Incomplete Dominance and Codominance

0:00Cross a red snapdragon flower with a white one. Is the offspring always a blend of its parents?

0:07Red plants are RR and white plants are WW: each plant carries two alleles, or versions, of the colour gene.

0:17Each parent passes on one allele, so every offspring is RW, and RW flowers are pink.

0:26This is incomplete dominance: neither allele is fully dominant, so a heterozygous plant, with two different alleles, has an in-between colour.

0:37Now cross two pink flowers. A Punnett square shows every way their alleles can combine.

0:43Each box takes one allele from the top and one from the side.

0:48So we expect one in four to be red, two in four to be pink, and one in four to be white.

0:56Red and white come back, so the alleles never blended. They were passed on unchanged.

1:02Codominance is different: both alleles show fully, side by side.

1:08Cross red cattle, RR, with white cattle, WW, and every calf is roan.

1:17A roan coat has red hairs and white hairs mixed together, not pink hairs.

1:23Crossing two roan cattle fills the same Punnett square.

1:27Again we expect one in four to be red, two in four roan, and one in four white.

1:35Blood type AB is codominance too: allele A from one parent and allele B from the other give red blood cells with both A and B markers.

1:46So offspring are not always a blend: incomplete dominance gives an in-between trait, and codominance shows both alleles fully.