Before I begin detailing Chapter 2 of Origin, I would like to take a brief look at our modern understanding of genetic inheritance. This serves two purposes, a basic grasp of Mendelian principles helps one to understand evolution in light of genetic inheritance, and also I have a genetics exam tomorrow morning (lucky me). The textbook I use for my class is Genetic Essentials: Concepts and Connections by B. A. Pierce, published by Freeman, 2010.
Gregor Mendel (1822-1884) was a little known contemporary of Charles Darwin. His research gained popularity years after his death, and was rediscovered in 1900. Mendel's now famous experiments were done on pea plants. He would take true breeding (homozygous) pea plants that express two different traits (denoted as the P generation) and cross them. The offspring (the F1 generation) would then be genetically heterozygous for whatever trait Mendel was isolating, but would only express the dominant phenotype. The subsequent generation (the F2 generation) derived from crossing the F1 generation would result in a phenotypic ratio of 3:1, meaning 3 plants would express the dominant phenotype to every 1 plant expressing the recessive phenotype.
Hope I haven't lost anyone. These are somewhat confusing terms if you haven't had a biology class in a while.
We receive one set of genetic information from our mother and one from our father. I find the easiest example of this to be sex. Sex is determined by what chromosomes we receive, XX being female, and XY being male. A mother always passes one of her X chromosomes to her offspring. A male will pass on either an X chromosome or a Y chromosome to his offspring. So we can use a Punnet Square to determine the different outcomes for what the offspring may turn out like:
Please excuse the crudeness of my MS paint punnet square. I'm also sure my professor for Sociology of Gender would just love that I used pink for females and blue for males. Anyway, as you can see, the sex ratio is 1:1 for sex.
Mendel was looking at alleles (different forms of the same gene). He focused on the phenotypic (physical) traits of parents and offspring and crossing them to see what happened.
So, the P generation would be YY (heterozygous dominant) and yy (homozygous recessive) for traits. Let's say that YY is yellow and yy is green. The F1 generation would all be Yy (heterozygous) and would all express a yellow color. The F2 generation would be YY, Yy, and yy at a ratio of 1:2:1, with YY and Yy expressing yellow while yy would express green. Mendel found this 3:1 physical expression ratio to hold true across his many experiments. It works as a basic model for predicting the inheritance of unlinked traits.
Punnet squares can also be applied to alleles that don't have complete dominance. An instance of incomplete dominance is when the phenotype of the heterozygous offspring is an intermediate between the two parents (ie a pea plant that is some color between green and yellow); co-dominance is when both phenotypes are expressed (ie a pea plant that is both distinctly yellow and green). In these cases, the F2 generation would have a phenotypic ratio of 1:2:1 (totally yellow, mix, totally green).
Mendel shows that traits that are not expressed can still be carried by heterozygous individuals and may be expressed in the next generation when two carriers reproduce. This ties in with Darwin's instances of animals reverting back to their "aboriginal states".
So how does this play into evolution? Sexual reproduction creates a huge amount of variation. Variation is necessary for natural selection to favor some individuals over others allowing for greater reproductive success and the propagation of the alleles that produce such traits.