Heredity

(Almost) Everything we have to Offer on the Subject

Heredity has a very interesting history. Many great scholars have speculated about the nature of heredity, and some have come close, but others have missed the mark entirely. In the end, the rules of heredity were laid down by Gregor Mendel, who was just an ordinary Austrian monk, except for his many genetic experiments. A look at how others thought heredity worked: Early in the 19th century, Jean Baptiste de Lamarck, a French zoologist, suggests that traits in organisms change as climate and habitat change. These traits are then passed down to the next generation. Charles Darwin, who is best known for his work on the theory of evolution (which was first published in 1859), suggests a heredity process called pangenesis, in which cells around the body give part of themselves in the human repoduction process. Darwin also thinks that traits that are expressed later in life can be transmitted to offspring. The German botanist August Weismann believes that traits expressed later on in life are not transmittable to offspring. Weisman, however, like Darwin, believes that there are heredity "particles" that affect traits. Gregor Mendel, our favorite Austrian monk, figures out many of heredity's rules using pea plants. He publishes his work in 1866, but no one really cares about until it is "rediscovered" in 1900. Thomas Hunt Morgan proposes a theory of heredity that includes chromosomes. Much research follows to find out how chromosomes act and work. Research is still going on in heredity, but no major new discoveries have been made. Who knows? Maybe you'll be the next revolutionary scientist in the field of heredity.

Heredity, as you should now know, from the Introduction, is how traits (or characteristics) are passed down from organism to organism. In other words, it is why "it runs in the family," why you have your father's eyes, and why everyone says you look like your sister.

As you may know, genes determine what you look like, how you may act, and many other things about you. Genes are generally the same (they contain the same basic information to keep the body running smoothly, usually), but have variations in less then 1% them called alleles. Alleles determine what kind of hair color you will have, how tall you will be, and many other things that describe your phenotype; that is, what you look like on the outside, to yourself and to others. For example, when you describe someone as short, with black hair and brown eyes, you are describing that person's phenotype. A person's genotype, on the other hand, uses alleles to describe what that person looks like on the inside. For example, instead saying that a person has brown hair, you'd say that the person has allele such-and-such. You would also say that they have the recessive allele this-and-that, which didn't show up on the phenotype (you will learn about recessive alleles later).

In general, there are two forms of a trait. These forms are called dominant and recessive. Dominant forms of an allele are alleles that take immediate power over the other allele that you have in your genes (remember, one set of chromosomes, which house the genes, is from your mother and the other is from your father, so for every trait except for possibly sex-linked traits you have two alleles). In other words, if you have a hanging earlobe allele, a dominant trait, it doesn't matter if you also have an attached earlobe allele, which is a recessive trait (you will learn about those very soon). You will have a hanging earlobe.

There are also recessive forms of alleles (like the attached earlobe). These are the exact opposite of a dominant allele. The only way they can be expressed is if both of the alleles you recieve for their trait are recessive. If a dominant allele is present, it takes over and the recessive allele, though still there, is not visible on your phenotype. In general, to have a dominant allele expressed, one or both of the alleles must be dominant. To have a recessive gene expressed, both of the alleles must be recessive.

The types of alleles discussed above occur on chromosomes that are autosomal (they are not sex chromosomes). The alleles and traits that are inherited from the sex chromosomes (sex-linked traits) act slightly differently. This is because in a male, the smaller Y chromosome that pairs with the X chromosome is not active. There are no genes or alleles that occur on the Y chromosome. This means that a male will show any sex-linked trait even if the trait is recessive because there is only one allele there. An example is the recessive disease hemophilia (let's call it h). If a man has the recessive allele for hemophilia (h) on the X chromosome, than he will have hemophilia. If he has the dominant allele for that gene (H, which does not cause hemophilia) then he will not have hemophilia. Females, on the other hand, will have two alleles for the gene that causes hemophilia since they have two X chromosomes. If they have two recessive h alleles, they will have hemophilia, but if one or both of their alleles are dominant H alleles, they won't have hemophilia. In this case, if the man has the dominant (H) allele for hemophilia, and the woman has both a dominant (H) allele and a recessive (h) allele for hemophilia, then their daughters will not have hemophilia, but 50% will carry the recessive allele for it. Fifty percent of their sons will have hemophilia. If the man has the recessive hemophilia allele, then 50% of their children will have hemophilia, and half of the remaining children will carry the recessive hemophilia allele.

There are exceptions to the general rules of heredity. Sometimes more than two alleles determine a trait. A good example of this is blood type, in which there are 3 different alleles, I^O, I^A, and I^B. With these blood types, only type O is recessive, so when type A and type B appear as alleles in the genetic makeup of a person, they are both expressed as blood type AB. This is called codominance. There is also a incomplete dominance. Here, both alleles blend together to form a new look, because neither one is completely dominant over the other. For example, consider two plants, one white and one red, that pollinate. The next generation of these plants would be pink if there was incomplete dominance. However, this does not mean that the alleles have changed. They can still be passed down to form traits that are not affected by incomplete dominance.

Other kinds of contradictions to the general rules of heredity inclue pleiotropy, in which alleles at the same point on a chromosome affect more than one trait. Epistasis is where in order for a trait to be useful, there must be two different alleles at different points on the chromosome acting together. Echolocation in animals is an example of this. Animals with ecolocation must be able to produce high-pitched squeaks, which they can do becuase of one gene, but must also be able to interpret the squeaks when they are bounced back off objects, a trait determined by a different gene. Complementary gene action is an action where there have to be two dominant genes for that trait to show, otherwise the recessive trait is shown. Finally, there is polygenic inheritence, in which a number of genes affect a part of the phenotype slightly, and all must work together to create the final result.

Introduction
Heredity
Punnett Squares