Genetic Diagrams II - Albino In the following example, the result of mating between a heterozygous albino man and another heterozygous albino woman is studied. It is known that the allele for normal pigment is dominant over the allele for albino.  Albino is a disease that causes people to be unable to produce pigments and their hairs and skins are colourless as a result. This is due to an effective gene that produces a defective protein. Let N be the dominant allele for normal pigment. Let n be the recessive allele for albino. Parent phenotype Heterozygous Normal Heterozygous Normal Parent genotype  Nn X Nn Gametes N n N n Possible offspring genotype NN Nn Nn nn Offspring phenotype Normal Normal Normal Albino Ratio of phenotype 3 normal : 1 albino Since the two fruit flies are heterozygous, their homologous chromosomes has the different alleles. When the homologous chromosomes separate during meiosis, the each parent will produce two types of alleles, N for normal pigment and n for albino. When the two gametes, (one from mother and one from father), fuse during fertilisation, three types of cells results. The ratio of normal child to albino child is 3 : 1.  Relating Topics - Paternal and Maternal Chromosomes - Random Assortment