12

key concepts

12.1

Inheritance of Genes Follows Mendelian Laws

12.2

Alleles Can Produce Multiple Phenotypes

12.3

Genes Can Interact to Produce a Phenotype

12.4

Genes Are Carried on Chromosomes

12.5

Some Eukaryotic Genes Are Outside the Nucleus

12.6

Prokaryotes Can Transmit Genes by Mating

What Are the Rules of Inheritance?

Red hair is the rarest natural hair color in humans, with a frequency of about 1 percent. It is most common in northern Europe, with Scotland leading at a frequency of about 15 percent. Throughout history people have attempted to find a correlation between red-headedness and other traits. Red hair and hot temper, for example, are often associated. A recent study by the Hamburg Research Institute in Germany on the sexual activity of hundreds of women came to the conclusion that redheads were having more sex than any other hair color group. Attempts have been made to prove that redheads have a lower tolerance to pain than do people with other hair color.

Given its rarity, it is not surprising that red hair was an early subject of investigation in genetics, the science of heredity. As you will see in this chapter, the foundations of modern genetics were laid late in the nineteenth century through careful experimentation on pea plants by Gregor Mendel. By the early twentieth century, biologists examining human families noticed that in most cases, red-haired children were born from (1) two red-haired parents, (2) one parent with dark hair and one with red, or (3) two dark-haired parents. This fit the pattern for recessive inheritance, in which the gene that determines red hair can be masked by a dominant gene for dark hair. Exceptions were noted, however, and how red hair is inherited was uncertain until the late 1950s, when Brenda Ellis took a genetics course from Professor Ralph Singleton at Wake Forest University in North Carolina.

After hearing her professor discuss theories about the inheritance of red hair, Ellis investigated her family tree dating back six generations, noting which relatives had red hair. She surmised that all the redheads in her family were the offspring of either two dark-haired parents or one with dark hair and one with red hair. Each of the dark-haired people had a red-haired ancestor—presumably a carrier of a gene that determines red hair. The genetic explanation that Ellis and her professor gave for the striking pattern of recessive inheritance of red hair in her family has since become widely accepted.

Sometimes, however, red hair pops up in families with no genetic history of red hair. In this chapter we’ll examine various patterns of inheritance and the genetic mechanisms by which they arise.

How do inherited characteristics such as red hair skip generations?