Mendel's model of complete dominance does not cover the whole of genetics: some flowers produce an intermediate color when crossed, some people show two alleles at once in their blood group, and some diseases affect almost only men because their gene travels on the X chromosome.
Codominance
On a Wisconsin dairy farm, a pure white cow is crossed with a pure black bull, and the calves are born with clearly defined white and black patches all over their bodies instead of being gray: how can two characteristics be expressed at the same time without one dominating or being diluted by the other?
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Explain what codominance is and give a human example in which two alleles are expressed simultaneously.
Explain what codominance is and give a human example in which two alleles are expressed simultaneously.
If a pure red-flowered four o'clock plant is crossed with a pure white-flowered one, all the offspring produce flowers of a uniform pink shade: why does neither color dominate the other, as happened with Mendel's pea plants?
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Explain what incomplete dominance is and describe which phenotype appears when crossing a red-flowered plant with a white-flowered plant if the trait follows this…
Explain what incomplete dominance is and describe which phenotype appears when crossing a red-flowered plant with a white-flowered plant if the trait follows this pattern.
In the four o'clock plant, flower color shows incomplete dominance: RR is red, Rr is pink and rr is white. Cross a pink-flowered plant with a white-flowered one and…
In the four o'clock plant, flower color shows incomplete dominance: RR is red, Rr is pink and rr is white. Cross a pink-flowered plant with a white-flowered one and state the genotypic and phenotypic ratios of the offspring.
In four o'clock plants, flower color shows incomplete dominance: red (RR), pink (Rr) and white (rr). Complete the Punnett square in the diagram, where two pink-flowered…
In four o'clock plants, flower color shows incomplete dominance: red (RR), pink (Rr) and white (rr). Complete the Punnett square in the diagram, where two pink-flowered plants are crossed, and state the genotypic and phenotypic ratios of the offspring.
At a school blood drive, donations are classified into four different groups (A, B, AB and O), a variety that could not be explained if only two gene variants existed for this characteristic: how many different alleles actually determine blood group in the human population?
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Explain what codominance is and give a human example in which two alleles are expressed simultaneously.
Explain what codominance is and give a human example in which two alleles are expressed simultaneously.
Color blindness and hemophilia: "criss-cross" inheritance
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In Andalusian hens, plumage color shows incomplete dominance: BB is black, Bb is blue and bb is white. Cross two blue hens and state the expected genotypic and…
In Andalusian hens, plumage color shows incomplete dominance: BB is black, Bb is blue and bb is white. Cross two blue hens and state the expected genotypic and phenotypic ratios in the chicks.
Back to the start: some traits do not follow Mendel's simple dominant-or-recessive rule. Heterozygotes can show both alleles (codominance) or a blend (incomplete dominance), blood type has three alleles, and genes on the X chromosome show up more often in males because they have only one X.
Summary
Let's practice! 2 medium exercises in this section