Variation and Mendel's experiments

Look around your class. Are your earlobes free (hanging below the point where they join the head) or attached? Count how many classmates have each kind. Both kinds run in families. Traits like these are inherited: passed from parents to children.

Not every trait is inherited. A tattoo, a scar, or muscles built at the gym are acquired during life. Your children won't be born with them.

Each generation inherits its parents' differences and adds a few new ones from small copying changes. Sexual reproduction mixes the DNA of two parents, so it makes far more variety than asexual reproduction. Not all variations survive equally well: the surroundings favour some of them.

In sexual reproduction, both parents give practically equal amounts of genetic material. So for each trait, a child has two versions of the instruction: one from each parent.

Mendel's experiment. Gregor Mendel crossed tall pea plants with short ones.

• First generation (F1): all the plants were tall. There were no medium-height plants.
• He let the F1 plants self-pollinate. Second generation (F2): most were tall, but about one quarter were short.

Where did the short plants come from, if none of the F1 plants was short?

Mendel's answer: each plant has two factors (today we say genes) for height. Write tall as T and short as t:

A trait that shows with one copy is dominant (T). One that shows only with two copies is recessive (t). The F1 plants were all Tt: tall, but carrying a hidden t.

When two Tt plants are crossed, each parent passes on either T or t, and which one meets which is a matter of chance. So we list every possible meeting:

Three tall plants to one short (3 : 1), and the genes come out as TT : Tt : tt = 1 : 2 : 1.

Read questions carefully: "how many are tall in F1?" (all of them) is a different question from "in F2?" (three quarters). And a question about the genes (TT : Tt) is not the same as one about how the plants look (tall : short).