Why do we look and act the way we do? Part
of the answer is based on what we have learned. Part of the answer is based on
traits we inherit from our parents. Our inheritance shows
itself in
What did we actually inherit from our
parents? We call units of inheritance genes. An Austrian genius, a monk
named Gregor Mendel, working in the late 19th centtury conceptualized the
transmission of discrete units of inheritance from aprents to offspring.
Mendel's study of garden pea plants revealed, for mankind, some of the
fundamental ideas relating to commonly observed human traits.
Mendel descovered that genes occur, in
populations, in different forms we call alleles. For example, pea plant
flower color can be pink or white. there is a gene for flower color. the gene
has 2 forms or alleles: one form produces a pink flower; the other form
produces a white flower. In the problem you will work on at the end of this
web exercise, we look at human eye color. We know that there is a gene for
human eye color. One allele of this gene produces brown eyes. Another allele
produces blue eyes.A third allele produces gray eyes.
Since living organisms usually have 2
parents, we inherit one allele from each parent. What traits are displayed
dependon how the alleles interact with each other in the new individual.
Mendel discovered that the pink allele, in combination with the white allele,
produced a pink plant indistinguishable from palnts with 2 pink alleles. He
therefore described the pink allele as dominantto the white allele. He
described the white allele as recessiveto the pink allele. The pink
allele could be represented as P;the white allele as p. So the
combination Pp (also called a hybrid) is as pink as PP. Only pp
appears white.
Since each parent provides only one allele to its offspring, the 4 possible offspring (each inside one of the 4 lower right hand boxes) ends up with 2 alleles. In this example, the male alleles are in bold helping you to follow where the genes (alleles) go. In this example from Mendel's work, when pink female plants and white male plants are crossed, all the offspring have pink flowers and all can be seen to be hybrids, that is, they have one allele for pink (P) and one allele for white (p).
Let's develop this in a logical pattern leading to a way to easily see this, called a punnett square. We can represent a cross or mating between a pink flowered female plant and a white flowered male plant in the following way:
When a cross occurs between the pink female and the white male, only one allele from each parent is contributed. The following punnett square represents all the possible combinations of alleles that can merge together from such a cross or mating:
male
female
Mate
p
p
P
Pp
Pp
P
Pp
Pp
Genes are located within the living cells that make up a living body. Depending on our size, each human is composed of between 60 trillion and 100 trillion living, microscopic units called cells. Smaller animals and plants would have correspondingly smaller numbers of cells, but still many. So, thinking about you and a trait you will be working with in the problem below, each of your cells possesses a gene for eyecolor. Eye color is expressed through pigments and their distribution in the colored ring called the iris of your eye. The eye color gene in cells making up regions of the body other than the eye may help determine pigment production in these organs, may contribute to functions we currently know nothing about, or the gene may be non-functional in these areas.
Genes are composed of a chemical called DNA. The DNA molecules are located inside each of our body's cells, in a region called the nucleus. Here the DNA acts, throughout the cell's lifetime, as a set of instructions to be read over and over again to direct the cell's internal activities. As this molecule is very large and fragile, it can be damaged by chemicals and radiation. A change in the structure of DNA, a change that can be inherited, is called a mutation. Mutations can alter the way a cell works. DNA is safeguarded from damage through its placement deep inside the cell's nucleus, by the presence of cellular repair machinery (repair enzymes), and sometimes by placement of radiation-absorbing pigment molecules around the nucleus (as in skin cells).
DNA's message is to instruct the cell how to build proteins, both structural and enzymatic. Structural proteins are like bricks and beams that are used to build a living cell's framework. Enzymatic proteins, usually just called enzymes, are molecular machines that carry out a single process or job. Working in a series, enzymes can carry out complex molecular tasks, like a factory does. A factory imports raw materials, processes them using many specialized machines, and finally turns out a finished product. Similary, cells import raw materials from food, like sugars, amino acids and fats, and, using enzymes, produce products like cell parts, hormones, or pigments found in skin or in a flower, or in the iris of one's eye.