Tay Sachs Disease, thalassemia and AR polycystic kidney disease. Ones that are specifically linked to the X-chromosome include Becker muscular dystrophy, Duchenne muscular dystrophy, hemophilia, and red-green color blindness. For these diseases, for a woman, two bad copies would be needed to show the effect. For a man, only one bad copy would be needed, since he has only one X chromosome!
There seems to be a widespread belief that susceptibility to a given malady may also be inherited. One of the reasons to study your ancestors is to find out what they died of, on the theory that you may be especially susceptible to the same problems. If your parents and/or grandparents are dead, doctors will usually ask what they died of when you have a major physical. In the Albro family, the commonest cause of “death of natural causes” has been lung diseases including pneumonia, “consumption”, and obstructive lung disease. Did we inherit a susceptibility to lung problems, or do we tend to choose a life style that favors such problems? No proof either way as yet.
The story of genes and matings and offspring and love and inheritance rights could never be told in all the web pages in the world. I hope this extremely brief (whether it seems brief or not!) summary of some basics has been helpful to you. If not, please tell me how I can make this more useful.