White comes back

About this video

Sometimes a child has a trait that neither parent shows, like a great-grandfather's red hair. Where was it hiding? This video follows the man who first explained it with careful counting.

From 1856, Gregor Mendel, a monk at the Abbey of St Thomas in Brno, spent about ten years crossing pea plants in the abbey garden: nearly 30,000 plants, following seven traits that each had two clear forms, such as violet or white flowers.

He crossed pure violet-flowered plants with pure white-flowered ones, and every one of their young had violet flowers. When he let those young pollinate themselves, white came back: he counted 705 violet plants and 224 white ones, about 3 to 1.

His explanation: each plant carries two versions of the instruction for a trait, one from each parent, and passes on just one to each young plant. Today we call them alleles, versions of a gene. Violet is dominant: it shows even with one copy. White is recessive: hidden when a violet version is there, but still passed on. A white flower appears only when a plant gets the white version from both parents, one chance in four, which is the 3 to 1.

Dominant only says which version shows; it says nothing about how common it is. Having extra fingers or toes is usually a dominant trait, yet it is rare. This video goes with the lesson Mendel's Peas in Biology Level 1 on Knaph.

Transcript

1. 0:00Sometimes a child has a trait neither parent shows, like a great-grandfather's red hair. Where was it hiding?
2. 0:09From 1856, the monk Gregor Mendel crossed pea plants in his abbey garden in Brno. Nearly 30,000 plants, seven traits, each with two clear forms.
3. 0:22He crossed violet-flowered plants with white-flowered ones. Every one of their young had violet flowers. White seemed to have vanished.
4. 0:34Then he let those young pollinate themselves. White came back. 705 violet, 224 white. About 3 to 1.
5. 0:48Each plant carries two versions of the gene, and passes on one. Violet is dominant, it shows with one copy. White is recessive, hidden but still passed on. White shows only with two copies, one chance in four.
6. 1:06Dominant doesn't mean common. Extra fingers or toes is usually a dominant trait, yet it is rare.
7. 1:18Two versions, one passed on. So a recessive trait can hide, and come back.