1st ESO

3rd ESO

4th ESO

Biology 2nd Baccalaureate

Index by courses



Skip navigation

3.3.1. Exceptions to Mendel's Laws


The mutations are changes that occur in DNA that make them appear new phenotypes and therefore not predictable.

Radiografía de una mano con seis dedos

By en:User:Drgnu23, subsequently altered by en:user:Grendelkhan, en:user: Raul654, and en:user:Solipsist (Original work by author) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

Lethal genes

Other times lethal genes appear, causing the death of individuals before they are born or in the first stages of life. This also modifies the expected phenotypic and genotypic ratios.

Multiple allelism

Mendel only worked with characters in which there were only two possible alleles, such as the yellow (A) or green color of the seed (a). But in some cases, there are more than two possible alleles or alternatives for that gene. For example, AB0 blood groups, in which there are three possible alleles for that gene: IA IB I0 . IA and IB are codominant and both dominate over I0. By combining these alleles in the pair of chromosomes in which they are located, 6 different genotypes and four phenotypes can be obtained, corresponding to blood groups A, B , AB and 0. In this topic, later, we will talk about the inheritance of blood groups AB0.

Genetics problems Solved problems of multiple alleles.

Quantitative inheritance

In other cases, the manifested character does not depend solely on a pair of genes, but rather several pairs of alleles contribute, in a summative way, to the appearance of the phenotype. Due to the summative nature of these alleles, many phenotypes appear with small differences between them, being able to be ordered progressively. An example of this type of inheritance would be that of hair color, skin, or height in people.

Exception to Mendel's 3rd Law: Linked Genes

As seen in Mendel's third Law, genes are passed on to offspring independently of each other. But there are genes that are located on the same chromosome, and these tend to be passed together and not independently as Mendel proposed. These characters are said to be determined by linked genes.


Legal warning






Follow us if it has been useful to you

Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.