MadSci Network: Genetics
Query:

Re: How does parentage determine one's hair color?

Date: Thu Oct 19 21:54:14 2000
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology, Roche Molecular Systems
Area of science: Genetics
ID: 970770141.Ge
Message:

Sorry to take so long responding Janie; I hope I can answer your question satisfactorily.

Your question is a very specific version of a question that puzzled people for a very long time. That question was something like, "how are traits passed from parents to offspring," and it was answered in the end of the 19th century by an Austrian scientist named Gregor Mendel. Today, Mendel is known as the ‘father’ of modern genetics because of his work describing the way that traits are passed from generation to generation.

The so-called mendelian traits we are talking about here, are the inherited distinguishing characteristics that make-up an organism. For example, Mendel studied plants that grew in both tall and short varieties, so both ‘tallness’ and ‘shortness’ are examples of inheritable traits. Other examples are the colors of flowers, or the shape of the seeds that the different plants produced. On the other hand, something like hair length in people is not a heritable trait, because you can easily cut your hair short, or leave it to grow long.

Mendel’s work demonstrated that each organism has two versions of a given trait, and that these traits are inherited equally from both parents. So an organism inherits half of the traits that its female parent has, and half that the male parent has, and therefore half of the organisms’s traits come from each parent. He also demonstrated that the traits he was studying were inherited randomly, or independently of each other, so that the height- determining trait that was inherited from one parent did not affect which flower-color-determining trait that was inherited from that same parent.

Mendel also showed that when you inherit two different or apparently contradictory traits for the same characteristic, one trait will allways "win out" over the other, and the organism will display the winning trait. So, thinking about the traits that determine how tall a plant will be, if a plant has inherited both a trait for tallness and a trait for shortness, it will always grow to be tall. The trait for tallness always wins out over the trait for shortness. We call the traits that always win out over other traits DOMINANT traits, and the traits that always lose out, RECESSIVE traits. Since dominant traits will always win out over recessive traits, and since you get two versions of each trait from your parents, this means that you have to have two versions of the same recessive trait in order to see the effect of those traits in the organism. So, short plants have inherited two copies of the trait for shortness from their parents.

Now, In the middle of the 20th century, the famous scientists Watson and Crick discovered the structure of the DNA molecule, and demonstrated how DNA worked as the physical carrier of mendelian traits. Just as organisms have two copies of a trait, organisms have two copies of the DNA molecule; and just as half of the traits are inherited from each parent, one of each copy of the DNA molecule is inherited from each parent. Among other things, Watson and Crick showed how traits could reside in the structure of the DNA, and in particular, in the sequence of nucleotide bases that make up the DNA molecule.

Today, we call the sequence of nucleotide bases that are responsible for determining a specific characteristic in an organism a gene. We know that a given gene contains (or encodes) the information necessary for making a particular enzyme, and that enzymes do most of the work necessary to keep an organism alive. When there is more than one version of a given gene, so that each different version can produce a slightly different enzyme, we call these versions alleles. It is these genes and their alleles that correspond to the traits that Mendel first observed being transmitted from parents to offspring.

For example, if a plant makes red flowers, then that means that it is producing an enzyme that makes a red pigment in flower petals. The trait would be for ‘red flowers’, and the gene would be ‘the gene that produces the enzyme that makes red pigment’. If a similar type of plant makes white flowers, then that means that the enzyme that would normally make red pigment isn’t working. It isn’t making any pigment, and the flowers are ‘blank’ or white in color. The trait would be for ‘white flowers’, and the gene that produces the enzyme that make red pigment would be defective. That means that there would have to be a change in the DNA, that results in a change in the enzyme such that it doesn’t work. Since two different versions of the same gene are alleles of one another, we have a ‘red’ allele, and a ‘white’ allele. Because the ‘red’ allele results in an enzyme that produces color, and the ‘white’ allele does not produce color, the ‘red’ allele is dominant, and the ‘white’ allele is recessive. Everytime an organism has both ‘red’ and ‘white’ alleles, it will have red flowers. It is only in those organisms that have two ‘white’ alleles, that you will see white flowers. (In some cases, an organism with two ‘red’ alleles will produce flowers that are darker red than an organism with one ‘red’ and one ‘white’ allele because they produce more pigment. This is called, incomplete dominance.)

Now lets switch from flower color to hair color. Obviously, hair-color is not an entierly heritable trait, because you can dye or bleach your hair to change the color. But lets consider only those people who do not alter their hair colors. We know that there are different colors of hair possible. They range from dark black to whiteish blond and also to bright red. In addition, there are many different shades of hair color possible, covering most of the ranges between these extremes. Can we explain all of this variation with simple mendelian genetics?

