diff --git a/src/structure/chromosome.py b/src/structure/chromosome.py
index a5349ad..a0cbc38 100644
--- a/src/structure/chromosome.py
+++ b/src/structure/chromosome.py
@@ -70,10 +70,14 @@ class Chromosome():
                 chromosome[index] = gene
         to set the indexed gene.
         """
+
+        # Single gene
         if isinstance(index, int):
             self.gene_list[index] = to_gene(gene)
+
+        # Multiple genes
         else:
-            self.gene_list[index] = (to_gene(item) for item in gene)
+            self.gene_list[index] = [to_gene(item) for item in gene]
 
 
     def __delitem__(self, index):
diff --git a/src/structure/population.py b/src/structure/population.py
index 8708ff5..5369ca4 100644
--- a/src/structure/population.py
+++ b/src/structure/population.py
@@ -125,13 +125,11 @@ class Population:
 
         # Just one chromosome
         if isinstance(index, int):
-            chromosome = to_chromosome(chromosome)
+            self.chromosome_list[index] = to_chromosome(chromosome)
 
         # Multiple chromosomes
         else:
-            chromosome = [to_chromosome(elem) for elem in chromosome]
-
-        self.chromosome_list[index] = chromosome
+            self.chromosome_list[index] = [to_chromosome(item) for item in chromosome]
 
 
     def __delitem__(self, index):