Homozygous gene pair are pair of allele that are identical e.g BB
Heterozygous gene pair are pair of allele that are not identical e.g Bb
Heterozygous gene pair could show complete dominance where the Allele for dominant will completely mask the effect of the reccessive allele and the individual will express the trait of the dominant allele.
Answer - They take in C02 (Carbon Dioxide) & Release O2 (Oxygen)
Reasoning - To be able to do Photosynthesis they Plants need C02 which is one of the important components to making Glucose. During that gas exchange they release O2 for C02.