Mendel's Pea Hybridization · PeaLab

Why peas?

From 1856 to 1863 Mendel grew peas in this very abbey garden for 8 years. The pea is self-pollinating and cleistogamous: pollen lands on its own stigma before the petals open, so in nature peas are usually pure-breeding. Peas also have many easily distinguished contrasting traits, and the flowers are large enough to emasculate and pollinate by hand.

The six rows hold six pure-breeding strains, each differing from the standard strain in stem height, flower colour, flower position, or pod shape and colour. Drag the timeline at the top right to watch them grow from sowing to flowering, podding and ripening.

Hand cross-pollination

To cross two strains, you must stop the pea from pollinating itself and bring pollen from another plant to its stigma:

Emasculate: at the bud stage (before the anthers ripen), part the petals and pluck out all 10 stamens with tweezers. Once the flower opens it has already self-pollinated, so it is too late.

Bag the flower to keep insects from bringing other pollen.

Wait about 1 day for the pistil to mature.

Pollinate by hand: pick up pollen from the father's ripe anthers with a brush and dab it on the mother's stigma.

Bag it again and hang a label recording the parents and the date.

Row Ⅰ is the round-seed strain (the mother, now in bud); row Ⅱ is the wrinkled-seed strain (the father, sown a week earlier and already flowering).

Crossing one pair of traits

Two crosses were made last season: row Ⅰ round ♀ × row Ⅱ wrinkled ♂, and row Ⅲ tall ♀ × row Ⅳ dwarf ♂.

Seed traits show up one generation before plant traits: the embryo inside a seed is already the next generation. So in the season of the cross, the seeds in the mother's pods are F₁ seeds, all round; the seeds set on F₁ plants are F₂, about 3 : 1 round to wrinkled. Stem height only shows once the seeds are sown and grow into plants, so it takes one more season.

Mendel's explanation: traits are determined by hereditary factors, which occur in pairs in body cells; when gametes form, the paired factors segregate into different gametes; at fertilization, male and female gametes combine at random.

Test cross: checking segregation

Mendel tested his hypothesis by deduction: if F₁ really carries two different factors (Rr) and makes equal numbers of R and r gametes, then crossing F₁ with a recessive homozygote (rr) should give round : wrinkled = 1 : 1.

Row Ⅰ holds F₁ plants (round); row Ⅱ is the wrinkled strain, sown a week earlier. Write down your prediction first, then do the experiment. Mendel's test cross gave 106 round and 101 wrinkled seeds.

Crossing two pairs of traits

Yellow round ♀ × green wrinkled ♂ (Ⅰ × Ⅱ, crossed last season). The F₁ are all yellow and round; when F₁ self, F₂ shows four phenotypes, yellow round, yellow wrinkled, green round and green wrinkled, in about 9 : 3 : 3 : 1. Green round and yellow wrinkled are new combinations not seen in either parent.

Each pair on its own is still 3 : 1. Mendel's explanation: when F₁ forms gametes, each pair of factors segregates and factors of different pairs assort independently, giving equal numbers of YR, Yr, yR and yr gametes. A test cross checks this: F₁ × green wrinkled should give 1 : 1 : 1 : 1.

Free exploration of seven traits

The garden is yours. Click “Sow” on the right of any row to plant a pure-breeding strain or seeds you have harvested, and drag the timeline to let them grow. Click a flower at the bud stage to pollinate it by hand; when the plants are ripe, click “Harvest” and sort and count on the workbench.

Try this: what colour is the F₁ seed coat from purple × white? Who decides pod shape and colour, the mother or the seed? Do two plant traits (say, stem height and flower position) also give 9 : 3 : 3 : 1 in F₂?

The hypothetico-deductive method

Looking back at how Mendel worked:

Observe and ask: F₁ shows only the dominant trait, F₂ shows the recessive one again in close to 3 : 1. Why?

Propose a hypothesis: traits are determined by hereditary factors; factors occur in pairs in body cells; when gametes form, the paired factors separate so each gamete carries one of each pair; at fertilization male and female gametes combine at random.

Deduce: if the hypothesis holds, F₁ × recessive homozygote → 1 : 1.

Test: the test cross gives 1 : 1.

Conclude: the law of segregation; the same reasoning for two pairs gives the law of independent assortment.

In 1909 Johannsen renamed “hereditary factors” genes. Today we know the R/r gene codes for a starch-branching enzyme: rr seeds hold less starch and more sucrose, and wrinkle as they dry.