Codominance refers to the expression of two genes simultaneously. Heter0zyg0us individuals express both phenotypes. So the answer to the question is D. Codominance.
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- Sex-linked ⇒ Refers to the gene that is linked to one of the sex chromosomes. In general, genes are linked to the X chromosome.
- Incomplete dominance ⇒ Refers to the cases in which the heter0zyg0us individual expresses an intermediate phenotype between the dominant and the recessive phenotypes. Neither of the involved alleles completely dominates over the other one.
- Complete dominance ⇒ When the dominant allele completely masks the recessive allele, we talk about complete dominance. This is the case of individuals that are heter0zyg0us for a particular gene and express the dominant trait. The dominant allele is hiding the expression of the recessive allele. Many genes show complete dominance.
- CODOMINANCE ⇒ Refers to those cases in which a phenotype expresses two alternative variants inherited from each h0m0zyg0us parent.
When two individuals, who are h0m0zyg0us and different for a trait, are crossed they produce heter0zyg0us individuals.
In codominance, Heter0zyg0te individuals, instead of showing an intermediate phenotype -which is the case of incomplete dominance-, express both alleles.
Their phenotype is an additive expression of their parent's genes. Both alleles are equally expressed.
A typical example of codominance is the case of speckled chicken.
Speckled chickens (BW) are the product of the cross of a black individual (BB) with a white individual (WW). Their progeny will express both genes as speckled feathers.
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