4 - There Are No “Genes For” Characteristics or Disease

Summary

If you were taught Mendelian genetics at school (see Figures 2.1 and 2.2) you should be aware that it is an oversimplified model that does not work for most cases of inherited characteristics. Human eye color is a textbook example of a monogenic characteristic. It refers to the color of the iris – the colored circle in the middle of the eye. The iris comprises two tissue layers, an inner one called the iris pigment epithelium and an outer one called the anterior iridial stroma. It is the density and cellular composition of the latter that mostly affects the color of the iris. The melanocyte cells of the anterior iridial stroma store melanin in organelles called melanosomes. White light entering the iris can absorb or reflect a spectrum of wavelengths, giving rise to the three common iris colors (blue, green–hazel, and brown) and their variations. Blue eyes contain minimal pigment levels and melanosome numbers; green–hazel eyes have moderate pigment levels and melanosome numbers; and brown eyes are the result of high melanin levels and melanosome numbers. Textbook accounts often explain that a dominant allele B is responsible for brown color, whereas a recessive allele b is responsible for blue color (Figure 4.1). According to such accounts, parents with brown eyes can have children with blue eyes, but it is not possible for parents with blue eyes to have children with brown eyes. This pattern of inheritance was first described at the beginning of the twentieth century and it is still taught in schools, although it became almost immediately evident that there were exceptions, such as that two parents with blue eyes could have offspring with brown or dark hazel eyes.