Previous tutorials, e.g. Dominance and Crossing Over, have described how genetic information is passed along from parents to offspring. Mendel summarised this in his first law, the principle of segregation

Mendel’s First Law

“The alleles of a gene exist in pairs but when gametes are formed, the members of each pair pass into different gametes. Thus each gamete contains only one allele of each gene.”

Incomplete Dominance

When a particular gene possesses both dominant and recessive alleles, it is possible for incomplete dominance to occur, where the organism at hand expresses a phenotype morphed by the expression of both the dominant and recessive alleles.

In essence, heterozygous (possessing opposing alleles Rr) organisms derived from homozygous (possessing the same alleles RR or rr) are created, they possess a phenotype different to that of both their parents.

Some of the following examples of monohybrid and dihybrid crossing illustrate this incomplete dominance.

Multiple Alleles

Diploid organisms naturally have a maximum of 2 alleles for each gene expressing a particular characteristic, one deriving from each parent. In some cases, however, more than two types of alleles can code for a particular characteristic, as is the case of genetic coding for blood type in humans. There are up to 6 possible genotypes that code for the four blood groups, A, B, AB, and O.

Example of a Cross

The following dihybrid cross involves two true-breeding pea plants, where two factors are looked at, the shape of the seed and the color of the seed.

More examples of dihybrid cross here.

Summary of Mendelian Genetics

The past few pages have elaborated on the work of Gregor Mendel and how his work has paved the way for predicting the characteristics of offspring. However, a degree of randomness is involved, when involving factors such as independent assortment during meiosis and the possibility of genetic mutations (explained in further pages).

In light of this, Mendel’s work allowed us to see that there is a degree of genetic inheritance from parents in offspring though modern biology indicates that more factors come into play to determine the final genotype and phenotype of an organism.

Sticking to the subject of genetics, the next tutorial looks at sex determination via chromosomes X and Y and some of the genetic traits inherited via these two chromosomes.

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