A heterozygous plant with genotype Aa self-pollinates. Find the probability that an offspring is tall for the following genetic systems.
1. Only plants that have two A alleles are tall (the allele A is recessive).
2. Plants that have either one or two A alleles are tall (the allele A is dominant).
3. Heterozygous plants are tall, and homozygous plants are short.
4. Half of heterozygous plants and one fourth of homozygous plants are tall (it depends on their position in the greenhouse).