Why do you share traits similar to the ones that your parents have? Why is it that a certain trait that your father has was passed to your siblings, but not to you? These questions were asked for hundreds of years. Traits are simply different forms of a character. For instance, AB and O are two different traits for the character blood type. Black hair and blonde are two different traits for the character hair color. Heredity is the passing of traits from the parents to the offspring, but it was not understood until a meticulous monk grew some peas.
Gregor Mendel was born to peasant parents in 1822. He was educated in a monastery and eventually became a monk. He was sent to the University of Vienna to study science and mathematics and had aspirations to become a teacher, but he failed to pass the certification exams. He returned to the monastery and eventually became an abbot. He was active in an informal science club. The club’s members were expected to perform some type of scientific investigation and report it back to the club. Mendel chose to try to repeat earlier experiments using pea plants, but he intended to keep better track of the results of the crosses. (Notebooks and detailed note taking are essential in science!) It was his training in science and mathematics that influenced his empirical (mathematical) approach to inheritance.
Mendel prepared an experimental design and set out to determine how these traits were passed from one generation to the next. By raising thousands of pea plants over many generations, strictly controlling how pollen moved between plants (by doing it himself), and keeping amazingly meticulous notes, Mendel was able to propose a theory of heredity. He did all this with knowing less about DNA than you do. Mendel presented his paper, Versuche über Pflanzenhybriden (Experiments on Plant Hybridization), at two meetings in 1865 and received some notice. When Mendel's paper was published in 1866, it was seen as essentially about hybridization rather than inheritance and had little impact and was cited about three times over the next thirty-five years. It was not until it was rediscovered and corroborated by experimentation in the 1900’s that it was seen as a critical tome on genetics, which today is one of the most important and famous theories in the history of science.
Mendel’s theory is composed of five hypotheses:
1. Parents do not transmit traits directly to their offspring. Instead, Mendel said, “factors” (we now call them genes) are transmitted to the offspring which allow for the expression of these traits.
2. Alternative forms of a given factor lead to alternative traits. Alternate forms of a gene are called alleles. An individual’s appearance, which is due to the combination of its alleles, is called its phenotype (physical appearance) and we call the actual combination of alleles (at the DNA level) its genotype (AA, Aa, aa).
3. In diploid species, each parent contains two copies (alleles) of the factor (gene) governing each trait. The copies may or may not be the same. If they are the same, the individual is said to be homozygous (same, e.g. AA or aa). If the copies are different, then the individual is said to be heterozygous (different, e.g. Aa).
4. The presence of an allele does not ensure that a trait will be expressed in the individual that carries it. Alleles can be dominant (upper case letter, e.g., A) or recessive (lower case letter, e.g, a). Dominant alleles will usually mask the phenotype coded by recessive alleles, but the recessive alleles will still be present and may still be passed to the offspring.
5. The two alleles that an individual possesses at each gene do not affect each other’s inheritance.
How did he test these hypotheses? He measured several traits of thousands of pea plants, and then he made crosses. That means he mated plants of certain traits in controlled combinations. Whenever he mated purebred