The genotype of an organism is its genetic makeup which is characterized by the presence of alleles of a gene.
The genotype can be broadly of two types :
1. Heterozygous - when an individual has 2 different variants of a gene known as alleles, then the person is said to be heterozygous for that gene.
2. Homozygous - if both the alleles of a gene are the same, then the person is said to be homozygous.
Since in the given situation you have two different alleles - yellow and green represented by Y and y, the individual is heterozygous.