Topic 1: Mendelian Genetics

The Law of Segregation

His second innovation was in counting results and keeping statistical notes. For example, in a particular experiment he used terms such as parental cross, F1 phenotype, F2 phenotypic ratio, and the F2 ratio. A literal translation of phenotype would be "the form that is shown". It is the physical appearance of a particular trait that we can see with our eyes.

In Mendel's pea plant experiment, some phenotypes that were exhibited were : round or wrinkled, yellow or green seed, red or white flower, and tall or dwarf plant. Mendel placed the phenotypes on his charts and compared the results to find answers in his study of pea plants and several other experiments.

He used terms such as the F1 and F2 generation in his pea plant experiment to create the law of segregation. In the F1 generation, we always see only one of the two parental phenotypes. But both parental phenotypes can still be created in the future generation. It is necessary to know that in the F2 generation a 3:1 ratio is always formed where the dominant trait is 3 times more likely to occur than the recessive trait.

As a result of the F1 and F2 phenotypes, Mendel coined 2 new terms: dominant and recessive. The dominant allele is the phenotype that is expressed in the F1 generation from the cross of two pure lines. It will express itself, even at the expense of an alternate allele. The recessive allele's expression is suppressed by the presence of a dominant allele. The phenotype that disappears in the F1 generation and reappears in the F2 generation is the recessive allele.

Mendel developed many conclusions from the pea plant experiment for the segregation law. His first conclusion was that hereditary determinants have a certain nature and these determinants are what we know as genes. Secondly, each parent has a gene pair in each cell of the parent's body. The dominant and recessive allele form the gene pair. His third conclusion was that one member of the gene pair segregates into a gamete, and thus each gamete has one member of the gene pair. His final conclusion was that gametes will always unite at random without taking any heed of the other gene pairs involved.

To review, an allele is one form of a certain allelic pair. A plant for example can be tall or short, and have tallness as its dominant allele. A body that contains only one allele in the allelic pair such as BB is termed homozygous dominant, while bb is homozygous recessive. Pure lines are homozygous for a particular gene. An individual that contains one of each kind of allele such as Bb is called a heterozygote. Apart from the phenotype, another point of interest for Mendel was the genotype--the specific allelic combination for a gene(s). A logical diagram called the Punnet Square was used by Mendel to find genotypic and phenotypic ratios between two individuals that were crossed and their offspring.

Thus, Mendel's first law showed that during gamete formation, each member of the allelic pair left the other to form the genetic structure of a gamete. How did he create this law? In order to confirm his hypothesis, he performed a backcross, which is a cross of the F1 hybrid to any one of the homozygous parent bodies. For pea plant height, the cross was DDxDd or Ddx dd.

He also performed a testcross - the cross of any individual to a homozygous recessive parent; which determines whether the individual body is homozygous dominant or just heterozygous. Most importantly, the Monohybrid cross - a cross between parents that differ at a single gene pair (AA x aa) -was necessary for the formation of the first law.

A Monohybrid is the offspring of two parents that are homozygous for differing alleles of an allelic pair. It is important to know that a monohybrid cross is not the cross of two monohybrids. Monohybrids are good for describing the relationship between alleles. When an allele is homozygous it will show its phenotype. It is the phenotype of the heterozygote that permits us to determine the relationship of the alleles. Generally, the dominant allele will make a gene product that the recessive is incapable of making; therefore the dominant allele will express itself as a phenotype whenever it is present.