7.1–7.5: Why do offspring resemble their parents?

Upon maturation, the baby Emperor Penguin will closely resemble its parents.

Can a gene be cruel? Of course not. But if one could, consider this candidate: in humans, there is a gene for an enzyme called FMO3 (flavin-containing mono-oxygenase-3), which breaks down a chemical in our bodies that smells like rotting fish. Some unfortunate individuals inherit a defective FMO3 gene and can’t break down the noxious chemical. Instead, their urine, sweat, and breath excrete it, causing them to smell like rotting fish. Worst of all, because the odor comes from within their bodies, they cannot wash it away no matter how hard they try. Called “fish odor syndrome,” this disorder often causes those afflicted by it to suffer ridicule, social isolation, and depression.

About 200 cases of fish odor syndrome have been reported, but researchers suspect that it may be under-reported because many people with the symptoms do not seek help. Currently, there is no cure for the disorder, although there are a few ways of reducing the odor such as reducing consumption of eggs, fish, and some meats that contain certain chemicals, including sulfur.

For individuals born with this malady, beyond their own suffering there looms a scary question: “Will I pass this condition on to my children?” They might also wonder how they came to have the disorder, particularly if neither of their parents suffered from it.

Fortunately, most of us do not have to worry about fish odor syndrome, but we do wonder about many other inherited traits that we may or may not pass down to our children. Where do we begin and our parents end (FIGURE 7-1)?

Figure 7.1: Where do we begin and our parents end? Family resemblances reveal that many traits are inherited.

In many cases, the answers to questions about heredity are simple. Recall from Chapter 6 that sexually reproducing organisms inherit one copy of each chromosome from each parent so that they carry two copies of every chromosome in every cell (except their sex cells). Humans, for example, have 23 pairs of chromosomes (46 individual chromosomes). At each location—referred to as a locus (pl. loci)—on the two chromosomes of a pair is the same gene: one copy from the mother and one from the father (FIGURE 7-2). As we noted in Chapter 5, each of the two copies of the gene is called an allele.

Figure 7.2: How we inherit our genes. Each human offspring inherits one maternal set of 23 chromosomes and one paternal set.

The gene for FMO3 is on chromosome 1. There is a normal version—or allele—of the gene for FMO3, which most people carry, and there is a rare, defective version that is responsible for fish odor syndrome. As long as a person has at least one normal version of the FMO3 gene, he or she will produce enough of the enzyme to break down the fishy chemical. But if a person inherits two copies of the defective version of the fish odor gene, one from each parent, that person will inherit the disorder (FIGURE 7-3).

Figure 7.3: Unlucky catch. A baby inheriting two copies of the defective FMO3 gene will develop fish odor syndrome.

When it comes to having children, there is a bright side, at least. Although individuals with fish odor syndrome carry two defective copies of the allele and will pass on one set of the bad instructions in every sperm or egg cell that they make, their children won’t necessarily inherit the disease. As long as the other parent supplies a normal version of the FMO3 gene, the child will not have fish odor syndrome. The unaffected children, though, will carry a silent copy of the fish odor allele, and if, when they have children, it comes together with a defective FMO3 allele from another person, the fish odor trait will be expressed. In this way, some alleles can exist in a population without always revealing themselves.

In this chapter, we explore how heredity works. Inheritance follows some simple rules that allow us to make sense of patterns of family resemblance such as facial features or hair texture and even to predict the likelihood that an offspring will inherit a trait. We’ll also examine why the behavior of some traits is easy to predict while many other traits have less straightforward patterns of inheritance, yet still can be studied experimentally and yield predictable patterns.