Phenotype: This is the "outward, physical manifestation" of the organism. Anything that is part of the observable structure, function or behaviour of a living organism.

Genotype: This is the "internally coded, inheritable information" carried by all living organisms. This stored information is used as a "blueprint" or set of instructions for building and maintaining a living creature. These instructions are found within almost all cells (the "internal" part), they are written in a coded language (the genetic code), they are copied at the time of cell division or reproduction and are passed from one generation to the next ("inheritable").

Traits: Traits are small parts of the phenotype of an organism, such as the red colour seen in red poppy petals which is passed on to offspring.

Gregor Mendel, an Austrian monk, revealed through numerous experiments with pea plants that offspring are simply not "blends" of their parents. Rather, he clearly demonstrated that traits tend be passed to offspring in a "particulate" fashion. Indeed, if the blending theory were true, then everyone would eventually look about the same. Who knows; perhaps the most favorable phenotypes would be similar to Elton John?!? (I'm a fan).

Mendel decided to perform some crosses with his plants to test the blending theory. First, he crossed tall plants (i.e. long stemmed) which had parents and grandparents that had all been tall, with short plants (short stemmed) which had parents and grandparents that had all been short. The plants he crossed were termed the P (parental) generation. He found that the offspring, or F1 (filial) generation, were 100% tall. However, when he crossed these F1 plants together he found that the F2 generation represented a mixture: 3/4 were tall, and 1/4 were short. Whenever he performed such crosses, he always came out with these mathematical ratios. These mathematical ratios are very important because changes in ratios can sometimes be indicators that evolution is occurring.

How can this "particulate" nature of heredity be explained? Well, Mendel had a background in mathematics and through years of crossing and eating peas he came up with the following hypothesis: that there must be 2 factors (now called alleles) for each trait (now called genes), and that these factors (alleles) behave as distinct "particles" when passed to the offspring. Some of these traits are dominant (i.e. when present they are expressed), whereas others are recessive and their expression can be "masked" by dominant alleles. Offspring receive one of the factors (an allele) from one parent, and receive the other factor from the other parent.

Lets assign letters to each trait and perform some genetic crosses using a single gene. This is termed a monohybrid cross (simply a cross between two individuals where we look at a single gene). In pea plants, tall is dominant and is represented by a big T. Any time a big T is present, the plant is tall. When no big T is present, the plant will appear short. Since Mendel's tall parent plants were all true breeding (i.e. only produced tall plants) both of their factors must have been big so they are represented as "TT." If any small factors were present, they would have certainly have shown up at some time during his observations of the parent and grandparent plants. This "TT" is the genotype (i.e., the genetic makeup representing the 2 alleles that are present). Because the short plants were true breeding (i.e. all offspring from crosses yielded 100% short plants), their genotype must have been "tt." We can visually show such a cross between two parents by using a Punnett's square:

(TT x tt)

Note that along the top we write in the two alleles that segregated into the different gametes from one parent prior to mating (light blue color). Along the left side, we write in the two alleles from the other parent (pale green color). The resulting diploid organisms (termed F1s) within the squares themselves represent the offspring. As one can see, 100% of all offspring are heterozygous (Tt). Since each organism possesses at least one big T, and big T is dominant to little t, whatever effects may be produced by the little t are totally masked. All offspring in this cross express the dominant characteristic for tallness. Thus, the genotype of all offspring is Tt, and the phenotype (what you see) is tall. In reality, most combinations of alleles in living organisms do not display complete dominance or recessiveness. Rather, both usually contribute to the outcome of the offspring and most characteristics are under the control of multiple genes. However, it is likely that Mendel spent considerable time observing his pea plants prior to his experiments and when the time came to get down and dirty with his experiments he probably chose only those characteristics that seemed to make sense to him.

Now then, lets perform Mendel's next experiments where he crossed the F1s to each other. You'll note from the above that all offspring were heterozygous (Tt). In this type of cross, he found that 3/4 of the F2 generation were tall, but that the recessive characteristic reappeared in 1/4 of the offspring. The cross can be visualized as follows:

(Tt x Tt)

Indeed, we can now see why 1/4 of the individuals were short since only one in four (lower right) possess the tt genotype; i.e. no big T is present in 25% of the offspring. Thus, the genotype of the F2 population is 25% TT, 50% Tt, and 25% tt (a ratio of 1:2:1). The phenotype is different, however. Since 3/4 possess at least one big T, they will be tall. So, the phenotypic ratio is 3:1 (tall vs short).

The totally recessive individuals are highly useful in genetics. Whenever you see one, you automatically know the entire genotype (i.e. both alleles) for that gene. For instance, if you have a tall individual, you know that at least one big T is present but you don't know if the second allele is "T" or "t." Not so for the short pea plants as they can only be "tt." These recessive offspring are extremely valuable in genetics. You not only know the genotype of the individual in question, but you also know that each of the parents carries at least one recessive allele whether you can see it or not (i.e., the offspring HAD to get a little "t" from EACH parent). These recessive individuals can be used to "test" an unknown plant and probe for any hidden, recessive alleles. This is called a testcross.

The results of Mendel's crosses allowed him to formulate his Law of Segregation, which states that each organism contains two factors (i.e. alleles) for each trait, and the factors segregate during the formation of gametes so that each gamete contains only one factor for each trait. What this means is that the alleles of an organism exist as pairs (we now know on separate, homologous chromosomes) and that one member of the pair enter different gametes (i.e., we now know that the homologous chromosomes separate during meiosis ).

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