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Exploring complex inheritance patterns like incomplete dominance and codominance.
If you mix red and white paint, you get pink. But if you mix a red bird and a white bird, why are the babies sometimes pink and other times speckled with both red and white feathers?
In Mendel's classic experiments, one trait was always the 'boss' (dominant) and the other was 'hidden' (recessive). However, nature is often more colorful! Incomplete dominance occurs when the phenotype of the offspring is somewhere in between the two parents. Think of it like a dimmer switch rather than an 'on/off' light switch. Neither allele is completely dominant over the other, resulting in a blended appearance in heterozygous individuals ().
In snapdragon flowers, red color () and white color () show incomplete dominance. 1. Parent 1 is Red: 2. Parent 2 is White: 3. When crossed, 100% of the offspring are . 4. Because neither color wins, the flowers appear pink.
Quick Check
If a 'Long-tail' cat and a 'No-tail' cat have kittens with 'Medium-length' tails, what type of inheritance is likely happening?
Answer
Incomplete dominance, because the offspring show a 'blended' intermediate trait.
While incomplete dominance is about blending, codominance is about co-existing. In this pattern, both alleles are expressed equally and at the same time. You don't get a mix; you see both original traits clearly. A classic example is the 'Erminette' chicken. If you cross a black chicken with a white chicken, the offspring aren't grey—they are speckled with distinct black and white feathers. Both alleles are 'strong' enough to show up.
Human blood types are a perfect example of codominance. The alleles for Type A () and Type B () are codominant. 1. A person with genotype has Type A blood. 2. A person with genotype has Type B blood. 3. A person who inherits one of each () has Type AB blood. 4. Their red blood cells actually display both A and B proteins on the surface simultaneously.
Quick Check
In a certain species of fish, crossing a blue fish with a yellow fish results in fish with blue and yellow scales. Is this incomplete dominance or codominance?
Answer
Codominance, because both the blue and yellow colors are visible and distinct, rather than blended into green.
To master genetics, you must distinguish between these three patterns. In Mendelian inheritance, the dominant trait masks the recessive. In Incomplete Dominance, the traits mix like paint (). In Codominance, the traits appear together like a mosaic (). Understanding these helps scientists predict everything from the color of a prize-winning horse to the likelihood of a child inheriting a specific blood type.
Suppose a mother has Type AB blood () and a father has Type O blood (). 1. The mother can pass on or . 2. The father can only pass on . 3. Possible offspring genotypes: (Type A) or (Type B). 4. Even though the mother is AB, none of the children will be AB or O!
A scientist crosses a white bull and a red cow. The offspring is a 'Roan' calf, which has both red and white hairs growing side-by-side. This is an example of:
If two pink snapdragons () are crossed, what percentage of the offspring will be pink?
In incomplete dominance, the 'recessive' trait is completely hidden by the 'dominant' trait.
Review Tomorrow
In 24 hours, try to explain to a friend the difference between a 'pink' flower and a 'spotted' flower using the terms 'incomplete dominance' and 'codominance'.
Practice Activity
Draw a Punnett square for a cross between a person with Type AB blood and a person with Type B blood (genotype ). What are the possible blood types of their children?