MadSci Network: Genetics

Re: Genetic Code and Phenotype

Area: Genetics
Posted By: Brian Foley, Post-doc/Fellow Molecular Genetics
Date: Mon Oct 7 14:12:32 1996
Message ID: 834904210.Ge

> How detailed is the genetic code in specifying its phenotype?  

> Consider hair on humans or fur on other mammals. Does the genetic code 
> specify the location of each hair or is there just some general place 
> coding indicating where hair will or will not grow?
Although some people have referred to the genome of an organism as a "blueprint", this is very misleading. A blueprint has physical characteristics resembling the final structure; each line on paper represent a wall or a pipe or a wire in the final building. An organism's genome is much more similar to a recipe than a blueprint. With a recipe, we have information about what ingredients to use, and how to mix them and cook them, but the final result looks nothing like the recipe. The flour and yeast and other ingredients all interact in a somewhat mysterious way to produce the bread or cake.

Mammals have many different tissue types. They all start from a single fertlized egg (zygote) and very early on in development, the cells that develop from the zygote differentiate into 3 types. One type will make the skin and nerves, one type will make muscles and bone, and the third will make organs and blood. A bit later, the three types further differentiate into individual organs and tissues.

Such a method of development is called hierarchical, meaning "different levels". Thus we can't have skin without first having the zygote form the three basic types of cells and then the skin/nerve type further splitting into skin and nerve types, and then the skin type spliiting further into hair follicles and sweat glands and other skin parts.

Very late in development, there are effects from hundreds of genes interacting together to produce final structures. The skin on the head is influenced by genes being expressed by brain tissue, and other head cells. The skin at the tips of the fingers is influenced by genes that regulate hand development so fingernails grow at the fingertips. It is at this stage that the number of hair follicles is worked out, and the amount of growth hormone receptors that will be part of the hair follicles which will later influence how fast the hair grows. Still much later in life, the development of sex hormones will infuence the growth of body hair (an head hair too in the case of male pattern baldness).

Although in some cases a single gene can have a major visual effect, such as causing male pattern baldness, it really takes the interaction of thousands of other genes to set the stage for that gene to take effect. Sort of like saving out some egg white to brush on the crust of a bread just before baking, it changes the looks of the loaf of bread a lot, but if the dough isn't just right and the temerature of the oven just right, the effect will not be seen.

> Also consider finger prints?  How similar or varied are the finger prints 
> of genetically identical individuals, for example, identical twins?
I am not a fingerprint expert, but I suspect that identical twins have very similar, close to identical fingerprints. A real fingerprint expert could probably still find differences between then though. Most parents of identical twins can tell them apart.

Brian Foley

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