“She’s got her father’s eyes.” “He’s got his mother’s nose.” From the moment a baby is born, expect children to look like their parents. But how does it happen?

An Austrian monk named Gregor Mendel took the first step toward our modern understanding of heredity in 1866, when he published a theory of inheritance based on his experiments with pea plants.

Mendel studied seven characteristics. For example, he crossed a tall strain of pea plants with a short strain. The first generation of crossbred plants were all tall. When he bred those plants together, though, three quarters of the offspring were tall and one quarter were short.

From this, Mendel deduced that there must be two hereditary factors, now called genes, at work for each inherited characteristic, one of which is “dominant,” and one of which is “recessive.” Each parent plant contained a pair of these genes. During reproduction, these two genes were separated, so that each parent plant contributed only one gene to the offspring.

In the first generation, all of the offspring had one tall (dominant) gene and one short (recessive) gene, and therefore grew tall. But in the next generation, it became possible for a plant to have either two tall genes, two short genes, or one of each. Since tallness was dominant, a plant with either two tall genes or one of each would grow tall, but a plant with two short genes would stay short. That made tallness three times more likely than shortness.

We now know that genes are made of lengths of deoxyribonucleic acid, or DNA, a molecule that is able to make perfect copies of itself. It’s structured like a twisted ladder, with the rungs made up of pairs of organic compounds called bases. The order these “base pairs” serves as a blueprint for the manufacturing of proteins, the basic building blocks of everything in our bodies, from bones to brain cells.

Genes form long, continuous strings called chromosomes. All chromosomes, and therefore all genes, occur in pairs–22 pairs in each human cell, plus another pair that determines gender.

When a cell divides, each daughter cell inherits a complete set of chromosomes. But when sperm and egg cells are formed, they inherit only half the chromosomes. When sperm and egg unite, each therefore contributes one half of one parent’s genetic material. Which half is pretty much a lottery. Mendel’s laws are statistical, not particular: it’s perfectly possible for two brown-eyed (a dominant trait) parents to have four blue-eyed (a recessive trait) children, even though statistically three of them should have brown eyes.

The sex chromosomes come in two types: X and Y. Women have two X chromosomes; men have an X and a Y. Men produce an equal number of X-carrying and Y-carrying sperm, which gives the offspring a 50/50 chance of being a boy or a girl.

The X chromosome in humans also carries genes that have nothing to do with sex. One example of such a “sex-linked” trait is red-green color blindness, which is far more common in men than in women.

Some inherited characteristics arise from the interplay of many different genes, or “polygenes.” Height, weight and skin color are examples. As well, these characteristics are influenced by the environment: nutrition, eating habits, time spent in the sun.

Inherited diseases are usually recessive genes. All humans carry some of these genes, and if a man and a woman with genes for the same disease located in the same place on their respective chromosomes have a child, that child runs a high risk of inheriting the disease.

Most of these bad genes are the result of mutation, a change in genes that can be caused by exposure to radiation or chemical agents, or sometimes just by a an accidental change in the chemistry of a cell. Mutation is the driving force behind evolution–it’s the only way new characteristics can appear. But most mutations are harmful: mutations of sex cells can result in deformed offspring; mutations in non-sex cells are a leading cause of cancer.

Fortunately for us, DNA is accurate in its reproduction of itself 999,999,999 times out of a billion, and therefore accurate in its task of overseeing the construction of our bodies, using blueprints traced from the blueprints used to build the bodies of thousands upon thousands of our ancestors.

In the case of living creatures, the whole is more than the sum of its parts. Thanks to the intricate mechanism of heredity, the whole is the sum of some of the parts of its parents and some of the parts of their their parents and…

So if you don’t like some of (or the sum of) your parts, take it up with your ancestors.

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