How do inherited characteristics such as red hair skip generations?

Hair color is determined by the pigment melanin, which is synthesized from the amino acid tyrosine (see Table 3.2). There are two forms of this pigment, eumelanin (black or brown, depending on how much accumulates) and phaeomelanin (red or blond). Pigment production is an excellent example of intercellular signaling at work (see Chapter 7). Skin cells called melanocytes have a receptor, MC1R (melanocortin 1 receptor), for the signal melanocortin. Receptor binding sets off a signal transduction pathway that activates the cells to make much more eumelanin than phaeomelanin.

As is common with recessive traits, red hair results from a mutation in a gene, in this case a mutation in the MC1R gene, located on chromosome 16. As a recessive trait it must be inherited from both parents to cause the hair to become red. The MC1R gene has multiple alleles: the wild type (R), which produces a normal receptor, and three recessive (r, mutated) alleles, each of which leads to red hair.

So what happens when a person inherits two of these recessive alleles? MC1R is not expressed, so it does not bind the signal that would activate the eumelanin pathway, and phaeomelanin is made instead. If enough phaeomelanin accumulates, red hair results. The type of red hair that is inherited—fair, strawberry blonde, or auburn—depends on which recessive allele is inherited.

Future directions

As biologists learn more about individual genes and their alleles, they are beginning to make predictions about how gene products interact with one another and with factors in the environment. You probably know that smoking can cause lung cancer, yet there are some people who smoke a lot and don’t get the disease. It turns out that these people often carry an allele that alleviates their encounter with the harmful substances in cigarette smoke. We all have genes whose products change environmental substances. In the case of cigarette smoke, the gene product of the typical allele turns the molecules in smoke into DNA-damaging, cancer-causing toxins. People who carry a mutant form of this gene are relatively resistant to getting lung cancer from smoking because the product of this mutant gene does not carry out the activation reactions. The genomes of all organisms carry such genes that are involved in environmental interaction. Describing these genes holds promise in understanding the relative contributions of environment and genetics to establishing a phenotype.