Fortunately, I think we can. Except in the case of the gray hairs that we develop as we age, the color of a hair shaft is determined by the presence of the compound melanin in the hair. Melanin is produced by pigment cells in your skin and hair follicles. These cells use enzymes to produce granules of colored melanin. When your hair starts to turn white or grey, it is because the pigment cells have stopped producing melanin granules, and also because the cells that make your hairs have started incorporating air bubbles into the hair shaft, resulting in more light being scattered.

Our pigment cells can actually produce two different kinds of melanin, resulting in two different color ranges. So-called eumelanin is brown in color; depending on how much melanin is packed into each granule and how densely packed the granules are in a hair shaft, the hair’s color will range from very light brown to dark black. So-called phomelanin is red in color; hair range from a yellow-blond color at low granule density to a dark red color at high granule densities.

Now, how does this work in mendelian terms? There are a few surprises; in fact, what I am going to describe is really the short version of the whole story. If you continue to study genetics, you will learn that it is a lot more complicated that simple mendelian inheritance. First of all, there must be at least two genes involved; one must encode an enzyme that makes eumelanin, and the other must encode an enzyme that makes phomelanin. It turns out that there are 4 different copies of the gene for eumelanin in human DNA. Each copy is on a different chromosme (chromosomes 3, 4, 10, and 18), and they can each be inherited independently from the others. Each copy of this gene has at two alleles, one which produces melanin (like the ‘red’ allele), and one that does not produce melanin (like the ‘white’ allele). That means that the more functional copies of the melanin gene that you have, the darker your hair will be. Since there are four distinct copies of this gene in your DNA, and since you have two full sets of DNA, you can have anywhere from zero functional copies of the melanin gene (if all the alleles you receive are ‘white’ alleles) to eight functional copies of the melanin gene. As in the case of the incompletely dominant ‘red’ allele above, having more melanin, as a result of having more functional genes, will result in darker hair.

But what about the phomelanin? The gene for the phomelanin enzyme is incompletely dominant as well, but it is only found at one place in the DNA. So, you only get one copy from each parent. If you have the ‘red’ allele for the phomelanin gene, your hair will have more of a red color than if you have two ‘white’ alleles for the phomelanin gene, but less red color than if you have two ‘red’ alleles for the phomelanin gene. This is further complicated by the fact that eumelanin will mask phomelanin when there is a lot of eumelanin. So, if you have eight functional eumelanin alleles, it will be hard to see any red pigment at all, regardless of how many ‘red’ phomelanin alleles you have. On the other hand, if you have a small number of functional eumelanin alleles, it will be easier to see the effects of any ‘red’ phomelanin alleles that you have.

So Janie, lets look at your question, "How does parentage determine hair color?" I think you can see that it isn't really parentage that determines your hair color, it is inheritance! You get half of your genes for eumelanin and phomelanin from each parent. If both of your parents have similar color hair, then that means that they have similar numbers of alleles for each type of melanin producing gene. If both of your parents have dark hair, then the chances are great that you will inherit a lot of functional alleles for eumelanin, and that you will have dark hair too. However, since all of the genes are inherited independently, there is a small chance that you will inherit mostly the ‘white’ alleles for eumelanin, and you could wind up with light colored hair. Similarly, if your parents both have red hair, you will probably inhert the ‘red’ alleles for phomelanin, and the fact that you can see the phomelanin in their hair means that they don’t have much eumelanin, so you will probably inherit a large number of ‘white’ eumelanin alleles. However, unless both of your parents have no functional eumelanin alleles, there is a small chance that you could inherit more functional eumelanin alleles than either of them has, and your hair would appear to be darker brown in color, rather than red.

To demonstrate that, lets say that the functional allele for eumelanin is called H and that the non-functional ‘white’ allele is called ‘h’. Since there are 4 copies of this gene, and a set of 4 from is inherited each parent, we could represent a person’s eumelanin alleles like this: HHHhhhhh, for someone with only 3 functional eumelanin alleles, and HHhhhhhh, for someone with only two functional alleles. But because the alleles are inherited independently of each other, an offspring of these two people could occasionally inherit all 3 functional alleles from one parent, as well as the 2 functional alleles from the other parent, resulting in a pattern that looks like HHHHHhhh. So, the child could have darker hair than their parents.

Well, I hope that helps you see what the issues are. If you want to do further research into the genetics of inherited traits, take a look at the Online Mendelian Inheritance in Man (OMIM) database. This database isn’t user friendly, but you can use it to search for genes and genetic disorders associated with a certain genes. For example, a search for ‘Albinism’ will show you all sorts of genes, found throughout the DNA, which result in some form of Albinism (loss of pigmentation).

You might also want to visit the Making A Face web page. This site describes a project that teachers can use to teach students about the genetics behind facial composition.


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