Children resemble their parents, yet no two siblings, apart from identical twins, are genetically the same. Explaining both facts, the likeness and the variety, is the work of genetics, the science of heredity.
Mendel's peas
Before the twentieth century, many people assumed that a child's traits were a blend of the parents', like mixing paint. Gregor Mendel (1822 to 1884), an Augustinian friar in the town of Brno, in what is now the Czech Republic, showed otherwise. Over about eight years he bred thousands of pea plants and counted their traits carefully.
When he crossed purple-flowered plants with white-flowered ones, all the offspring had purple flowers. White seemed to vanish. But when he bred those offspring with each other, white came back: in the second generation, purple and white appeared in a ratio of roughly 3 to 1. Mendel concluded that each trait is controlled by a pair of hereditary factors, one from each parent, that do not blend. One factor can be dominant, hiding the effect of a recessive one, which can reappear in later generations.
Mendel published in 1866, but almost no one noticed. His work was rediscovered in 1900, and his factors became what we now call genes.
Some useful words
- A gene is a stretch of DNA that carries instructions, usually for making a protein.
- An allele is one version of a gene. The flower-color gene in peas has a purple allele and a white allele.
- Your genotype is the set of alleles you carry; your phenotype is the trait that can actually be observed. A pea plant with one purple allele and one white allele has a mixed genotype but a purple phenotype.
The double helix
By the 1940s experiments had shown that genes are made of DNA (deoxyribonucleic acid). Its structure was solved in 1953 by James Watson and Francis Crick in Cambridge. Their model depended heavily on X-ray images of DNA made by Rosalind Franklin at King's College London, including the famous “Photo 51,” which her colleague Maurice Wilkins showed Watson without her knowledge. Franklin died of cancer in 1958, aged 37. Watson, Crick and Wilkins shared the Nobel prize in 1962, which is never awarded after death, and her contribution was long underrated.
DNA is shaped like a twisted ladder, the double helix. The rungs are pairs of four chemical bases: adenine (A), thymine (T), guanine (G) and cytosine (C). A always pairs with T, and G with C. Because each side of the ladder fits the other exactly, a cell can copy DNA by unzipping it and building a new partner for each strand.
From gene to protein
Crick summarized how genetic information flows in what he called the central dogma: DNA → RNA → protein. A gene's DNA is copied into a messenger molecule of RNA, which travels to a ribosome. The ribosome reads the message three bases at a time; each three-letter “word,” called a codon, specifies one building block of a protein. Proteins then do most of the work in the body, from digesting food to building muscle.
The Human Genome Project, completed in 2003, read nearly all of our roughly 3 billion base pairs. It found only about 20,000 genes that code for proteins, far fewer than many scientists had expected.
Mutations
A mutation is a change in the DNA sequence. Mutations arise from copying errors, radiation and certain chemicals. Most have little or no effect; some are harmful; a few turn out to be useful. Mutations are the ultimate source of new genetic variety.
Sickle cell disease shows how complicated their effects can be. It is caused by a change in a single base in a gene for hemoglobin, the protein that carries oxygen in red blood cells. People with two copies of the sickle allele have the disease. People with one copy, called carriers, are usually healthy and are partly protected against malaria, which is why the allele is common where malaria has long been widespread.
Genes are not destiny
Mendel chose pea traits controlled by single genes. Most human traits, such as height or skin color, are not like that: they are shaped by many genes acting together and by the environment. Height depends on hundreds of genes and also on childhood nutrition. Nature and nurture are not rivals; they work together.
Worked example: two carriers start a family
Ama and Kwame are both healthy carriers of the sickle cell allele. Call the usual allele A and the sickle allele S. Each of them has the genotype AS. What are the chances for each child?
Each parent passes on one allele, chosen by chance. A simple grid, called a Punnett square after the geneticist Reginald Punnett, lists the four equally likely combinations:
- AA: no sickle allele at all. Chance: 1 in 4.
- AS: one allele of each kind, a carrier like the parents. This can happen two ways, A from the mother and S from the father or the reverse. Chance: 2 in 4, or 1 in 2.
- SS: the child has sickle cell disease. Chance: 1 in 4.
This is Mendel's 3 to 1 pattern again. Three of the four combinations produce children without the disease, and one produces a child with it. Two points are worth noticing. First, the odds apply to each pregnancy separately: having one child with the disease does not make the next one safe. Second, genotype and phenotype differ: a carrier child has the S allele but not the disease. Genetic counselors explain exactly this kind of calculation to families, so they can make informed choices of their own.