MORE BIOLOGY!
Now, we need to have a little more biology. Every organism is made up of many cells. The activity of each cell seems to be controlled by a structure located generally in the center of each cell called a nucleus. Within the cell's nucleus are contained the chromosomes (at least in higher plants and animals), and within every chromosome are the cell's genes. Chromosomes are made up of a chemical known as DNA (deoxyribonucleic acid) and some protein.
The chromosomes form a binary (two-part) system, and Mendel was lucky that they do. The characters that Mendel followed are alternative states in this binary system. In pea, Mendel examined tall versus dwarf plant size.
He examined purple versus white flower color.
Mendel compared smooth versus wrinkled seeds. These characters are called phenotypes, and a plant's phenotype is generally something visible: white flowers, dwarf size, wrinkled seed.
In Mendel's system, phenotypes exist as two-part alternative states, and a geneticist would call the gene that controls each alternative state an allele. In this system, there are two alleles possible for any one gene at any one time. When both alleles are the same, the genotype (or genetic combination) is called homozygous ("homo" meaning "same"). When the alleles are different,
the genotype is considered heterozygous ("hetero" meaning "different").
Back to Mendel. His original plants were homozygous, and for all of the characters he examined. How could he tell? Because each generation resembled the one before it. Inbred species (or inbred plants) produce progeny that very closely resemble the parents. The more the inbreeding (the more homozygosity), the more close the resemblance. So Mendel crossed two different homozygous genotypes, which appeared to be phenotypically uniform. And he produced F1 seed. So what happened? For every character in which his parental plants differed, the F1 was heterozygous! When you cross different homozygous lines, you get heterozygotes! Even more important, in the F1, every plant is the same. Every plant in the F1 generation is identical to every other plant. Every plant is a heterozygote, and all of the alleles are mixed at random. But every individual plant in the F1 population is uniform, and identical to every other plant. To a geneticist, the F1 generation has zero variability (every plant is the same), but also has the maximum heterozygosity (every possible combination of alleles is present).
As tedious as it was to get to this point, this is probably the most critical point of any discussion about saving seed. F1s are uniform, but in their seed is a tremendous amount of variation. Back to Mendel again. What happened with Mendel's F1? It was uniform, without variation. And "only the strong characters were visible." You just read that the F1 is a heterozygote, with two different alleles present for each gene. Why then is only the strong character visible? The real answer is very complicated, and there are actually many correct real answers. The simplest explanation is that the strong character hides the weak character. A geneticist calls this strong character the dominant character, and the allele causing the dominant character the dominant allele. Alternatively, the weak character is called the recessive character, and the recessive character is caused by the recessive allele.
In Mendel's original cross, tall is dominant over dwarf. Purple flowers are dominant over white flowers. Smooth seeds are dominant over the recessive wrinkled seeds. In Mendel's F1, the dominant characters are the ones that are visible. The dominant characters make up the phenotype of the F1. Mendel created an F2, and found all possibilities in variation. An F2 has the maximum amount of genetic variation possible. How does this occur? Through independent assortment. The alleles in each F1 are mixed randomly during meiosis, so that each sperm or egg cell is different from every other. These highly variable cells fuse during fertilization to produce every possible combination. If the F1 contains every possible combination of alleles, and those combinations are hidden by dominance, then the F2 expresses every possible combination, including those recessive characters previously hidden.
I have asked you to think about only three genes, and only two alleles for each gene. Reality is much more complicated. Other possibilities do exist. Neither allele may be dominant, and a cross between a red flower and a white flower may give you an in-between pink as a result. This is called co-dominance, or non-dominance. In many cases, each allele present gives an incremental response, so that an additive pattern develops. This can be extended across many genes controlling the same phenotype, so that additive effects can be very minor for each allele, but can accumulate and produce major effects. This is considered quantitative inheritance --- but at each gene, the inheritance is precisely the same as that for Mendel's genes. There can be many possible alleles for each gene. Only two at a time in any individual, but many possible combinations do exist.
And finally, three is about as many possibilities as I can handle in my mind at one time. Three genes, and two alleles each, gives a total of 64 different possibilities in an F2. The reality is that there are hundreds of thousands of genes operating in a plant at the same time, and that there may be many possible alleles in a given population of plants. The combinations that are actually possible are truly mind-boggling.