Linkage was one of the first phenomena to break the Mendelian laws, but there were many additional conditions that Mendel would have puzzled over, such as incomplete dominance. Usually in a heterozygous organism (one with a dominant allele and a recessive allele at the same locus), the pheno-type is controlled by the dominant allele, and the trait from the recessive allele will be masked. When incomplete dominance occurs, the dominant trait is weakened, and the heterozygotes look as though they have a trait partway between the recessive and dominant traits. For example, if a red-flowered snapdragon, RR, is crossed with a white-flowered snapdragon, rr, all the first-generation offspring are heterozygous, Rr. If the trait were dominant, then all the flowers in the offspring would be red. However, the trait displays incomplete dominance, and all the flowers are pink.
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