diff --git a/src/initialization/chromosome_structure/chromosome.py b/src/initialization/chromosome_structure/chromosome.py
index 1880f54..fecc278 100644
--- a/src/initialization/chromosome_structure/chromosome.py
+++ b/src/initialization/chromosome_structure/chromosome.py
@@ -1,12 +1,16 @@
 class chromosome:
-    # fitness = Empty, genes = [gene,gene,gene,etc]
-    def __init__(self, genes = []):
-        self.genes = genes
+    
+    # fitness = Empty; genes = [gene, gene, gene, etc.]
+    def __init__(self, genes = None):
+        if genes is None:
+            self.genes = []
+        else:
+            self.genes = genes
         self.fitness = None
 
     def add_gene(self, gene, index = -1):
         if index == -1:
-            index = len(self.genes) - 1
+            index = len(self.genes)
         self.genes.insert(index, gene)
 
     def remove_gene(self, index):
diff --git a/src/initialization/population_structure/population.py b/src/initialization/population_structure/population.py
index fd80c28..7973218 100644
--- a/src/initialization/population_structure/population.py
+++ b/src/initialization/population_structure/population.py
@@ -1,7 +1,11 @@
 class population:
-    # population = [chromosome,chromosome,etc]
-    def __init__(self, chromosomes = []):
-        self.chromosomes = chromosomes
+    
+    # fitness = Empty; population = [chromosome, chromosome, etc.]
+    def __init__(self, chromosomes = None):
+        if chromosomes is None:
+          self.chromosomes = []
+        else:
+          self.chromosomes = chromosomes
         self.fitness = None
 
     def get_closet_fitness(self,value):
@@ -10,7 +14,7 @@ class population:
 
     def add_chromosome(self, chromosome, index = -1):
         if index == -1:
-            index = len(self.chromosomes) - 1
+            index = len(self.chromosomes)
         self.chromosomes.insert(index, chromosome)
 
     def remove_chromosome(self, index):