Genes and alleles.

Step 1 of 4

Two copies

In $1856$⁠ Gregor Mendel, a monk in the town of Brno, began growing peas in the monastery garden. Over eight years he crossed thousands of plants and counted their offspring, one trait at a time: flower colour, seed shape, plant height. From those counts he worked out how inheritance works, decades before anyone knew what a gene was.

Today we know that every plant, like every person, has two copies of each gene, one from each parent, on the two chromosomes of a homologous pair. A gene can come in different versions, alleles. The pea’s gene for flower colour has an allele for purple, written P, and an allele for white, written p.

A plant’s two alleles are its genotype, and its flower colour, what you see, is its phenotype. A plant with two identical copies, PP or pp, is homozygous, and a Pp plant is heterozygous.

Step 2 of 4

Pure-breeding

Mendel started from pure-breeding varieties: plants that, crossed among themselves generation after generation, give flowers of only one colour. A pure purple variety is PP, and a pure white one pp.

Here one parent is PP and the other pp. Cross the parents is fixed until you answer.

Step 3 of 4

White comes back

A dominant allele shows even when there is only one copy of it. A recessive allele shows only when no dominant copy is beside it, that is only in pp. A Pp plant looks exactly like PP, but it is a carrier of the white allele.

Mendel crossed the plants of his first generation, all purple, with each other. Cross the parents is fixed until you choose an answer, and will then open at Punnett square.

Step 4 of 4

Dominant does not mean common

The square on the board shows the four ways the parents’ alleles can meet. In the next Lesson you will build squares like it yourself.

Before that, a common slip of words: “dominant” sounds like “strong” or “common”, but it means neither.