Module 6 Lesson 3 - 2
Lesson 3 — Incomplete Dominance, Codominance, and Multiple Alleles
Incomplete Dominance
Read pages 594
Four o'clock flower is a plant native to tropical South America. As the name suggests, the flowers open in the late afternoon and close in the morning. Interestingly, when pure breeding red flowers are crossed with pure breeding white flowers, all the
F1 generation have pink flowers.
Why are there three phenotypes (red, white and pink) instead of two as was the case in all monohybrid crosses studied so far? This is a curious genetic phenomenon inconsistent with Mendel's observations. What is the explanation?
P1 Generation Phenotype = Red Flowers x White Flowers
P1 Generation Cross = RR x WW
| R | R | |
| W | RW | RW |
| W | RW | RW |
F1 Generation Phenotype = 100% Pink Flowers
F1 Generation Genotype = 100% RW
The phenotype of the pink flowers from the F1 generation is a blend of the two parents. It is an intermediate type in which the two alleles (red and white) blend to produce an intermediary pink flower.
When two of the F1 pink flowers are crossed, they produce red, pink, and white flowers.
F1 Generation Phenotype = Pink Flowers
F1 Generation Cross = RW x RW
|
|
R | W |
| R | RR | RW |
| W | RW | WW |
F2 Generation Phenotype = 1 Red : 2 Pink : 1 White Flowers
F2 Generation Genotype = 1 RR : 2 RW : 1 WW
More Examples of Incomplete Dominance
- Combining deep purple eggplants with white eggplants results in eggplants of a light violet color.
- Crossing a black and a white chicken produces a blue-feathered bird.
- When a long-furred Angora rabbit and a short-furred Rex rabbit reproduce, the result is rabbits with fur longer than a Rex, but shorter than an Angora.
- Pups of long-tailed and short-tailed dogs have medium-length tails.
- A cream colored horse mated to a horse with a red coat produces horses with golden coats such as palominos and buckskins.
- In humans, a parent with straight hair and one with curly hair have a child with wavy hair.