Mendel’s Genetics[2]: The monohybrid cross


  • Phenotype

Any character (trait) which can be shown to be inherited, such as eye color, leaf shape or an inherited disease, such a cystic fibrosis, is referred to as a phenotype.

Description: A fly may be described as having a red-eyed phenotype. A child may be described as displaying the cystic fibrosis phenotype.

  •  Genotype

The pattern of genes that are responsible for a particular phenotype in a individual is referred to as genotype.

  • Dominance

In hybrids between two individuals displaying different phenotypes, only one phenotype may be observed. This phenotype is referred to as the dominant trait and the un-shown one the recessive.

For instance, if the wife has wide eyes while the husband has small eyes, and their little girl has wide eyes, then the wide eyes are dominant to small eyes.

  •  Pure-breeding lines

Organisms which have been inbred for many generations in which a certain phenotype remain the same.Pedigree breeds of dogs or cats are commonplace examples of pure-breeding lines.

A puppy from two purebred dogs of the same breed, for example, will exhibit the traits of its parents, and not the traits of all breeds in the subject breed’s ancestry.

  • Homozygous: Individuals with two identical copies of a gene.

“True breeding (pure-breedind) organisms are always homozygous for the traits that are to be held constant.”

  • Heterozygous: Individuals with two different copies of the gene.
  • Alleles: The different variants of a gene.


Mendel made a cross between two pure-breeding lines of pea plants, one of which had violet petals and the other white petals. The hybrids produced in this cross were referred to as the F1 (first filial) generation.

In Mendel’s experiment, the ratio of violet pedals and white ones in the second filial were very close to 3 to 1, which applied to the theoretic reasoning shown above.

He did many other experiments focusing on different types of genotypes of the pea plants and the results were shockingly similar. The hidden phenotype in the first filial reappeared in the second filial and the ratio of the dominant to the recessive phenotype were all close to 3 to 1.

The 3:1 ratio is referred to as the monohybrid ratio and is the basis for all patterns of inheritance in higher organisms.

One simple extension of the 3:1 phenotype ratio is a 1:1 ratio, produced when a heterozygous F1 individual is crossed to the homozygous-recessive parent. The process is known as testcross.

Testcross is useful in any condition when it is necessary to determine whether an individual is heterozygous or homozygous. Conceivable that if F2 all have dominant phenotype, then the tested parent is homozygous-dominant; if F2 have a 1:1 ratio of dominant and recessive phenotype, then the tested parent is heterozygous.