Genome is the sum of all the genes occurring in the cells of a given individual. In contrast, genotype is the sum of all the alleles of a particular individual. The nuclear genome is the sum of all the genes occurring in the nucleus of the cell; the cytoplasmic or organelle genome is the sum of the genes contained in the DNA of cell organelles, mitochondria or plastids. The genomes of males and females of a certain species can differ in the presence or number of sex chromosomes, i.e. chromosomes whose occurrence or number determines whether the individual will develop as a male or female, see Y Chromosome.