Mendel's monohybrid cross

A monk and his peas

More than a hundred and fifty years ago, a monk named Gregor Johann Mendel grew peas in his monastery garden. He wanted to know the rules by which traits pass from parents to young. His experiments laid the base of the science of heredity, genetics.

He chose the garden pea, Pisum sativum, for good reasons:

• it grows quickly and makes many seeds, so there are many plants to count;
• it shows clear contrasting traits, such as tall or short, with nothing in between;
• it normally pollinates itself, but can also be crossed by hand.

He studied seven pairs of contrasting traits:

Mendel also did something new for his time. He counted the plants of each kind, and looked for patterns in the numbers.

The monohybrid cross

A cross that follows one pair of traits is a monohybrid cross. Mendel crossed a pure-breeding tall plant with a pure-breeding short plant. Pure-breeding means it gives only the same kind when it pollinates itself.

1. The parents. Tall crossed with short.
2. The first generation, F1. Every plant was tall. None was of middle height, and none was short.
3. The second generation, F2. He let the F1 plants pollinate themselves. Now both kinds came back: about three tall plants for every short one.

In one experiment he counted $787$ tall plants and $277$ short ones. That is close to $3 : 1$.

So the trait for shortness had not vanished in the F1. It was there, hidden, and showed up again in the F2.

Mendel's explanation

Mendel explained his results with a few simple ideas. We now call his "factors" genes.

• Each trait is controlled by a pair of gene copies in every plant. One copy comes from each parent.
• A copy can come in different forms. Here, T stands for tall and t for short.
• When both forms are present, one shows and hides the other. The one that shows is dominant. The hidden one is recessive. Tall is dominant over short.
• When gametes form, the two copies separate, so each gamete carries just one copy.

Some words you will need:

• Genotype: the pair of copies a plant carries, such as TT, Tt or tt.
• Phenotype: what the plant looks like, such as tall or short.
• Homozygous: both copies the same, TT or tt. Such a plant is pure-breeding.
• Heterozygous: two different copies, Tt.

A Tt plant is tall, just like a TT plant. Their phenotypes are the same, but their genotypes differ.

The Punnett square

A Punnett square shows every way the gametes can meet. Write the gametes of one parent along the top, and those of the other down the side. Fill each box with the copies from its row and column.

For the F1 cross Tt with Tt, each parent makes T and t gametes in equal numbers. The square gives $1$ TT, $2$ Tt and $1$ tt. Three of the four boxes hold a T, so they are tall. One is tt and short. That is the $3 : 1$ ratio.

Pick the genotypes of two parents below and see what share of the young comes out tall.

Try it in Python

The program crosses two Tt plants. Each gamete gets one copy, so it pairs every gamete of one parent with every gamete of the other.

Practise with new numbers

Common mistakes

• "The F1 plants are of middle height." They are all tall, because tall is dominant.
• "The recessive trait is lost in the F1." It is hidden, not lost. It comes back in the F2.
• "A tall plant is always TT." A tall plant may be TT or Tt. Only a short plant has a known genotype, tt.
• "A gamete carries both copies." The two copies separate, so each gamete carries just one.
• "3 : 1 means exactly 3 tall for each short." It is the expected share. Real counts come close but rarely exact.

Remember these

• Mendel used garden peas with seven pairs of contrasting traits, and counted the offspring.
• Pure tall crossed with pure short gives an F1 all tall. The F1 selfed gives an F2 of $3$ tall to $1$ short.
• Each trait has a pair of gene copies, one from each parent. The dominant copy hides the recessive one.
• The F2 genotypes are $1$ TT : $2$ Tt : $1$ tt.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Pure violet-flowered peas are crossed with pure white-flowered peas. All the F1 plants have violet flowers. Which colour is dominant?

2. What are the genotypes of a pure tall plant, a pure short plant, and their F1 offspring?

3. Two plants are both tall. Can they have different genotypes? Explain.

4. Why did the short trait reappear in the F2, though no F1 plant was short?

5. Out of every four F2 plants of Tt crossed with Tt, how many do you expect to be pure-breeding?