Mendelian inheritance

Mendelian inheritance is the set of laws, originally recognised by Gregor Mendel, according to which traits are inherited from generation to generation. It was later consolidated by Thomas Morgan Hunt's chromosome theory of inheritance to give a solid grounding for classical genetics.

According to Mendel there are two principle laws governing inheritance:

1. The law of segregation: members of a gene pair (i.e. alleles) segregate from one another in the formation of gametes. This was later confirmed by the discovery of meiosis: during meiosis I, homologous pairs of chromosomes (maternal and paternal) are separated into gametes.

2. The law of independent assortment: members of different gene pairs are transmitted independently of one another in the formation of gametes: so the segregation of one gene pair should bear no impact on the segregation of another. As a simple example, the inheritance of eye colour should be independent of the inheritance of height. Mendel discovered that in the inheritance of a single trait, the offspring segregate with a 3:1 dominant : recessive ratio, and in two-trait inheritance there is segregation with a 9:3:3:1 ratio (equivalent to each gene segregating with a 3:1 ratio).

This phenomenon has since been realised to be more complex. Genes can be physically linked on chromosomes, causing co-segregation of alleles and deviation from the expected Mendelian ratios. Nevertheless, entire chromosomes are independently inherited; during metaphase I of meiosis, homologous pairs orient randomly along the metaphase plate and gametes may acquire any combination of maternal and paternal chromosomes.

Interestingly, Mendel observed independent assortment in 7 traits in pea plants; pea plants in fact carry 7 chromosomes.

A Mendelian trait is one that is controlled by a single locus and shows a simple Mendelian inheritance pattern. In such cases, a mutation in a single gene can cause a disease that is inherited according to Mendel's laws. Examples include sickle-cell anemia, Tay-Sachs disease, cystic fibrosis and xeroderma pigmentosa. This can be contrasted with the many loci that are subject to non-Mendelian inheritance.