Being polyploid has a fitness advantage but is physically complex. During polyploid plant evolution, some duplicate genes retain their ancestral function, which affected the plant phenotype in allelic dosage or functional redundancy. However, how duplicated genes whose products needed to form functional complexes coped with deleterious mutations remained unclear. Here, we report a yellow green leaf-2 (ygl2) mutant with yellow-green leaves derived from a cross of Shaan 3025 (S3025) and Shi 4185 (S4185) that was controlled by a combination of Tachli-7A null and Tachli-7B truncation, whereas Tachli-7A null or Tachli-7B truncation individually resulted in normal leaf colour. Our results indicated genetic complementarity between TaCHLI-7A and TaCHLI-7B is responsible for normal chloroplast development. Furthermore, TaCHLI-7D was conserved in ygl2, S3025 and S4185 at both sequence and expression levels. Furthermore, two-thirds of the total mRNA abundance in S4185 with Tachli-7A null was sufficient for chlorophyll synthesis, indicating that redundant mRNA dosage was the reason for genetic complementarity. Particularly, Tachli-7A null can be retained in several modern cultivars with no disadvantage under field conditions, probably because the redundant mRNA dosage is expected to buffer the gene imbalance caused by the imperfect relationship between different copies of TaCHLIs and their molecular interactors. Furthermore, the loss of TaCHLI-7A seems to preserve the minimum dosage and maximise simplification. Our findings provide evidence of homoeologs loss and functional mechanism during polyploid evolution.