Introduction
A good understanding of the principles of Mendelian inheritance is a prerequisite to the conceptual understanding of evolutionary theory. Indeed, though Darwin himself subscribed to the "blending" theory of inheritance, it has since been determined that evolution by natural selection requires discrete genes. For those visitors who are unfamiliar with the basic principles of Mendelian genetics, we recommend a reading of the following introductory material.
Definitions and Terms
It would be helpful to be familiar with the following terms before reading this section.
- Allele: One alternative of a pair or group of genes that could occupy a specific position on a chromosome.
- Chromosome: A linear strand of DNA harboring many genes.
- DNA: Deoxyribonucleic acid; the molecule in which genetic information is encoded.
- Dominant: An allele producing the same phenotypic effect whether inherited heterozygously or homozygously; an allele that "masks" a recessive allele.
- Gene: A unit of genetic information that occupies a specific position on a chromosome and comes in multiple versions called alleles.
- Genotype: The genetic constitution of an organism.
- Heterozygous: Having a genotype with two different and distinct alleles for the same trait.
- Homozygous: Having a genotype with two of the same alleles for a trait.
- Phenotype: The physical or observable characteristics of an organism.
- Recessive: An allele producing no phenotypic effect when inherited heterozygously and only affecting the phenotype when inherited homozygously; an allele "masked" by a dominant allele.
The "Blending" Theory of Inheritance
Before Mendel's work, the most popular theory of inheritance stated that the qualities of the parents blended to form the qualities of the child. Under this theory, one tall parent and one short parent would produce a child of medium height. Most ordinary observations seemed to support this hypothesis, which rejected the notion of discrete units of inheritance (ie, genes). However, this theory was poorly equipped to deal with such phenomena as two brown-eyed parents giving birth to a blue-eyed baby. Though Darwin himself subscribed to the blending theory, it would clearly dilute any "favorable" characteristics acquired by mutation, thereby halting the evolutionary process. Only with the introduction of Mendel's work did the theory of evolution acquire a concrete, consistent framework of heredity.
Mendel's Experiments
Gregor Mendel, the Austrian monk famous for his experiments with pea plant characteristics, was the first to identify discrete units of heredity and thus discredit the blending theory. Mendel used characteristics of pea plants and four o'clock flowers to analyze the hereditary patterns of these traits. His historic experiments led him to the conclusion that inherited characteristics were carried in discrete, independent units (later named genes). In Mendel's interpretation, hereditary characteristics occurred in pairs of factors that had specific relationships. Mendel devised two fundamental principles of inheritance:
- Mendel's Principle of Segregation: The factors of inheritance (genes) normally are paired, but are separated or segregated in the formation of gametes (eggs and sperm).
- Mendel's Principle of Independent Assortment: Each factor's distribution in the gametes is not related to the distribution of any other factor. (This principle is not strictly true due to the organization of genes on chromosomes.)
Mendel also defined and described the relationships between the different factors of inheritance and their effects on the observed characteristics of the organism.
Mendel's Observations
Mendel made numerous important observations in his exhaustive study of pea plants' characteristics. He elaborated an important distinction between dominant and recessive traits through his work with pea plants.
By studying the characteristics of pea plants, such as their height, seed shape, seed color, flower position, and other traits. this discussion will use height as a primary example. Mendel first crossbred one tall, true-breeding plant with one short, true-breeding plant. Contrary to the blending theory, all the offspring were tall. In terms of genotype, the original tall plant was TT (two dominant alleles; homozygous), the short plant was tt (two recessive alleles; homozygous), and the second-generation plants were Tt (one dominant and one recessive allele; heterozygous).
When Mendel next allowed these plants to self-fertilize, he found that the short trait reappeared in the third generation. The ratio of short to tall plants was almost exactly 3:1. Their genotypes were as follows - 1 short (tt) : 2 tall (Tt) : 1 tall (TT).
