The Law of Segregation
Goes like so: During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.
The way I figure it, Mendel probably got really bored crossing pure dominant trait pea plants with pure recessive trait pea plants (over & over & over again) & getting nothing but pea plants with the dominant trait as a result. Except for gaining more & more evidence for his Law of Dominance, this probably grew tiresome. So, at one point he takes the offspring of a previous cross & crosses them. Ooooooooh ............
Recall that his original cross for the tall & short pea plants was:
TT x tt
tall x short
So, he takes two of the "F1" generation (which are tall) & crosses them. I would think that he is figuring that he's gonna get all tall again (since tall is dominant). But no! Low & behold he gets some short plants from this cross! His new batch of pea plants (the "F2" generation) is about 3/4 tall & 1/4 short. So he says to himself,
"Greg ol' boy, the parent plants for this cross each have one tall factor that dominates the short factor & causes them to grow tall. To get short plants from these parents, the tall & short factors must separate, otherwise a plant with just short factors couldn't be produced. The factors must SEGREGATE themselves somewhere between the production of sex cells & fertilization."
I think it's easier to picture this law by using a p-square. Our cross is two hybrid parents, Tt x Tt.
The punnet square would look like this:
Now, when completing a Punnet Square, we model this "Law of Segregation" every time. When you "split" the genotype letters & put one above each column & one in front of each row, you have SEGREGATED the alleles for a specific trait. In real life this happens during a process of cell division called "MEIOSIS". Meiosis leads to the production of gametes (sex cells), which are either eggs or sperm. Sometimes the term "GAMETOGENESIS" is used instead of meiosis. Scientists love vocabulary (sorry).
You can see from the p-square that any time you cross two hybrids, 3 of the 4 boxes will produce an organism with the dominant trait (in this example "TT", "Tt", & "Tt"), and 1 of the 4 boxes ends up homozygous recessive, producing an organism with the recessive phenotype ("tt" in this example).
Parent Pea Plants
(Two Members of F1 Generation)
Tt x Tt
tall x tall
A helpful thing to recognize:
Any time two parents have the same phenotype for a trait
but some of their offspring look different with respect to that trait,
the parents must be hybrid for that trait.