The laws of genetic inheritance were first laid down in the 1860s by an Austrian Monk called Gregor Mendel. He noticed that if pea plants producing purple flowers were pollinated by white flowered pea plants, the resulting seeds all produced plants with purple coloured flowers. However, crossing these F1 offspring with each other produced an F2 population of plants in which three-quarters produced purple flowers and one-quarter produced white flowers, i.e. a 3:1 ratio.
How does this inheritance of flower colour in pea plants relate to us? Sexual reproduction in humans occurs with the fusing of two gametes (the oocyte and sperm) each with one half of the pairs of 23 chromosomes. The resulting zygote therefore inherits one copy of a chromosome, with the respective genes, from each parent. When an identical mutated gene or allele is inherited from each of the two parents, the individual is referred to as a homozygote as they have two copies of the same gene; a heterozygote refers to the presence of only one copy of a particular allele. A Mendelian trait or disease is one that is controlled by a single gene mutation and shows a simple Mendelian inheritance pattern – either dominant or recessive. When an organism has two different variants (alleles) for a trait, the allele that is expressed, overshadowing the expression of the other allele, is said to be dominant (i.e. the purple flower – denoted by P in the diagram). The gene whose expression is overshadowed is said to be recessive (i.e. the white flower – denoted by p in the diagram).
Other patterns of inheritance occur if a gene is on the X chromosome (sex-linked) or mitochondrial DNA (mitochondrial inheritance). Furthermore, there are many disorders that involve a number of different genes (referred to as polygenic) or are influenced by other genes in combination with the environment, which are termed multifactorial and inherited in a non-mendelian fashion.