Punnett Squares: Simulation
These results can be simulated with a Punnett square, a calculation device used to determine the possible genotypes of offspring given the genotypes of the parents. The parents' genotypes are represented by letters, one allele in each cell of the upper row (traditionally the mother) and rightmost column (traditionally the father). The offspring's genotypes are then calculated by observing the intersection of the mother's and father's individual alleles (mush like a multiplication table). Use the interactive Punnett square below to simulate Mendel's results or experiment with combinations of your own. (If you do not select a genotype, the simulation will assume Mendel's original cross: a true-breeding short plant with a true-breeding tall plant.)
Codominance (Incomplete Dominance)
Some inherited traits do not exhibit strict Mendelian dominant/recessive relationships. The simplest example of this phenomenon is called codominance, or incomplete dominance. This pattern is displayed in the colors of four o'clock flowers. When a white and a red flower are cross-fertilized, the second generation is all pink. However, when a pink flower is allowed to self-fertilize, the white and red attributes return. The color ratios for this third-generation cross are - 1 white : 2 pink : 1 red. This pattern is due to the fact that three alleles, instead of the usual two, determine color in four o'clock flowers. If red color is designated R and white color r, then pink color (not red or white) is the phenotypic effect of genotype Rr. (This is one type of pattern formerly used in support of the blending theory of inheritance.)
A slightly more complicated multiple-allele system determines blood type in humans. The three alleles are: A, B, and O, corresponding to A, B, and O type blood respectively. The A and B alleles are dominant over the O allele, which is always recessive. However, there is an additional twist: the A and B alleles exhibit incomplete dominance and, when inherited together, give rise to AB blood type. Yet another catch is waiting, however: humans also have the Rh factor, a special type of protein found in most (but not all) human blood. Rh factor's presence is inherited in the ordinary Mendelian fashion, with Rh-positive dominant over Rh-negative. In total, this means humans have 8 simple blood types.
Sex-Linked Traits
Sex-linkage is another common alternative-inheritance pattern. In sex-linked traits, such as color-blindness, the gene for the trait is found on the X chromosome (a sex chromosome). Sex-linked traits affect primarily males, since they have only one copy of the X chromosome (male genotype: XY). Females, who have two copies of the X chromosome, are affected only if they are homozygous for the trait. Females can, however, be carriers for sex-linked traits, passing their X chromosomes on to their sons. Sex-linked inheritance works as follows: if a female carrier and a normal male give birth to a daughter, she has a 1 in 2 chance of being a carrier of the trait (like her mother). If the child is a son, he has a 1 in 2 chance of being affected by the trait (for example, colorblindness). If a female carrier and an affected male give birth to a daughter, she will either be affected or be a carrier. If the child is a son, he will either be affected or be entirely free of the gene. See the following Punnett squares (The letters X and Y represent their respective normal chromosomes; X underlined represents the colorblindness allele).
Another example of a sex-linked trait is hemophilia, made famous by the "Queen Victoria pedigree" of the European nobility. Beginning with Queen Victoria of England (in whom it was probably a spontaneous mutation), the hemophilia gene spread quickly throughout the European rulers (who intermarried as a matter of course). The disease, which prevents blood from clotting properly and renders a minor injury a life-threatening event, claimed several young men of the royal line. Especially since male heirs were preferred over female as successors to the thrones of Europe, the spread of such a debilitating disease was a major problem.
The most famous case in the line was that of Alexei, son of Czar Nicholas II and Czarina Alexandra of Russia. Alexei's illness directly contributed to the downfall of his parents' regime and helped to usher in the Russian Revolution, which allowed the Bolsheviks to seize power and establish Communist rule in Russia.
Limitations of the Mendelian System
The simple system of Mendelian genetics is very powerful and serves to explain the inheritance patterns of numerous traits. However, many traits are controlled by many genes acting in tandem, and thus do not obey strict Mendelian patterns (although their constituent genes may). Furthermore, many human traits are strongly influenced by the environment as well, and therefore their phenotypes cannot be said to be Mendelian (though the genetic components may be). In sum, Mendelian patterns are important, but cannot be applied universally. Individual traits must be researched to find out if they obey typical Mendelian patterns.